The Rise and Reign of Mammals by Brusatte

Ref: Stephen Brusatte (2022). The Rise and Reign of Mammals. Firefly Books.

___________________________________________________________________________

Summary

• THIS IS NOT a book about humans. We, Homo sapiens, are but one of more than 6000 species of mammals alive today. Viewed from the longer perspective of mammal evolution, we are a single point, among millions of species over more than 20My. It’s not all about us. The tale of mammal history told in these pages- extending back to those scaly critters in the coal swamps, enduring mass extinctions and dinosaurs and cruel climates- was not some mere backstory to our inevitable coronation. Submarine-size whales, woolly mammoths and sabertooths, ocean-rafting monkeys, and echolocating bats are all special in their own right. As, of course, are we: a big-brained, nimble-handed, two-legged primate whose intelligence and capacity for destruction are unparalleled among mammals. That, and we are the only ones that can contemplate our own origins. 

• In this book, I tell the story of mammal evolution, as we know it now. Roughly the first half of the book covers the early stages of the mammal lineage, from the time they split from the reptiles until the extinction of the dinosaurs. This was when mammals acquired nearly all of their signatures—hair, mammary glands, and so on—and morphed, piece by piece, from an ancestor that looked like a lizard into something we would recognize as a mammal. The book’s second part lays out what happened after the dinosaurs died: how mammals seized the opportunity and became dominant, adapted to constantly changing climates, rode along on drifting continents, and developed into the incredible richness of species today: runners, diggers, flyers, swimmers, and big-brained book readers. 

• Therapsids evolved from pelycosaurs, which stemmed from that “scaly critter” that split into the synapsid and diapsid lines during coal forest times, which in turn can trace its ancestry back to those tetrapods that evolved from fishes, crawled onto the land, and developed amniotic eggs. 

• Mammals: All animals that evolved a new jaw-closing joint between the dentary bone of the lower jaw and the squamosal bone of the upper skull.

___________________________________________________________________________

Tetrapod - Mammal Evolution

• Tetrapods (~390 Ma): Four-legged amphibians that first stepped onto land ~390 Ma; however, they still needed to return to the water to lay their eggs. Early tetrapods included Edaphosaurus, Echinerpeton, and Amniotes.

• Amniotes (~340 Ma).

• Diapsids (~325 Ma): Reptiles; Evolve from amniotes with two windowlike openings behind the eye socket to provide room for bigger and stronger jaw muscles. These would eventually evolve into lizards, snakes, crocodiles, dinosaurs, birds, and turtles (which closed up their holes). 

• Synapsids (~325 Ma): Evolve from amniotes with a single windowlike opening behind the eye socket that would diversify into an array of species, including >100 My in the future, mammals.

• Archaeothyris: An early synapsid that lived during the late carboniferous (~306 Ma) and grew to ~50cm in length. 

• Anteosaurus: An early synapsid that grew up to 5m long and weighed ~.5t (similar to a polar bear). It was among the largest synapsid predators that ever lived, before the advent of modern mammals.

• Pelycosaurs (308-260 Ma): The first big evolutionary wave of the synapsid lineage and were the first to diversify and spread around Pangaea; include the carnivorous Dimetrodon (299-273 Ma), vegetarian Edaphosaurus, and pudgy caseids.

• Therapsids (~299-201 Ma): Early animals that evolved from a midsize, carnivorous pelycosaur, and weighed ~50-100 kg. Therapsids include the dicynodonts, dinocephalians, gorgonopsians and many other subgroups. Early therapsids began to raise their metabolism, become warm-blooded, grow faster, bite stronger, and grow hair (most likely as a sensory aid- like whiskers, or as a gland-based waterproofing system).

• Gorgonopsians (270-252 Ma): The terrors of the middle and late Permian; they ranged in size from a small dog to monsters like Inostrancevia, which was 3.5m long, weighed ~300 kg with a .7m-long head, and had enlarged canines, of the saber-tooth style.

• Dicynodon (254-251.9 Ma): The most diverse Permian therapsid subgroup.

• Cynodonts (260-125 Ma): An early stem-lineage mammal that evolved a secondary palate- a hard roof of the mouth- which divides the mouth from the nasal passage giving air its own dedicated pathway to the lungs; cynodonts could eat and breathe at the same time. Cynodonts survived the Permian extinction. They diversified during the remaining 50 My of the Triassic. One of the cynodont lines, which included Thrinaxodon, the tusked dicynodonts, the head-butting dinocephalians, and saber-toothed gorgonopsians, led to mammals. Over the course of cynodont evolution, the quadrate, articular, and many smaller bones at the back of the jaw withered in size while a new, stronger dentary-squamosal jaw joint- the feature that defines mammals- replaced them. Include Charassognathus (the oldest known cynodont, ~ the size of a squirrel), and Thrinaxodon.

• Unknown Link (includes Oligokyphus- closest mammal cousins)

• Mammals

• Mammals: Animal species with a dentary-squamosal jaw hinge.

• Morganucodon (~200 Ma): One of the earliest mammals which developed a dentary-squamosal jaw hinge. During cynodont evolution the dentary progressively got bigger and stronger, while the postdentary bones atrophied, until it became necessary to build a new dentary-squamosal jaw hinge. In Morganucodon and the first mammals, there were two jaw joints: the new dentary-squamosal one and the ancestral quadrate-articular one. The dentary-squamosal joint was doing most of the jaw closing and was the primary source of the strong bites and precise chewing of these animals. The quadrate and articular joint, however, was still bearing some of the jaw load. At the same time, the quadrate and articular bones had moved backward to the point where the quadrate contacted the stapes bone—that ancestral middle ear bone that transmits sound from eardrum to cochlea in reptiles, amphibians, and birds. Therefore, in Morganucodon-type mammals, the quadrate-articular joint was serving dual functions: it was transmitting sound to the ear, but also participating in jaw closing.

• Unknown Mammal Link(s)

• Docodonts (176-145 Ma): One of the first great radiations of mammals that did not survive past the Cretaceous. 

• Haramiyidans: One of the first great radiations of mammals that did not survive past the Cretaceous. 

• Unknown Mammal Link

• Monotremes (‘One-holed’- Greek): Modern egg-laying mammals; include the platypus and echidna that lay eggs and have a single all-purpose orifice for urination, defecation, and reproduction.

• Unknown Mammal Link

• Multituberculates (~167-34 Ma): An early mammal group that evolved after the decline of the docodonts and haramiyidans and took advantage of the angiosperm evolution. They may have evolved from haramiyidians, as both groups had similar molar teeth, with long cusp rows, and chewing strokes, in which their jaws moved backward in a grinding motion. Variants include Kryptobaatar (modern Gobi Desert), Sloanbaatar, Kamptobaatar, Catopsbaatar, Nemegtbaatar, Bulganbaatar, Chulsanbaatar, Nessovbaatar, Tombaatar

• Therians

• Therians: A cretaceous era mammal that evolved a tribosphenic tooth- a special type of molar tooth, ideal for cutting, grinding, and pulverizing the hard exoskeletons of insects in one chewing stroke to extract the nutrients within. Major therian groups include placentals and marsupials.

• Metatherian: Evolved from tribosphenic therians during the Cretaceous; carnivores, which knifed through muscle and sinew with sharp, modified tribosphenic molars. Include modern-day marsupials like kangaroos and koalas that give birth to feeble young that develop further in a pouch. Metatherians were hammered by the end K-T extinction; they managed to hold on in Europe, Asia, and N. America for a few tens of millions of years, but then vanished across the North.

• Didelphodon: A massive mammal that weighed ~5 kg, about the size of its distant cousin, the opossum. 

• Marsupials (~125 Ma): Modern pouched mammals that give birth to live young which develop further in a pouch. Includes koalas, kangaroos, wallabies, bandicoots, possums, and Tasmanian devils (~250 total species).

• Deltatheridium: A type of metatherian with bulging jaw muscles, big piercing canines, large upper and lower molars, and reduced talonid basins, but enlarged shearing cusps and crests, which resembled blades.

• Eutherians: Evolved from tribosphenic therians during the Cretaceous and survived the K-T extinction; insectivores that ate bugs with tribosphenic teeth; include placentals (like us) that give birth to well-developed young. Include Ectoconus (~ the girth of a pig), Wortmania (~the size of a badger), Eoconodon, and Pantolambda.

• Placentals: A mammal sub-group divided into four groups: Afrotheria, Xenarthra, Laurasiatheria, and Euarchontoglires. All of today’s key placental mammals had emerged and were thriving in the Eocene.

• Afrotherians (~100 Ma): An early mammal group predominantly from Africa that includes golden moles, tenrecs, hyraxes, elephants, aardvarks, and manatees.

• Sirenians: Afrotherian mammals that migrated globally along the Tethyan.

• Xenarthrans (~60 Ma): An early mammal group with ‘xenarthrous articulations’ (extra joints) between their vertebrate, which strengthen and stabilize their backbones; incorporates mostly South American species like anteaters, sloths, and armadillos.

• Unknown Mammal Link

• Euarchontoglires (~90 Ma): A mammal group distributed widely across Europe, North America, and Asia, but with members south of the equator, too; includes our primate cousins (and us), rabbits, and rodents.

• Laurasiatherians (~90 Ma): A mammal group distributed widely across Europe, North America, and Asia, but with members south of the equator, too; includes dogs, cats, pangolins, and odd and even-toed hoofed mammals, whales, and bats.

• Unk Mammal Link

• Carnivores (dogs, cats)

• Unk Mammal Link

• Perissodactyls: Odd-toed hoofed ungulates that evolved in Africa; hindgut fermenters, which break down the cellulose in plant matter in their intestines, after it has passed through the stomach; include 20-odd species of odd-toed horses, rhinos, and tapirs and several extinct species such as Brontotheres, Chalicotheres, and Sifrhippus.

• Sifrhippus: An early WY perissodactyl. A tiny horse with hooves perched on long limbs, but its more flexible shoulder and pelvic joints provided greater maneuverability as it galloped through the dense undergrowth- like its close cousin, the Messel mare Eurohippus, which lived later in the Eocene, after the warming pulse.

• Artiodactyl: Even-toed hoofed ungulates that evolved in Africa; ruminators with a multiplex stomach of four chambers that chew their cud, swallow their food, process it in the first two stomach chambers, regurgitate it, chew it some more, and pass it back through the entire stomach. By doing so, they extract maximum nutrition out of every morsel they eat, a great trick when eating tougher or lower-quality vegetations, like grasses; include even-toed cows, camels, deer, pigs, whales, and dolphins.

• Diacodexis: The earliest-appearing artiodactyl; it looked like a deer, albeit one the size of a rabbit with a body customized for speed: its limbs were long, thin, and capped with hooves. Its main ankle bone, the astragalus, had a deep groove at each end, ensuring that the foot could extend and flex in a fore-aft direction without rotating laterally. This “double pulley” condition is a hallmark of today’s artiodactyls- from cows to camels- and permits them to run fast without dislocating their ankles.

___________________________________________________________________________

Intro: Our Mammalian Family

• Mammals arose ~325 Ma, starting when scaly critters in the coal swamps broke away from the reptile line.

• There are >6K mammal species on Earth today.

• All modern-day mammals belong to one of three groups: the egg-laying monotremes like the platypus, marsupials that raise their tiny babies in pouches like kangaroos and koalas, and placentals, which give birth to well-developed young, like us.

___________________________________________________________________________

Making a Mammal

• Today’s mammals and birds are fully warm-blooded, which means three things. They control their body temperatures internally, and have high body temperatures and constant body temperatures, regardless of whatever their external environment throws at them. For mammals, this process began with therapsids in the Permian, as they dealt with the seasonal climates in their higher-latitude homes. Their finer control of body temperature and faster growth were probably central to their survival—particularly of the Thrinaxodon-type cynodonts—at the end-Permian. Then, during the Triassic, these cynodonts continued to move toward total warm-bloodedness. 

• Turbinates: Small structures inside the nose that cleanse, warm and moisten the inhaled air that passes through the nostrils into the lungs. We have them- inside our skull, in between our nostrils and the back of our throat. They ensure that even our coldest, driest breaths on the most frigid of winter days are warmed and humidified to rain-forest-like conditions within a split second. They also work in reverse, recouping precious water as we breathe out. 

• The lateral temporal fenestra is the hole that evolved ~325 Ma in the Carboniferous and defines the synapsid lineage—merged with the eye socket, creating a single, master chamber for the muscles. There was one major problem, though: the joint with the upper skull was still located on the articular bone, which was reducing to obsolescence. The bone of the upper skull that it connected to, the quadrate, was also dwindling. Some cynodonts developed a brace on the surangular bone of the lower jaw to strengthen the contact with the upper skull, but because the surangular was also shrinking, it didn’t do much good. No engineer would design a jaw-closing system this way, and the only reason it worked was because these cynodonts were getting so small that they didn’t have to endure the fierce bite stresses of their larger ancestors. Somehow, the cynodonts needed to find a solution. They did—and it is what defines mammals…The dentary-squamosal joint was a game-changer. Suddenly the jaws, which had been so tenuously hitched to the skull, were firmly fastened. This new joint- operated by the jaw muscles that had gotten bigger earlier in cynodont evolution- was now capable of generating much stronger bites. It could also deliver much more controlled bites, orchestrated by the divided temporalis, masseter, and pterygoideus muscles, the three strings of a puppeteer. These jaws could focus the strongest bite forces on particular teeth at particular times. This unlocked a completely new way of eating, which is extremely rare among animals: chewing. By chewing their food into mush, these early mammals could do most of the processing in the mouth- essentially beginning digestion before the food hit the stomach. It was yet another way to take in more calories, more efficiently. 

• Mammals: All animals that evolved a new jaw-closing joint between the dentary bone of the lower jaw and the squamosal bone of the upper skull.

• Mammary Glands: The largest and most complex glands in our skin, produce milk, which mammalian mothers use to nourish their young, a process called lactation. There are two leading ideas for the development of mammary glands today. The first is that skin glands began secreting antimicrobial fluids, to help protect the newborns from bacterial infections, and this later developed into a full-on food source. The second is that milk was initially used to keep the tiny eggs of mammals moist, so they wouldn’t dry out, but then the hatchlings started to eat it, and natural selection turned it into their nourishment…As soon as they started feeding milk to their young, the first mammals developed enlarged brains with a novel structure, the neocortex, for sensory processing. 

• Ossicles: Bones of the ear which include the malleus (hammer), incus (anvil), and stapes (stirrup). The ossicles form a chain between the eardrum and inner ear: the eardrum contacts the hammer, which has a mobile joint with the anvil, which touches the stirrup, which strikes the cochlea, the soft part of the inner ear that actually processes sound and sends auditory signals to the brain. The ossicle chain serves three key functions. Like a telephone line, it transmits sound from the eardrum (receptor) to the cochlea (processor). Like a bunch of bullhorns lined together, it amplifies this sound. And like a power plug converter you may bring on vacation to another country, it converts airborne sound waves to waves that flow through the liquid inside the cochlea. It is the motion of these microscopic waves that trigger teeny hairs in the cochlea, whose movements are then transformed to an electrical signal, which is relayed by nerves to the brain, producing what we sense as “sound.”

• Birds, reptiles, and amphibians can all hear. They can all take sound waves and convert them to liquid waves in their cochlea’s. But they can’t hear anywhere near as well as mammals, and across such a wide range of frequencies, because they have only a single ear bone to do it all- the stapes, the equivalent to the stirrup. 

• The hammer and anvil are the articular and the quadrate. They are jaw bones. 

• In humans, the hammer and anvil become disconnected from the jaw during the 8th month in the womb. 

• Tribosphenic Molar: Constructed over many evolutionary steps. Its foundation was the molar shape of the first Triassic and Jurassic mammals like Morganucodon, which looked like a mountain with three peaks when viewed from the side. To these three cusps on the lower molars, therians added another three. The six cusps divided into two distinct regions: a “trigonid” set of three pointy spires at the front of the tooth and a “talonid” basin rimmed by three more subtle cusps at the back of the tooth. Meanwhile, the upper molars transformed too: they sprouted a big new cusp on their tongue-ward side, called the “protocone,” which snugly fit into the talonid basin of the corresponding lower molar when the jaws closed. 

• The two types of tribosphenic teeth are not equivalent. There is a therian version, and a monotreme version, which evolved independently, probably both during the Middle Jurassic, and for a similar reason: to increase shearing capability, and maybe to add a little bit of grinding. The therian version evolved in the north, enabled marsupial and placental antecedents to flourish during the Cretaceous Terrestrial Revolution, and persists today as a highly adaptable tooth design, including in our own mouths. The monotreme version evolved in the south, seems to have spread widely below the equator during the Jurassic and Cretaceous, but then essentially disappeared, its only remnant the ghostly teeth of the platypus that disintegrate as babies leave the nest.

• Placenta: A temporary organ that exists only during pregnancy, connecting fetus to mother. Placentas are not unique to mammals; they have evolved ~20x, in a variety of species that traded egg-laying for live birth, including even some fishes. An egg is essentially its own care package, with a yolk that includes all the nutrients a growing embryo needs to develop. Once a mother lays her eggs, she can protect them, but she can’t really break through the egg shell to provide additional nourishment. Live birth, however, requires that an embryo (and then fetus) grows inside its mother until it emerges into the world. It needs access to food and O during this time, plus a way to expel waste. The placenta does the trick, acting as the baby’s pantry, lungs, and excretory system at the same time. Then, after childbirth, it is simply discarded—as what we unceremoniously call the “afterbirth.” 

• Hair: Likely first appeared in Permian era therapsids- dicynodonts and cynodonts, first as a sensory aid (like whiskers), a display structure, or as part of a gland-based waterproofing system, and was later repurposed as a body coating to retain heat by early therapsids. 

___________________________________________________________________________

Carboniferous (355-299 Ma)

• During the Carboniferous, two types of trees make up the bulk of Earth’s canopy, both of which grew to around 30m high. One was called Calamites, and it looked like an emaciated Christmas tree, with a straight bamboo-esque trunk that spit out intermittent bunches of branches with whorls of needle-shaped leaves. The other was Lepidodendron, whose 2m-thick trunks were naked except for a thicket of branches and leaves only at the very tip-top- a mop of foliage on a giant stalk. They grew remarkably fast, going from spore to sapling to the pinnacle of the canopy in only 10 or 15 yrs, before dying, being buried and turned to coal, and replaced by another generation…The growth of so many giant trees drew CO2 out of the atmosphere, and with less of this GHG to insulate the planet, temperatures plummeted. Over tens of millions of years, the ice cap waxed and waned in size, a conductor controlling the global sea level. Ice would melt, seas would rise, swamps would drown, trees would die and get buried. Then the ice would grow, sucking up water from the seas, lowering the sea level, making room for swamps to thrive. Back and forth it went. We know this because Pennsylvanian rocks often form barcode sequences called cyclothems, repeated series of thin layers formed on land and in the water, with coal seams tucked in between. 

• Over many millions of years, Gondwana drifted northward, at about the rate our fingernails grow, before colliding with Laurasia. This was the beginning of the birth of Pangea. Today’s modest Appalachian Mountains are a remnant of this once-towering chain.

___________________________________________________________________________

Permian (299-252 Ma)

• There was a massive drop in diversity as the Carboniferous transitioned to the Permian, as many coal-forest tetrapods went extinct. This likely didn’t happen all at once, but over several million years, as the drylands gradually replaced the coal forests in the tropics, in a march from west to east. This habitat transition produced more open landscapes, which favored migration, an ideal environment for amniotes.

• End Permian Mass Extinction: Occurred ~252 Ma; megavolcanoes, fed by a hot spot of magma deep in the Earth’s mantle under Siberia, and on the northern fringes of the Pangaean subcontinent erupt basalt, CH4, CO2, S, and other noxious gases for several hundred thousand years, inundating several million square miles of North Pangaea with lavas and causing runaway global warming as increasing GHG’s in the atmosphere trapped heat by absorbing radiation and beaming it back to Earth. Over a few tens of thousands of years, temperatures increase ~5-8 C and the oceans acidify, starving the water of O and causing ~90% of all species, maybe more, go extinct. In the oceans, there is widespread death of shelled invertebrates and other sea life. As the climate changed, the forests withered, and about 70-90% of surface vegetation disappeared. This caused the entire ecosystem to fall like a house of cards. The diverse Permian forests, dominated by the seed fern Glossopteris and evergreen gymnosperms, collapsed, as plants endured their second and final mass extinction—the only one after the Carboniferous Rainforest Collapse some 50 My earlier. They were replaced by ferns and club mosses- much smaller relatives of the coal swamp Lepidodendron trees- which grew fast from spores rather than seeds, allowing them to better cope with the intense seasonality and fluctuations in rainfall. As the climate changed, the forests withered, and about 70-90% of surface vegetation disappeared. This caused the entire ecosystem to fall like a house of cards. 

___________________________________________________________________________

Triassic (252-201 Ma)

• Thanks in part to the dentary-squamosal joint, Triassic mammals became food-processing machines. Their jaws could move up and down, back and forth, and side to side. They developed a chewing motion, consisting of three actions in sequence. First the lower jaw would move upward and inward to approach the upper jaw (the power stroke) as the mouth closed, then downward as the mouth opened (recovery stroke), and then slightly outward (the preparatory stroke) before going through the process again and again. All this was enabled by the unique ability of the lower jaw to roll outward and inward as the jaws opened and closed. 

• Triassic Extinction (~202 Ma); Supercontinent Pangaea begins to split apart down its middle with North America separating from Europe and South America from Africa. Before water fills the gaps between the diverging landmasses, the earth hemorrhaged lava. For ~600K yrs, four violent pulses of mega-volcanic activity erupt along the future Atlantic seaboard, increasing GHG concentrations in the atmosphere and leading to global warming. The temperature spike causes oceans to acidify, starves shallow waters of O, and triggers ecosystem collapse, with ~30% of species dying out.

___________________________________________________________________________

Jurassic (201-145 Ma)

• Jurassic and Cretaceous mammals were hugely diverse. Their only limitation was size: most were about the size of shrews or mice, and none that we know of was bigger than a badger.

___________________________________________________________________________

Cretaceous (145-66.7 Ma)

• As these early mammals were competing with one another for resources in the Jurassic, the tribosphenic molar was a useful gadget for tiny insect-eaters, but not yet a game-changer. It was only during the Cretaceous Terrestrial Revolution, many tens of millions of years later, that the explosion of angiosperms sparked a diversification of insects, generating an all-you-can-eat bug buffet. 

• The Cretaceous terrestrial revolution (125-80 Ma); mammals split into placentals, marsupials, and monotremes, and angiosperms evolve. The revolution was a changing of the guard from primeval communities to a more modern world, of forests alive with colorful flowers, fragrant fruits, buzzing insects, chirping birds, and most importantly for our story, a multitude of new mammals, including the immediate ancestors of today’s placentals and marsupials. 

• At the time of the Cretaceous terrestrial revolution, north and south were separated by a wide equatorial waterway, called the Tethys Sea. 

• DNA family trees of mammals imply that primates originated in the Cretaceous, so maybe our primate ancestors actually survived the asteroid instead of just taking advantage of it.

• At the end of the Cretaceous the continents were near their present positions. South America was widely separated from North America, but remained tenuously tethered to Antarctica, which nearly touched Australia. India was an island continent off the east coast of Africa, plowing rapidly northward. There were no ice caps, so sea levels were high, and Europe was nothing but a speckle of islands poking out of a tropical sea. Another sea lapped far onto North America, at times stretching from the Gulf of Mexico to the Arctic, bisecting the continent into a mountainous western slice called Laramidia and an eastern portion called Appalachia. The European islands were convenient stepping-stones between North America and Asia, but there was a wide oceanic barrier between these northern lands and the southern continents.

• At the end of the Cretaceous, ~66 Ma, mammals were ubiquitous, although still small. Vintana, weighing ~9 kg, was by far the largest- regardless, an easy snack for a tyrannosaur or other meat-eating dinosaur. Therians and multituberculates were ensconced in the north, from the Asian heartlands to the North American mountains to the European islands. Among them were insectivores that demolished bugs with their tribosphenic teeth (eutherians), herbivores that feasted on flowers and fruits and other angiosperm parts (multituberculates), and the odd carnivore, which knifed through muscle and sinew with its sharp, modified tribosphenic molars (metatherians). To the south, across the crystal blue waters of the Tethys Sea, were other mammals that filled similar roles: the monotreme-line insect-eaters with copycat tribosphenic molars (australosphenidans); other insectivores with long snouts (dryolestoids); and herbivores (gondwanatherians). 

• End Cretaceous (K-T) Extinction: ~66 Ma; an asteroid (possibly a comet)- came from the far reaches of the solar system, beyond the orbit of Mars, perhaps even farther. It was ~10 km wide (the size of Mount Everest), and about 3x as wide as Manhattan. It collided with what is now the Yucatán Peninsula of Mexico, impacting with the force of >1B nuclear bombs, punching a hole in the crust 40 km deep and >160 km wide. That scar is still visible today as the Chicxulub Crater. Almost instantaneously, everything within 1000 km of ground zero was vaporized, wildfires and hot glass rain during the next few hours and days, a nuclear winter that followed lasted for decades, which was followed by a few millennia of global warming. ~75% of species died out.

• Within ~375,000 to 850,000 years of the asteroid, as temperatures stabilized and ecosystems recovered, mammals were thriving in Montana. 

• Within ~200K yrs, multituberculate mammals suffer a modest extinction, metatherians are nearly wiped out, and eutherians diversify in scale. The Paleocene mammal survivors were smaller than most cretaceous mammals, and their teeth indicate that they had generalist, omnivorous diets.

___________________________________________________________________________

Paleocene (66-56 Ma)

• “Archaic” placentals were building the first mammal-dominated ecosystems in the Paleocene.

• Teilhardina: A Paleocene period WY primate with big eyes, nails on its fingers, toes to grip branches, and lithe ankles allowed it to move gracefully through the canopy.

• Purgatorious: A stem primate; a type of plesiadapiform; the oldest-known animal on the primate bloodline, after it diverged from the other major groups of mammals ~62 Ma. These first primates ate more vegetation with their modified molars, but they had moved from the ground into the trees- a refuge from the chaos and climate swings and post-dinosaur predators.

• Rodents: Originated during the Paleocene, before the PETM. Today there are >2000 species of rodents (~40% of all mammals). 

• Paramys: An early rodent that looked like a mix between a squirrel and a prairie dog that mostly lived in trees. They possessed two signatures of modern rats, mice, beavers, and kin: the ability to chew food by sliding the jaws forward and backward and ever-growing incisors for gnawing.

• Carnivorans: Members of the dog and cat group that originated during the Paleocene, before the PETM. Carnivores evolved a new dental utensil for cutting flesh and breaking bone: enlarged cheek teeth (either premolars or molars) that resemble knife blades. There are four of these so-called carnassial teeth in the mouth, one on each side of the upper and lower jaws, and the corresponding upper and lower pairs work against each other as carnivorans bite. With their new razor teeth, carnivorans supplanted the “archaic” meat-eaters, and they have remained at the top of the food chain—as lions, tigers, hyenas, and wolves.

• Sparassodonts: South American metatherian carnivores. Sparassodonts like Thylacosmilus were eventually replaced by jaguars and other actual placental predators that moved southward, 

• Eoconodon: A Paleocene era “archaic” condylarth from modern New Mexico. 

• Hoof: A modified ungual, or last bone, on each finger or toe.

• Macrauchenia: Primitive hoofed animals.

• Toxodon: Primitive hoofed animal.

• Pyrotherium: A primitive hoofed animal with upper and lower tusks and a trunk, like an elephant, on the body of a hippo.

• Homalodotherium: A primitive hoofed animal with scythe claws on its hands.

• Thoatherium: A primitive hoofed animal which walked on only a single hoofed toe, like a modern-day horse.

• Ungulates: Originated only in South and Central America. Darwin’s Ungulates persisted for well over 60 My. They nearly made it to the present, but the last survivors were among the victims of the Ice Age extinctions around 10 Ka.

• Darwin’s Ungulates—at least most of them—were close cousins of horses, rhinos, and tapirs. They likely evolved from “archaic” placental ancestors, like the New Mexican condylarths.

• Paleocene-Eocene Thermal Maximum (PETM): Over a 20K yr period, CO2 released from Atlantic mid-oceanic ridge volcanoes increases global GHG levels, raising global temperatures between 5-8C with the average temperatures of equatorial regions rising to >40C and the arctic to 25C. Large swaths of low-latitude waters are too hot to support much, if any, life. The surge of global warming peaks and abates within 200K. Conifers withered and were replaced with trees better able to withstand heat, particularly those of the bean family, which migrated ~600-1500 km northwards from the tropics. Over the following 10-27K yrs, scores of new mammals appeared in the Bighorn Basin. Chief among these were the first members of three modern groups, what we call the PETM Trinity: primates, even-toed artiodactyls, and odd-toed perissodactyls. It was either during or just after the PETM that Primates evolved.

• The PETM was part of the disassembly of Pangea, which had begun some 140 My earlier, when the first mammals were scurrying. As magma percolated through the crust, on its way to the surface, it fanned out into thousands of horizontal sheets called sills, which literally baked organic matter they came into contact with. Like an engine burning gasoline, this released GHGs: CO2, and the more potent CH4. Trillions of tons of C leaked into the atmosphere, raising the CO2 level between 2-8x beyond that of the already scorching Paleocene. Temperatures spiked, leaving a telltale chemical fingerprint in the rocks: a sharp decline in the ratio of the heavier isotope of O (18O), which has more neutrons than the lighter isotope (16O). From lab experiments, we know that the ratio of these two isotopes is a paleothermometer—and it pinpoints the temperate rise, of five to eight degrees Celsius, right at the Paleocene-Eocene boundary. 

• In a landmark study published in 2012, Ross surveyed the Bighorn Basin mammal fossils. He found that about 40% of the Paleocene locals got smaller during the PETM, most of which then bounced back and grew larger. Horses got progressively smaller as the world went hot, and then got bigger as the world cooler.

___________________________________________________________________________

Eocene (56-33.9 Ma)

• As the Paleocene gave way to the Eocene, plesiadapiforms diversified from their Purgatorius origins. They became a successful group of >150 known species, which lived across North America, Europe, and Asia. Features of the hands and feet that permitted better branch-grabbing evolved alongside dental features enabling better fruit-chewing. They were different from their Purgatorius-type ancestors in two main ways. First, they became even better arborealists, able to leap through the branches. To do it, they turned their claws into flat nails, added an opposable thumb and long toes to the opposable big toe and long fingers of their ancestors, and fashioned a more constrained ankle that still could move in many directions but was now stable enough for jumping and sticking the landing. Second, they became much smarter and sharper eyed. Their brains not only got bigger but were reorganized, with a larger neocortex for sensory integration, and larger visual regions that developed as the olfactory regions truncated, reflecting a trade of smell for eyesight. Their bulbous, forward-facing eyes could see in 3D, and some in technicolor. Bigger eyes and brain plus atrophied nose meant a flatter face, with less of a snout. With these new adaptations, primates spread and diversified with an intensity that matched the extreme Eocene temperatures. Quickly they hopped southward to Arabia and Africa. Their family tree swelled, spinning out lemurs—with lower incisors and canines modified into a comb to groom their fur- and then New World monkeys, which made their ridiculous cross-ocean journey from Africa to South America. At least once, and maybe twice, lemurs also rode the waves, hitching an easterly ride on currents streaming off Mozambique and landing on an island that would become their playground, and later their only sanctuary: Madagascar. 

• Lemurs: Primates with lower incisors and canines modified into a comb to groom their fur. 

• Archaeoindris: A giant lemur ~the size of a gorilla that weighed 200kg.

• The migration of primates and rodents from Africa to South America was one of those fluke events that changed the course of mammalian history. It is because of this unlikely journey that we have New World monkeys and caviomorph rodents today. These monkeys- some 60+ species- are part of the fabric of Central and South American jungles. Some, like the howler monkeys, drown the rain forests in their cacophony of screams. Others, like the spider monkeys, are the only primates that hang from trees with prehensile tails, while the pygmy marmosets, half a foot long and weighing less than half a pound, set the record for primate smallness. The caviomorphs are even more diverse, with hundreds of species burrowing, climbing, running, and swimming across basically every environment South America can throw at them. Among them are the velvet-furred chinchillas and the largest rodents alive today, the dog-size capybaras, plus extinct ones that were much bigger, like the cow-size Josephoartigasia. And guinea pigs, were descendants of the Eocene rafters. 

• During the Eocene, the main groups of placental mammals- primates, even and odd-toed hoofed species, carnivorans like dogs and cats, elephants, bats, whales—began to proliferate during the Eocene.

• The signature gargantuan brains of modern placental mammals first appear, with voluminous neocortices that swell over much of the surface of the cerebrum.

• Marsupials: Australia Metatherians that evolved in the Eocene.

• How did marsupials get to Australia? Metatherians continued their trek, using as superhighways the tendrils of land connecting South America to Antarctica to Australia. For whatever reason, the South American placentals made incursions into Antarctica and at least one group appears to have reached Australia, but they didn’t establish a lasting foothold there—leaving the metatherians alone to mingle with the monotreme natives. As Australia broke free in the Eocene, it became a laboratory where marsupials could do as they pleased. Many converged on placentals; there are marsupial versions of anteaters, moles, wolves, lions, and groundhogs. 

• Onychonycteris: The oldest and most primitive bat, evolved in the early Eocene, ~52.5 Ma.

• Teilhardina: An early WY primate with big eyes, nails on its fingers, toes to grip branches, and lithe ankles which allow it to move gracefully through the canopy.

• Eurohippus: An Eocene era horse. 

• Ailuravus: An Eocene era herbivore, which would have looked like a squirrel.

• Darwinius: An Eocene era primate tree-dweller with opposable thumbs. 

• Eomanis: An Eocene era anteater, one of the first pangolins.

• Lesmesodon: An Eocene era carnivore, part of the dog and cat group.

• Messelobunodon: An Eocene era Artiodactyl, related to cows, sheep, and deer, with an even number of toes.

• Macrocranion: An Eocene era fish-scavenger of the hedgehog lineage.

• During the Paleocene and early Eocene hothouse, Antarctica was still connected to both Australia and South America, ever so tenuously. After millions of years of earthquakes, Antarctica snipped free, on both sides, in the late Eocene. Water rushed in to fill the gaps, creating a new frigid-water current that circled the South Pole, preventing warmer waters from reaching the Southern Ocean. The circumpolar current behaved like an air conditioner, plunging Antarctica into a deep freeze and feeding glaciers that rapidly frosted the polar lands. For the first time in hundreds of millions of years, since the Carboniferous- Permian age of distant mammal ancestors, large ice sheets advanced across a continent.

• Baleen and toothed whales first appear during the Eocene- Oligocene boundary.

___________________________________________________________________________

Oligocene (33.9-23 Ma)

• The Oligocene saw a shift to cooler temperatures. As Antarctica was orphaned over the South Pole and southern glaciers nucleated, primates were decimated in Europe and eradicated in North America. For unclear reasons, they were not replaced by any of the South American New World monkeys, which after their improbable transatlantic odyssey suddenly grew tired of wandering, making it as far north as Central America before going no farther. 

___________________________________________________________________________

Miocene (23-5.3 Ma)

• Livyatan melvillei: An early whale that lived ~12 Ma in the Miocene. It had an 18m long body and a 3m long head and shared the same waters as Megalodon- and no doubt the shark would have feared the whale.

• During the Miocene, some apes in Africa became gorillas, and one population reached SE Asia and became orangutans.

___________________________________________________________________________

Pliocene (5.3-2.6 Ma)

• In the Pliocene, the cooling and drying trend continued. Now, what had been happening in fits around the world properly hit Africa: the tropical forests withered into pockets, replaced in many areas by open grassland. Some apes stayed in the shrinking forests, like gorillas and chimps. But the hominins took another path: out of the forests and onto the open range. Early hominins started walking on two legs before sprouting big brains and learning to shape tools from stone. Walking upright seems to have enabled these other human innovations- probably by liberating the hands from locomotor tasks and enabling these hominins to eat new calorie-rich foods that could be transformed into brain tissue.

• In the Pliocene, the coolhouse became an icehouse. Glaciers crept across the N. continents, and drier, open grasslands spread farther. The browsing horses all went extinct, not only in N. America but around the globe, leaving only the grazing equines. These became the horses of today- the genus Equus, which originated ~4-5 Ma in N. America. Then Equus declined even further, going extinct in N. America ~10 Ka, a victim of climate change and overhunting by humans.

• Ardipithecus ramidus (ardi-ground, ramidus- root): A hominin that arose ~4.4 Ma after proto-humans split from apes. 

• Australopithecus: An early hominin that lived slightly after Ardipithecus with a new human profile- tall, dignified, legs and back and neck and head aligned and balanced on two arched feet. 

• From Ardipithecus and Australopithecus, a rich family tree of hominins blossomed. It wasn’t a simple, ladderlike tree of Ardipithecus evolving into Australopithecus, which begot modern humans, in a tidy sequence of grandmother-daughter-granddaughter. Our family tree is more like a bush, with a lush and thorny tangle of ancestors and cousins. The bush was firmly rooted in Africa- the homeland of humanity- for the first few million years of our history. 

___________________________________________________________________________

Pleistocene (2.6 Ma- 10 Ka)

• Ice Age: Comprises dozens of cycles of glacial advances and retreats over the past 2.7 Ma (mostly during the Pleistocene). The Ice Age has been a roller coaster of cold snaps when glaciers spread from the poles far onto the continents (glacials) and warm spells when the ice melted back (interglacials)…We are still in the Ice Age. We’re just in an interglacial period. Before long, we’re due to lurch back into a glacial phase. However, all the GHG’s we’re pumping into the atmosphere will probably suppress it: one positive, and unexpected, side effect of global warming. 

• The roots of our current Ice Age go back to the Eocene-Oligocene transition, 34 Ma, when Antarctica became stranded over the South Pole and frosted with glaciers, crossing a climatic threshold that turned a hothouse world into a coolhouse. This provided a baseline of cool global temperatures, which got even colder in the Pliocene, between 3.3-2.7 Ma. Another threshold was crossed, and a big ice cap nucleated at the North Pole. With ice sheets on both poles, itching to grow and move, the earth officially entered an ice age.

• The cold snap was probably due to the rise of the Himalayas, Andes, and Rockies over the last few tens of millions of years. As mountains grow taller, they are inevitably mowed down by erosion. When rocks are eroded, they dissolve and react with CO2, forming new minerals- effectively locking up CO2 and preventing it from warming the atmosphere. Taller mountains mean more erosion, and thus more CO2 sequestered from the atmosphere, weakening the greenhouse effect and cooling the earth. 

• As the celestial cycles synchronized, global temperatures crashed >12 C from their interglacial high. Cold air holds less moisture, so the world became not only cooler, but drier. The northern ice cap bloated, sucking water from the oceans to make into more ice, dropping sea levels by over 100m. Large areas of continental shelf were exposed, connecting hitherto detached landmasses—like Asia and North America over the Bering Strait, or Australia and New Guinea. A vast belt to the south of the ice sheets—stretching from Britain and Spain, across the entire Asian continent, and jumping the Bering Land Bridge to continue across North America—was transformed into a new ecosystem. This so-called Mammoth Steppe was a dehydrated prairie, chilled by frosty winds sweeping off the glaciers, where only the heartiest grasses, low bushes, wildflowers, and small herbs could grow. Trees, if present, were restricted to the edges of frigid rivers that bled off the glacial front. Average winter temperatures were -30C- and perhaps even colder. 

• Great American Interchange: The interchange of Northern and Southern American animals after North and South America reconnected after the formation of the Isthmus of Panama ~2.7 Ma. Only a few gained a foothold up north, including armadillos, sloths, and opossums. North American mammals pushed their way into a new neighborhood, seizing the South American rain forests and grasslands and displacing the natives. Many hoofed mammals swarmed southward, including camels, tapirs, deer, and horses. Pressure from these invaders—whose descendants include llamas and alpacas, two of the most iconic South American mammals today- is probably among the reasons Darwin’s Ungulates met their end. Many predators also moved south, the ancestors of today’s jaguars and cougars, and southern wolves and bears.

• Wooly Mammoths (‘Mammuthus primigenius’): Approximately the size of a modern-day African elephant: males stood ~3m tall at the shoulder and weighed up to 6t; females were slightly smaller. Mammoths originated in Africa during the Pliocene, ~5 Ma, before the North Pole ice cap began to crawl onto the continents. A couple million years later, they leapt north and fanned across Europe and Asia, spinning out new species as they pushed farther into new territory. ~1.5 Ma, one of these mammoth species traversed the Bering Land Bridge during a glacially induced drop in sea level and found itself in North America, where it became the Columbian mammoth. Then, a little over a million years later, that same ancestral Asian stock wandered into North America again during another sea level fall. These became the woolly mammoths- one of the last megafauna immigrants into North America. Woolly mammoths met the incumbent Columbian mammoths and the two reached an understanding: the woollies kept mostly to the steppes on the fringes of the glaciers, whereas the Columbians preferred the warmer grasslands to the south. Occasionally they would intermingle in the North American midlands, and they still shared enough DNA that they could successfully interbreed- as shown by preserved genetic material of both species. 

• Mastodons: One of two American elephants with cusped teeth for snipping and grinding leaves.

• Sabretooth Tiger (‘Smilodon’- scalpel tooth): A cat, but part of an archaic family that branched off the cat tree >15 Ma. Early on, their domain was Europe and Asia, but during the Pliocene- before the ice sheets began to dance- one of them made it to North America. This migrant got bigger and thirstier for new territory, and as the ice caps grew, it split into two species: Smilodon fatalis and an even larger one called Smilodon populator that took part in the Great American Interchange, marching across the Panamanian land bridge into South America ~1 Ma. About the dimensions of a modern African lion, but bulkier and up to 400 kg in weight, Smilodon populator was one of the largest cats of all time. It had a worthy adversary: Smilodon fatalis. At ~280 kg, fatalis was about the same weight as a modern Siberian tiger.

• Dire Wolves: Untamed dogs that were slightly larger than today’s wolves, with much stronger bites. Dire wolves were true pursuit predators: they chased prey over long distances, with limbs optimized for speed but unable to grab or pounce. To snatch and kill, it was up to their jaws alone. 

• Homo erectus: An early hominin of the Homo genus; stood taller, more upright on longer legs, with shorter arms that signaled a divorce from the trees, a flatter face, and a much bigger brain than the hominins that had come before. It was a runner, which chased prey over long distances. Homo erectus was also the first hominin to move widely, and as far as we currently know, the first to leave Africa.

• Different Homo populations reached different islands, becoming at least two new species: Homo luzonensis on Luzon (now part of Philippines) and Homo floresiensis on Flores (now Indonesia). When they settled the islands, these humans did what mammals often do when marooned: they got smaller. The Flores dwarf is nicknamed the Hobbit, and for good reason. Barely 1m tall as an adult, it weighed 25 kg and had a minuscule brain that reverted to the size of a chimp’s. But it was well suited to its environment, living for many hundreds of thousands of years in isolation, going extinct ~50 Ka.

• While Homo erectus was off exploring, the genus Homo continued to evolve in Africa, too. Some of these Homo populations meandered around the Mediterranean coasts, into the Middle East, the Caucasus, the Balkans, and across Europe. Others stayed in Africa, at least for a while. About 300 Ka, in what is now Morocco, the first crumbling bones of Homo sapiens entered the fossil record.

• There was a constellation of early sapiens populations across Africa, and they mated and migrated within a continent-wide petri dish, mixing their combinations of anatomical features until, sometime between 100-40 Ka, our classic modern human body plan became fixed. 

• When the wave of sapiens departed Africa, and first entered Europe and Asia, they did not find themselves on virgin territory, unclaimed by other humans. No, they would have encountered at least two other species, both close relatives and also classified in the genus Homo: the Neanderthals in Europe, and the Denisovans in Asia. These humans were offshoots of those earlier Homo wanderings, which were ambling around before Homo sapiens became a fixed species, with our classic body plan. 

• Although three different species, Neanderthals, Denisovans, and H. sapiens were related closely enough for genetic exchange to occur…East Asian and Oceanian people today share between 0.3-5.6% of their genes with Denisovans, and all non-African people are 1.5-2.8% Neanderthal. Modern-day Africans have, mostly, descended from the sapiens populations that remained in Africa, so their ancestors would not have met the European Neanderthals.

• Near Time Extinction: Hypothesis for the largest mass mortality event since the KT-extinction 66 Ma. It far eclipses the killing power of both the PETM global warming spike and the Ice Ages, and it is the only mammal extinction over this time that focuses intensely on large-bodied species. As Homo sapiens entered new continents, we surged across the land as a tidal wave, hunting and killing the large mammals until none of them were left. 

• Over the last 50,000 years, extinctions of large mammals were concentrated during quick warming episodes, 

___________________________________________________________________________

Holocene (10 Ka- Present)

___________________________________________________________________________

Extreme Mammals

• Flapping flight is no easy task, which is why evolution has managed to do it just three times in the entire history of vertebrate life. Pterodactyls stretched their ring finger to support a giant sail of skin. The dinosaur ancestors of birds lengthened their entire arms to anchor an airfoil of feathers. Bats, on the other hand (pun intended), elongated most of their fingers to create a hand-wing. This bat wing is an ingenious design: the skin extending between the fingers is thin and flexible, and it flaps as big muscles attaching to the breastbone contract. 

• When a bird breathes in, some of the O-rich air goes directly across the lungs, while the rest is shunted into the air sacs. Then, when the air sacs contract, the still-oxygenated air inside is passed across the lung during exhalation, meaning birds- and the giant dinosaurs with the same lungs- take in O while breathing in and out. This means dinosaurs got more O with each breath than a similarly sized mammal. The air sacs extend through the body and even into the bones, acting as an air-conditioning system, and lightening the skeleton. The end result: large dinosaurs were more efficient breathers, could cool their bodies easier, and had lighter and more limber skeletons. 

• Bats: The only mammals that can fly using powered flaps of their wings, and one of only three types of backboned animals (pterodactyls and birds) to figure it out. Comprise ~20% of all mammals alive now with 1400 total. Bats cluster near carnivorans (dogs and cats) and hoofed mammals (odd-toed perissodactyls and even-toed artiodactyls). Bats evolved from tree-living glider ancestor, which became a flapping flier by lengthening its fingers into a hand-wing.

• Onychonycteris: The oldest and most primitive bat, evolved in the early Eocene, ~52.5 Ma.

• Echolocation: Bat’s conduct throat-echolocation via two methods: 1) a big coiled cochlea in the ear to hear the echoes and 2) a firm linkage between the throat and the ear- provided by an expanded throat bone called the stylohyal that wraps around the ring bone of the eardrum- so that the nervous system can compare outgoing squeaks to incoming echoes, and to support the prominent larynx muscles that do the squeaking. 

• The three vampire species, which live in Central and South America, are the only mammals that feed exclusively on blood, a most unusual diet called hematophagy. They find their sleeping victims with the one-two punch of echolocation and a brain tuned to detect breathing rhythms. They’ll quietly fly toward their unknowing target in the dark and land nearby, creep slowly on all fours, use a heat sensor in their nose to locate a place on the skin where blood is flowing underneath, and then attack with their pointy teeth, lapping up the oozing blood with their tongue. Their drinking binges last for ~30 min, and they’re careful not to remove too much blood, so as to keep their host alive for the next feed. The victims are usually birds and cows and horses, but vampires have been known to attack humans. 

• Indohyus: An early land mammal that experimented with the water. 

• Pakicetus: Evolved from Indohyus; a carnivorous land mammal ~the size of a large dog, with the long snout and sharp-toothed snarl of a wolf.

• Ambulocetus: Evolved from Pakicetus; a land/water mammal ~the size of a large sea lion with a longer and more-tube shaped body, shorter limbs, and broader hands and feet that looked like paddles with an evolved hearing aid: a fat pad in the lower jaw that connected to the bulla bone, which gathered underwater vibrations and sent them to the ear. Ambulocetus- like today’s whales- heard through its jaws.

• Rodhectus: Evolved from Ambulocetus; a protocetids that was the first worldwide whales. Their hands and, especially, their feet were comically oversized.

• Basilosaurus: Evolved from Rodhectus; a gargantuan proto-whale some 17m long, weighing ~5t. 

• Whales (Cetacea): A mammal with the single lower jawbone and three middle ear bones that define mammals, they have mammary glands and feed their young with milk, and although their skin is smooth, they maintain vestiges of hair—as whiskers around the mouth, which in some species are present only in babies. Furthermore, whales are placental mammals: they birth large (often very large) and well-developed offspring, nourished by a placenta. The closest living relatives of whales are hippos, which evolved in the Miocene.

• Toothed (Odontocetes): Include sperm whales, killer whales, narwhals, dolphins, and porpoises. Toothed whales make HF clicks and whistles by squeezing air through phonic lips, a fleshy constriction in the nasal passages just underneath the blowhole. A blob of fat, called the melon, which bulges out from the forehead acts as an acoustic lens and focuses the sounds, which then reverberate and echo back, detected by a specialized cochlea in the ear. 

• Sperm Whales: Toothed whales with the largest brain of any animal on Earth; at 10kg, it has the second highest brain to body size (after humans).

• Baleen (Mysticetes): Diverged from Odontocetes; include blue whales, right whales, minkes, and humpbacks. Baleen whales filter feed by straining small prey from the water. The feeding dance of some species is something to behold: they will lower their jaws, open their mouth into a gaping chasm, gulp an unholy volume of seawater, and use their tongues and throat muscles to squeeze the water out of the mouth and through the baleen, all so they can catch thousands of plankton at a time.

• Blue Whales: The bulkiest elders are just over 30m long and weigh ~100-110t. Adults can dive to depths >315m and hold their breath for >1 hr. They eat upwards of 2t of krill daily. Their low-pitch vocalizations are the most powerful in the animal kingdom, able to reverberate for >1500km through the abyss. ~99% of the blue whale population was exterminated by whaling. Of a community that once numbered in the hundreds of thousands, only a few tens of thousands remain.

• Elephants: A placental afrotherian mammal, of which only three species survive today; scattered across sub-Saharan Africa, India, and SE Asia. Elephants converse over long distances by using low-frequency infrasound vocalizations or seismic communication. Elephant jaws are conveyor belts: new molars erupt at the back and gradually move forward, getting worn down as they chew and then falling out of the front of the jaw, replaced by the next tooth behind.  Modern elephants include the African bush elephant (the largest living land mammal, standing ~3m tall with males weighing ~5-7t).

• Eritherium: An extinct elephant that lived in the middle Paleocene (~60 Ma). It stood ~20cm tall and weighed ~5kg.

• Phosphatherium: Evolved from Eritherium; ~3x larger than Eritherium with full-on corrugated teeth, procumbent incisors, and a hint of tusks.

• Daouitherium: Evolved from Phosphatherium; weighed ~200kg and lived ~55 Ma; had tusks and procumbent incisors.

• Numidotherium (Algerian): Evolved from Daouitherium; stood >1m tall and weighted ~300kg. It’s at this point that elephant’s acquired their high foreheads, and their nostrils shifted backward, to fasten a small proboscis like that of a tapir (the proboscis later tuned into a trunk).

• Paleomastodon: Evolved from Eocene/Oligocene elephants; weighed 2.5t with downturned tusks while its lowers stuck out from the jaw horizontally, to extend beyond the uppers in a goofy underbite.

___________________________________________________________________________

Mammals and Changing Climates

• Recall that the Paleocene world, after the asteroid felled the dinosaurs 66 Ma, was a hothouse, with jungles covering much of North America and no ice at the poles. At the Paleocene-Eocene boundary, 56 Ma, the hothouse boiled even further, in a spasm of global warming. Temperatures declined somewhat during the remainder of the Eocene, but it was still a hothouse. The jungles endured, the poles remained free of ice. Then, ~34 Ma, as the Eocene transitioned to the Oligocene, the world turned. The hothouse flipped to a coolhouse, which would eventually become an icehouse…It took ~300K yrs- at most- for global temperatures to lurch downward. Higher latitudes cooled an average of 5 C, but the effects were even more pronounced inland, deep in the continental interiors, like the areas that would become the American Savanna. Here, temperatures dropped by 8 C. As land and sea cooled, climates became more seasonal, more variable, and a hell of a lot more unpredictable…The culprit for all this upheaval was a one-two-three punch of coincidences. First, CO2 in the atmosphere was gradually decreasing, meaning there was ever-less GHG’s to insulate the earth and keep it warm. Second, summers were becoming cooler than usual, probably due to fluctuations in the earth’s orbit around the sun. And third, and perhaps most critical, the continents were still moving. Gondwana, the last remnant of old Pangea, was finally completing its messy divorce. 

___________________________________________________________________________

Epilogue: Future Mammals 

• > 350 mammals have gone extinct since Homo sapiens started meandering late in the Ice Age. Of these, about 80 have died over the last 500 yrs. 

• Very soon- the next century at the latest- we are projected to reach a climate state like that of the Pliocene, before the Ice Age. If we keep emitting GHG’s, then we will reach Eocene climates within a few centuries.

___________________________________________________________________________

Misc Quotes

• “(I) prefer the tranquil pursuits of science to the pugilism of politics”-Thomas Jefferson.

• Europeans arrived in the Chicago area in the 1600s, and confronted the Potawatomi, the lakeshore was a swamp, with the stench of wild onions- a plant some of the natives called “shikaakwa”, which the French wrote as Chicago.

• “He was a ‘character’ in the truest sense, as opposed to just a ‘jerk,’ which seems to be the case for many others in academia who garner that description.”

• In their exploration of the American West, Lewis and Clark’s tasks were many, but Jefferson personally asked them to find animals “deemed rare or extinct,” to prove Cuvier wrong…After he returned from his western expedition, Clark was given another presidential commission to gather megafauna bones from a place called Big Bone Lick in KT. Clark was wildly successful and returned with >300 bones, which Jefferson scattered across the floor of the East Room of the White House.

___________________________________________________________________________

Terminology

• Amphicyonids: An extinct group of dead dog relatives nicknamed the “bear dogs”; include Amphicyon, which was ~2.5m long and weighed ~600kg. 

• Arthropleura: An early Carboniferous millipede that grew to >2m in length.

• Bergmann’s Rule: Animals living in warmer areas are often smaller than their contemporaries in colder climes. The reasons are not entirely understood, but it is probably, in part, because smaller animals have a higher surface area relative to their volume than plumper animals and can thus better shed excess heat.

• Borophagines: An extinct group of dogs that included Epicyon, which acted like hyenas in wolves’ clothing, chasing down prey, then dismembering it with bone-crushing bites. 

• Bulla: Cavernous bones- one on each side of the head, which insulate the middle ear bones as they transmit sound to the cochlea, and then the brain. Whales need to hear underwater, a more challenging medium than air. Hence, they need hearing aids, and the thickened rind of the bulla and its curved shelf enhance sound detection.

• Carrier’s Constraint: A constraint plaguing amphibians and reptiles that wiggle their bodies sideways, left to right, during their sprawling walks. Such lateral flexion means that one lung is always expanded while the other is compressed, making it difficult for these animals to move and breathe at the same time, greatly limiting their speed and agility. 

• Cold-Blooded Animals: Rely on their environment to heat their bodies- they are at the mercy of changes in the weather, changes in the seasons, even temperature swings from night to day, or between sun and shade. 

• Diphyodonty: Animals that grow a series of baby teeth during the formative years, and then a permanent array of adult teeth. 

• Endotherms: Warm-blooded animals that produce their own heat- often by packing more energy-producing mitochondria into their cells- and thus can maintain a body temperature warmer than their environment. 

• Entelodonts: So-called hell pigs with enormous heads, fat bodies, humped backs, and slender legs for running, tipped with hooves—a dreadful combination of strength and speed. The biggest of them, Daeodon, stood 2.1m tall at the shoulder, and weighed ~450kg.

• Estivating: Lying dormant, perhaps for several weeks or months, to conserve energy and ride out the dry season.

• Fossils: Direct evidence of species that used to be alive.

• Grass: Include >11K species alive today that cover ~40% of Earth’s land surface as savannas, prairies, and grasslands; include wheat, corn, rice, and bamboo, to name a few. Grasses first appeared in the late Cretaceous. Grasses grow quickly and tolerate harsher conditions allowing them to replace the forests, bit by bit, an army slowly marching across the earth and claiming it as their own. 

• Phytoliths: A hard protein that gives grass its rigid shape. With dirt, it functions as sandpaper, filing down the teeth of grazing mammals as they eat. This is not trivial: grazers today lose ~3mm of tooth every year. 

• Hypsodonty: Tall teeth, which prolong the period that a mammal can chew. At least 17 groups of hoofed mammals independently evolved hypsodonty.

• Great Transformation (Simpson): The changeover from forests to grasslands introduced a new set of mammalian characters: grazers like the horses and rhinos, hypercarnivores like the hell hounds and hell pigs, speed demons, jumpers, hoppers. The grasslands spawned new niches, to add to those already present in the forests. Cooler temperatures, open spaces, and grasses worked in tandem to turn a limited cast of mammals into a larger, more diverse, more specialized, and more interesting cavalcade of species compared to the jungle dwellers. 

• Two families thrived in unison, for nearly 20 My: the grass specialists of the equine group, and leaf-browsers called anchitheriines. 

• Lessemsaurus: A 9m long, 10t long-necked dinosaur that lived at the end Triassic; primitive kin of the colossal sauropods like Brontosaurus.

• Lilliput Effect: A decrease in body size in animals that survive a mass extinction and prosper afterward.

• Lophs: A type of tooth common in elephants that gives the grinding surface of the tooth a corrugated appearance, perfect for pulverizing plants.

• Meganeura: An early Carboniferous pigeon-sized dragonfly that had four large, translucent wings.

• Morass: An area of muddy or boggy ground; a complicated or confused situation.

• Pantolambda: A Paleocene mammal around twice the size of Ectoconus, and about the bulk of a Vechur cow (today’s smallest cattle breed) that lumbered across New Mexico, some 64 Ma, as one of the first large mammals to expand into the ecological niches left vacant by the K-T extinction.

• Pelycosaur: A large extinct reptile of the late Carboniferous and Permian periods, typically having a line of long bony spikes along the back supporting a sail-like crest.

• Taeniodonts: A Paleocene mammal that lived in New Mexico; among the first mammals to develop high-crowned teeth, a godsend for eating tough vegetation, like dirt-encrusted roots and tubers. 

• Thylacoleo: An early placental lioness which weighed ~160 kg and with premolar guillotines so powerful that they doubled as bolt cutters, for crushing bone as well as slicing flesh.

• Trilobites: Extinct bug-like arthropods with hard exoskeletons, which mobbed the oceans hundreds of millions of years before dinosaurs and mammals lived on the land.

• Waif Dispersal: Long-distance animal exoduses; a waif being a slightly derogatory name for a homeless or orphaned child that leaves their wretched life for a faraway place.

___________________________________________________________________________

Chronology

• 2015: Geneticists map the entire wooly mammoth genome- an inventory of >3B base pairs (Mammals by Brusatte).

• 2008: The lone finger bone of a young female Denisovan is found in a cave in Siberia. Its genome is sequenced in 2010 and is found to have a genetic code that diverged from sapiens and Neanderthals, a new species (Mammals by Brusatte).

• Late 1990s: The first DNA-based animal genealogies are published (Mammals by Brusatte).

• 1944: Warsaw Uprising; under the guise of Soviet help, the Polish in Warsaw rise up against Nazi occupation but are violently quelled with ~200K killed and the city mostly destroyed. Soviet help never arrives (Mammals by Brusatte).

• 1936: Extinction of the thylacines, dog-mimic hunters known as the “marsupial wolf” when the last species dies in a Tasmanian zoo (Mammals by Brusatte).

• 1905: Broom invents the term “therapsid” to categorize the Karoo “mammal-like reptiles” (Mammals by Brusatte).

• 1876: Baldwin, a member of the 1874-Wheeler survey, returns to the San Juan area of New Mexico, under a new commission by Othniel Charles Marsh of Yale University to collect fossils. Thus, began the “Bone Wars” feud between Cope and Marsh (Mammals by Brusatte).

• 25 Jul, 1874: The Wheeler Survey, named for cartographer George Wheeler and commissioned by the USC to chart the expanse of land west of the 100th meridian in the US (Western USA), sets out from the railway terminal in Pueblo, CO, heading south. While making their map, the crew was also to take a census of the Native American tribes, appraise sites for railroads and military installations, and seek out mineral resources. Leading the team was a zoologist named H. C. Yarrow, but the man really in charge- through the brute force of his personality- was a paleontologist from Philadelphia, Edward Drinker Cope, who acted as the team’s geologist (Mammals by Brusatte).

• 1854: London hosts the Crystal Palace Exhibition on dinosaurs, which introduces the Victorian masses to the prehistoric world (Mammals by Brusatte).

• 1845: Owen’s publishes a description of some of Bain’s fossils, one of which he names Dicynodon (Mammals by Brusatte).

• 1787-1788: The British launch the First Fleet to one of their newest colonies: Australia. Six ships carried prisoners, three transported supplies, and two provided naval muscle. One of these escort ships, the HMS Sirius, was captained by Hunter. When the fleet arrived in early 1788, and found their new home considerably less inviting than promised, Hunter set out to find more hospitable land on Australia’s treacherous SE coast. He explored the Parramatta River, the main tributary of a bay that was well-protected, easy to access, and surrounded by ample fresh water and rich soil. This would become the center of their penal colony- a place they called Sydney (Mammals by Brusatte).

• ~1000: Extinction of Madagascar’s elephant birds, flightless birds that weighed 500-730 kg (Mammals by Brusatte).

• ~4 Ka: Extinction of wooly mammoths on Wrangel Island (Mammals by Brusatte).

• 6 Ka: Equus (the horse genus) is domesticated by human hunters in Asia (Mammals by Brusatte).

• ~10 Ka: Extinction of Equus in N. America by humans (Mammals by Brusatte).

• ~10 Ka: Wooly mammoths and saber-toothed tigers become rapidly rare. Some woollies managed to escape to Wrangel Island, a speck of frozen ground a little smaller than Jamaica, above the Arctic Circle north of Siberia. There, they did what mammals often do on islands, since at least the time of dinosaurs: they got smaller. The Wrangel woollies managed to hold on for several thousand years, but they were not well. The island could support a population of only a few hundred, maybe a thousand at most. They accumulated genetic defects, which spread like wildfire through such a small community. Their sense of smell atrophied, their hair lost its luster and became a boring satin (Mammals by Brusatte).

• ~12-10 Ka: Agricultural revolution; Neolithic (late Stone Age) humans domesticate plants. Human lifestyles quickly change from nomadic hunting and gathering to a more sedentary existence, with many people settling into towns and villages (Mammals by Brusatte).

• ~23 Ka: Humans domesticate wolves during the bitter depths of the most recent glacial maximum. Wolves learned to live alongside us, to help us hunt, to become indispensable companions on the steppe. Not only did they live with us, but we took care of them, controlled their breeding, modifying their genes to make them more docile. These wolves became dogs: the first domesticated mammal species. When the Siberian sapiens crossed the Bering Land Bridge into the new frontier of North America, they brought their dogs with them (Mammals by Brusatte).

• 26 Ka: Peak of the last ice age (Mammals by Brusatte).

• ~30 Ka: Homo sapiens cross the Bering land bridge into North America during a glacial interval, then rapidly cross into South America (Mammals by Brusatte).

• 40 Ka: Extinction of the Neanderthals, Denisovans, and H. erectus (Mammals by Brusatte).

• 40-10 Ka: The last glacial advance (Mammals by Brusatte).

• 50 Ka- Present: Near Time (‘Megafauna’) extinction; one of the largest mass mortality events since the KT-extinction. It far eclipses the extinctions of the PETM and the ice ages, and was focused primarily on large-bodied land animals. As H. sapiens enter new continents they hunt and kill large mammals until few were left. Megafauna extinctions were concentrated during quick warming episodes (Mammals by Brusatte).

• ~50 Ka: Extinction of Homo floresiensis on the island of Flores (modern Indonesia). H. floresiensis stood barely 1m tall as an adult, weighed only 25 kg, and had a miniscule brain the size of a chimps (Mammals by Brusatte).

• ~50 Ka: Homo sapiens arrive in Australia (Mammals by Brusatte).

• ~60-50 Ka: Homo sapiens emigrate from Africa in many pulses (Mammals by Brusatte).

• ~130-11 Ka: The last ice age, which peaks 26 Ka (Mammals by Brusatte).

• ~195 Ka: The Denisovans arrive in Asia, settling there (Mammals by Brusatte).

• ~300 Ka: Dating of the earliest Homo sapien bones from a cave in Morocco (Mammals by Brusatte).

• 750 Ka: A Homo erectus population lives on the outskirts of Beijing as “Peking Man” (Mammals by Brusatte).

• ~1 Ma: Extinction of Chalicotheres, an odd-toed Perissodactyl species (Mammals by Brusatte).

• 2 Ma: Homo erectus reaches the southern tip of Africa, where they mingle with the last-remaining Australopithecus and departs Northern Africa into the Middle East, then Europe, and then Asia (Mammals by Brusatte).

• 2.7 Ma: Beginning of the ice ages; a roller coaster of cold snaps when glaciers spread from the poles far onto the continents (glacials) and warm spells when the ice melts back (interglacials) (Mammals by Brusatte).

• ~2.7 Ma: Formation of the Isthmus of Panama; North and South America merge damming the cross-ocean current that had flowed through the Gulf of Mexico, linking the Pacific to the Atlantic. Flow was rerouted and more Atlantic water streamed northward, bringing more moisture to the North Pole. More moisture meant more raw material to freeze into ice, and so the glaciers swelled. The merger of the two continents facilitated the Great American Interchange of animals from both continents (Mammals by Brusatte).

• Only a few animals gained a foothold up north, including armadillos, sloths, and opossums. North American mammals pushed their way into a new neighborhood, seizing the South American rain forests and grasslands and displacing the natives. Many hoofed mammals swarmed southward, including camels, tapirs, deer, and horses. Pressure from these invaders- whose descendants include llamas and alpacas, is probably among the reasons Darwin’s Ungulates met their end. Many predators also moved south, the ancestors of today’s jaguars and cougars, and southern wolves and bears (Mammals by Brusatte).

• 2.8 Ma: Evolution of the Homo genus in Africa (Mammals by Brusatte).

• 3.4 Ma: Dating of the earliest stone tools that mark a carnivorous diet; the beginning of the archaeological record

• Eating meat was a game-changer. Meat has a lot more calories than leaves and bugs, and those calories fueled larger brains. Gluttonous, energy-rich meals also meant that these humans could spend less time looking for food, and less time and energy grinding nutrition out of roots and foliage. Their teeth and chewing muscles grew smaller, bestowing our welcoming smiles and lean faces. More idle time meant more opportunities for socializing, communicating, teaching, learning—the origins of our culture. Meanwhile, toolmaking was a game-changer, too. For the first time, humans didn’t need to wait for natural selection to produce new utensils—teeth, or claws, or whatever—for obtaining new foods. We could make the cutters, scrapers, and pounders ourselves. This diversity of weaponry, alongside our flexible and ever-expanding appetites, helped us become supremely adaptable. Around two million years ago, humans were living in a kaleidoscope of African environments: meadows, woodlands, lakesides, grasslands, dry steppes, and fire-charred landscapes (Mammals by Brusatte).

• 3.5 Ma: Numerous hominin species are living together across Eastern and Southern Africa (Mammals by Brusatte).

• ~4.4 Ma: Life of Ardipithecus ramidus, a hominin that arose after proto-humans split from apes (Mammals by Brusatte).

• ~5 Ma: Evolution of the horse genus Equus in North America (Mammals by Brusatte).

• ~5 Ma: Grasslands advance across Australia. Wombats and Kangaroos become grazers, evolving stretched  hypsodont teeth to deal with the phytoliths and grit (Mammals by Brusatte).

• ~5 Ma: Evolution of the Wooly Mammoth (Mammuthus primigenius) in Africa (Mammals by Brusatte).

• 5.3-2.6 Ma: Pliocene period; the earth continues cooling and drying. Tropical forests in Africa wither into pockets, replaced in many areas by open grassland. Some apes stayed in the shrinking forests, like gorillas and chimps. But the hominins took another path: out of the forests and onto the open range (Mammals by Brusatte).

• ~7-5 Ma: Hominins and chimps diverge in the late Miocene (modern humans share 98% of our genes with chimps). It appears genes kept flowing between the human and chimp branches until around 4 Ma, in the early Pliocene (Mammals by Brusatte).

• ~12 Ma: Life of Livyatan melvillei, an early large whale with an 18m long body and a 3m long head (Mammals by Brusatte).

• 12 Ma: Volcanic eruption of Yellowstone Supervolcano in modern ID during the middle Miocene. Ash is blasted across much of North America (Mammals by Brusatte).

• ~15 Ma: Evolution of hippos (Berkeley.edu).

• 20 Ma: The Arabian plate merges with the Eurasian plate, reducing the Tethys to its western arm- the Mediterranean, and re-connecting Africa to Eurasia. Africa had been isolated from Eurasia for nearly 100 My after splitting from the rest of Gondwana during the Cretaceous (Mammals by Brusatte).

• 23-5.3 Ma: Miocene period (Britannica).

• 33-23 Ma: Open habitat grasses engulf the continental interiors, creating entirely new ecosystems with an all-you-can-eat buffet of greenery that grew, constantly, from the soil. Horses and many other mammals evolve from browsing to grazing (Mammals by Brusatte).

• ~33.9 Ma: Baleen and toothed whales first appear at the Eocene-Oligocene boundary (Mammals by Brusatte).

• 34 Ma: Extinction of Multituberculates (an early mammalian group) (Mammals by Brusatte).

• 33.9 Ma: Eocene/Oligocene cooling; over ~300K yrs- global temperatures decline due to a decrease in atmospheric CO2, Milankovitch cycles, and the separation of Gondwana. The effects are more pronounced inland, deep in the continental interiors, like the American Savanna, temperatures drop by 8 C (Mammals by Brusatte).

• ~33.9 Ma: Antarctica moves atop the South Pole and becomes covered in glaciers, causing the Earth to slowly cool. Primates are decimated in Europe and eradicated in N. America (Mammals by Brusatte). 

• ~50 Ma: Evolution of Whales (Berkeley.edu). 

• ~52.5 Ma: Evolution of Onychonycteris, the most primitive bat (Mammals by Brusatte).

• 53-50 Ma: The Tethys Sea, a narrow stretch of tropical water, lies between the Asian and Indian landmasses (Mammals by Brusatte).

• ~55 Ma: Dating of the oldest Australian metatherian fossils, from which Australian marsupial radiation begins (Mammals by Brusatte).

• ~55 Ma: Evolution of primates (Nature).

• ~56 Ma: Plesiadapiforms (pan-primates) diversify from their Purgatorious origins, becoming a successful group of more than 150 known species, which lived across N. America, Europe, and Asia. Features of the hands and feet that permitted better branch-grabbing evolved alongside dental features enabling better fruit-chewing. They were different from their Purgatorius-type ancestors in two main ways. First, they became better arborealists, able to leap through the branches. To do it, they turned their claws into flat nails, added an opposable thumb and long toes to the opposable big toe and long fingers of their ancestors, and fashioned a more constrained ankle that still could move in many directions but was now stable enough for jumping and sticking the landing. Second, they became much smarter and sharper eyed. Their brains not only got bigger but were reorganized, with a larger neocortex for sensory integration, and larger visual regions that developed as the olfactory regions truncated, reflecting a trade of smell for eyesight. Their bulbous, forward-facing eyes could see in 3-D, and some in technicolor. Bigger eyes and brain plus atrophied nose meant a flatter face, with less of a snout. With these new adaptations, primates spread and diversified with an intensity that matched the extreme Eocene temperatures. Quickly they hopped southward to Arabia and Africa. Their family tree swelled, spinning out lemurs- with lower incisors and canines modified into a comb to groom their fur- and then New World monkeys, which made their ridiculous cross-ocean journey from Africa to South America. At least once, and maybe twice, lemurs also rode the waves, hitching an easterly ride on currents streaming off Mozambique and landing on an island that would become their playground, and later their only sanctuary: Madagascar (Mammals by Brusatte).

• 56-34 Ma: Eocene period; today’s main groups of placental mammals- primates, even and odd-toed hoofed species, carnivorans like dogs and cats, elephants, bats, whales- began to proliferate (Mammals by Brusatte).

• ~56 Ma: Evolution of rodents. Today, there are >2000 species (~40% of all mammals) (Britannica).

• 56 Ma: Paleocene-Eocene Thermal Maximum (PETM); over a 20K yr period, CO2 released from Atlantic mid-oceanic ridge volcanoes increases global GHG levels, raising global temperatures between 5-8C with the average temperatures of equatorial regions rising to >40C and the arctic to 25C. Large swaths of low-latitude waters are too hot to support much, if any, life. The surge of global warming peaks and abates within 200K. Conifers withered and were replaced with trees better able to withstand heat, particularly those of the bean family, which migrated ~600-1500 km northwards from the tropics. Over the following 10-27K yrs, scores of new mammals appeared in the Bighorn Basin. Chief among these were the first members of three modern groups, what we call the PETM Trinity: primates, even-toed artiodactyls, and odd-toed perissodactyls. It was either during or just after the PETM that Primates evolved (Mammals by Brusatte).

• The PETM was part of the disassembly of Pangea, which had begun some 140 My earlier, when the first mammals were scurrying. As magma percolated through the crust, on its way to the surface, it fanned out into thousands of horizontal sheets called sills, which literally baked organic matter they came into contact with. Like an engine burning gasoline, this released GHGs: CO2, and the more potent CH4. Trillions of tons of C leaked into the atmosphere, raising the CO2 level between 2-8x beyond that of the already scorching Paleocene. Temperatures spiked, leaving a telltale chemical fingerprint in the rocks: a sharp decline in the ratio of the heavier isotope of O (18O), which has more neutrons than the lighter isotope (16O). From lab experiments, we know that the ratio of these two isotopes is a paleothermometer—and it pinpoints the temperate rise, of 5-8C, right at the Paleocene-Eocene boundary (Mammals by Brusatte).

• ~60 Ma: Evolution of undulates, hoofed mammals, as the toes evolve from multiple digits to a single large toe  (Mammals by Brusatte).

• ~60 Ma: Evolution of carnivorans, members of the dog and cat group. Carnivores evolved a new dental utensil for cutting flesh and breaking bone: enlarged cheek teeth (either premolars or molars) that resemble knife blades. There are four of these so-called carnassial teeth in the mouth, one on each side of the upper and lower jaws, and the corresponding upper and lower pairs work against each other as carnivorans bite. With their new razor teeth, carnivorans supplanted the “archaic” meat-eaters, and they have remained at the top of the food chain—as lions, tigers, hyenas, and wolves (Mammals by Brusatte).

• ~60 Ma: Evolution of Xenarthrans, an early mammal group with ‘xenarthrous articulations’ (extra joints) between their vertebrate, which strengthen and stabilize their backbones; incorporates mostly South American species like anteaters, sloths, and armadillos.

• ~62 Ma: Purgatorius diverges from the other major mammal groups as the oldest-known animal of the primate bloodline. These first primates ate more vegetation using modified molars, but moved from the ground into the trees- a refuge from the chaos and climate swings and post-dinosaur predators (Mammals by Brusatte).

• ~64 Ma: Life of Pantolambda, an early Eutherian mammal about the size of a Shetland pony that lumbered around modern NM and was one of the first large mammals to expand into the ecological niches left vacant by the K-T extinction (Mammals by Brusatte).

• 66-56 Ma: Deposition of the Nacimiento formation (modern NM); the world’s premier record of Paleocene mammals (Mammals by Brusatte).

• 66-56 Ma: Paleocene period (Britannica); jungles cover much of N. America, neither pole has ice (Mammals by Brusatte).

• ~66 Ma: End Cretaceous (K-T) Extinction; an ~10 km wide (the size of Mt. Everest and possibly a comet) from the far reaches of the solar system, beyond the orbit of Mars (perhaps even farther) collides with what is now the Yucatán Peninsula of Mexico, impacting with the force of >1B nuclear bombs, punching a hole in the crust 40 km deep and >160 km wide. That scar is still visible today as the Chicxulub Crater. Everything within 1000 km of ground zero is almost instantly vaporized, wildfires and hot glass rain during the next few hours and days, a nuclear winter that follows lasts for decades, which is followed by a few millennia of global warming. ~75% of species died out. Within ~200K yrs, multituberculate mammals suffer a modest extinction, metatherians are nearly wiped out, and eutherians diversify in scale. The Paleocene mammal survivors are smaller than most cretaceous mammals, and their teeth indicate that they had generalist, omnivorous diets (Mammals by Brusatte).

• Within ~375,000 to 850,000 years of the asteroid, as temperatures stabilized and ecosystems recovered, mammals were thriving in Montana (Mammals by Brusatte).

• ~67-64 Ma: A massive river drains the ancestral Rocky Mountains flowing eastwards into a seaway that cuts N. America in half (Mammals by Brusatte).

• ~90 Ma: Evolution of Euarchontoglires, a mammal group distributed widely across Europe, North America, and Asia, but with members south of the equator, too; includes our primate cousins (and us), rabbits, and rodents (Mammals by Brusatte).

• ~90 Ma: Evolution of Laurasiatherians, a mammal group distributed widely across Europe, North America, and Asia, but with members south of the equator, too; includes dogs, cats, pangolins, and odd and even-toed hoofed mammals, whales, and bats (Mammals by Brusatte).

• ~100 Ma: Evolution of Afrotherians from Therians; an early mammal group predominantly from Africa that includes golden moles, tenrecs, hyraxes, elephants, aardvarks, and manatees (Mammals by Brusatte).

• ~125 Ma: Evolution of Marsupials, modern pouched mammals that give birth to live young which develop further in a pouch. Includes koalas, kangaroos, wallabies, bandicoots, possums, and Tasmanian devils (~250 total species) (Mammals by Brusatte).

• ~125 Ma: Evolution of angiosperms (‘flowering plants’) during the Cretaceous terrestrial revolution (Mammals by Brusatte).

• 125-80 Ma: The Cretaceous terrestrial revolution; mammals split into placentals, marsupials, and monotremes, and angiosperms evolve. The revolution was a changing of the guard from primeval communities to a more modern world, of forests alive with colorful flowers, fragrant fruits, buzzing insects, chirping birds, and most importantly for our story, a multitude of new mammals, including the immediate ancestors of today’s placentals and marsupials. At the time of the Cretaceous terrestrial revolution, north and south were separated by a wide equatorial waterway, called the Tethys Sea (Mammals by Brusatte).

• 130-120 Ma: Cretaceous era Jeholodens mammals form a community in modern Central Asia called the Jehol Biota (Mammals by Brusatte).

• 160 Ma: Life of Juramaia, the oldest known mammal with tribosphenic teeth. Juramaia was a shrew-sized branch-climber (Mammals by Brusatte).

• 166-157 Ma: Jurassic era mammals form a community called the Yanliao Biota in modern Central Asia (Mammals by Brusatte).

• ~167-34 Ma: Evolution of multituberculates, an early mammal group that evolved after the decline of the docodonts and haramiyidans and took advantage of the angiosperm evolution. They may have evolved from haramiyidians, as both groups had similar molar teeth, with long cusp rows, and chewing strokes, in which their jaws moved backward in a grinding motion. Variants include Kryptobaatar (modern Gobi Desert), Sloanbaatar, Kamptobaatar, Catopsbaatar, Nemegtbaatar, Bulganbaatar, Chulsanbaatar, Nessovbaatar, Tombaatar (Mammals by Brusatte).

• 171-163.5 Ma: Middle Jurassic; the first birds take to the air as pigeon sized creatures (Mammals by Brusatte, Wiki).

• 176-145 Ma: Life of Docodonts, one of the first great radiations of mammals that did not survive past the Cretaceous (Mammals by Brusatte).

• ~200 Ma: Evolution of Morganucodon, one of the earliest mammals which developed a dentary-squamosal jaw hinge. During cynodont evolution the dentary progressively got bigger and stronger, while the postdentary bones atrophied, until it became necessary to build a new dentary-squamosal jaw hinge. In Morganucodon and the first mammals, there were two jaw joints: the new dentary-squamosal one and the ancestral quadrate-articular one. The dentary-squamosal joint was doing most of the jaw closing and was the primary source of the strong bites and precise chewing of these animals. The quadrate and articular joint, however, was still bearing some of the jaw load. At the same time, the quadrate and articular bones had moved backward to the point where the quadrate contacted the stapes bone- that ancestral middle ear bone that transmits sound from eardrum to cochlea in reptiles, amphibians, and birds. Therefore, in Morganucodon-type mammals, the quadrate-articular joint was serving dual functions: it was transmitting sound to the ear, but also participating in jaw closing (Mammals by Brusatte).

• 202 Ma: Triassic Extinction; Supercontinent Pangaea begins to split apart down its middle with North America separating from Europe and South America from Africa. Before water fills the gaps between the diverging landmasses, the earth hemorrhages lava. For ~600K yrs, four violent pulses of mega-volcanic activity erupt along the future Atlantic seaboard, increasing GHG concentrations in the atmosphere and leading to global warming. The temperature spike causes oceans to acidify, starves shallow waters of O, and triggers ecosystem collapse, with ~30% of species dying out (Mammals by Brusatte).

• ~220 Ma: Cynodont’s, a Therapsid sub-group, diversify with species such as tritylodontids and mammals (Mammals by Brusatte).

• 252 Ma: End-Permian mass extinction; megavolcanoes, fed by a hot spot of magma deep in the Earth’s mantle under Siberia, and on the northern fringes of the Pangaean subcontinent erupt basalt, CH4, CO2, S, and other noxious gases for several hundred thousand years, inundating several million square miles of North Pangaea with lava and causing runaway global warming as increasing GHG’s in the atmosphere trapped heat by absorbing radiation and beaming it back to Earth. Over a few tens of thousands of years, temperatures increase ~5-8 C, and the oceans acidify, starving the water of O and causing ~90% of all species, maybe more, to go extinct. In the oceans, there is widespread death of shelled invertebrates and other sea life. On land, the forests withered, and ~70-90% of surface vegetation disappeared, causing the ecosystem to fall like a house of cards (Mammals by Brusatte).

• The diverse Permian forests, dominated by the seed fern Glossopteris and evergreen gymnosperms, collapsed, as plants endured their second and final mass extinction- the only one after the Carboniferous Rainforest Collapse some 50 My earlier. They were replaced by ferns and club mosses- much smaller relatives of the coal swamp Lepidodendron trees- which grew fast from spores rather than seeds, allowing them to better cope with the intense seasonality and fluctuations in rainfall (Mammals by Brusatte).

• 260-125 Ma: Evolution of Cynodonts, an early stem-lineage mammal that evolved a secondary palate- a hard roof of the mouth- which divides the mouth from the nasal passage giving air its own dedicated pathway to the lungs; cynodonts could eat and breathe at the same time. Cynodonts survived the Permian extinction- persevering through the volcanoes, global warming, aridification, monsoons, forest collapse, ecosystem implosion, and the 5 My slog of recovery. They would continue to diversify during the remaining 50 My of the Triassic. One of the cynodont lines, which included Thrinaxodon, the tusked dicynodonts, the head-butting dinocephalians, and saber-toothed gorgonopsians, led to mammals. Over the course of cynodont evolution, the quadrate, articular, and many smaller bones at the back of the jaw withered in size while a new, stronger dentary-squamosal jaw joint- the feature that defines mammals- replaced them (Mammals by Brusatte).

• 273 Ma: Pelycosaurs, primitive synapsids that evolved during the Permian, crater in diversity as their tropical habitats become more arid (Mammals by Brusatte).

• 299-273 Ma: Life of Dimetrodon, an early Pelycosaur (Mammals by Brusatte).

• 299-201 Ma: Evolution of Therapsids from a midsize, ~50-100 kg carnivorous pelycosaur. Therapsids include the dicynodonts, dinocephalians, gorgonopsians and many other subgroups. Early therapsids began to raise their metabolism, become warm-blooded, grow faster, bite stronger, and grow hair (most likely as a sensory aid- like whiskers, or as a gland-based waterproofing system) (Mammals by Brusatte).

• ~306 Ma: Life of Archaeothyris, an early synapsid that grew to ~50 cm in length (Mammals by Brusatte).

• 307-303 Ma: The Carboniferous rainforest collapse; Earth’s climate becomes drier, temperatures swing cold and hot, and the ice caps melt, eventually disappearing for good in the ensuing Permian. The coal swamps are devastated, as the soaring Calamites, Lepidodendron, and Sigillaria trees find it harder to grow in the more arid conditions. They are replaced by conifers, cycads, and other seed-bearing plants, which were more drought resistant. The ever-wet rain forests give way to more seasonal, semiarid drylands in the tropics, while other parts of Pangea became parched deserts. The collapse extinguishes half of the Pennsylvanian plant families (one of two mass extinctions recognized in the plant fossil record, the other occurred at the end of the Permian) (Mammals by Brusatte).

• 308-260: Pelycosaurs spread around Pangaea as the first big evolutionary wave of the synapsid lineage; they include the carnivorous Dimetrodon (299-273 Ma), vegetarian Edaphosaurus, and pudgy caseids (Mammals by Brusatte).

• ~325 Ma: Two types of trees make up the bulk of Earth’s canopy- the Calamites with a straight bamboo-esque trunk that grow intermittent bunches of branches with whorols of needle-shaped leaves and the Lepidodendron, whose 2m thick trunks were bare except for a thicket of branches and leaves at the very top. They grew fast, reaching 30m in height in only 10-15 yrs, before dying, being buried, replaced, and later turned to coal (Mammals by Brusatte).

• ~325 Ma: Evolution of reptiles (diapsids with two openings in the skull behind the eye) and mammals (synapsids with one opening in the skull behind the eye) from amniotes. At the time, there was an ancestral stock of small, scale-covered critters that lived in lush swamp forests, which were frequently inundated by rising seas. They split apart, with one side of the family tree leading to reptiles and the other toward mammals (Mammals by Brusatte).

• ~340 Ma: Evolution of amniotes from amphibian ancestors in the Pennsylvanian period (bio.libretexts.org) as a more specialized type of tetrapod; named for their amniotic eggs, whose internal membranes surround the embryo to protect it and prevent it from drying out. These new eggs allowed the amniotes to lay their eggs inland. It was from amniotes that the reptile (diapsid) and mammal (synapsid) lines arose in ~325 Ma (Mammals by Brusatte).

• ~390 Ma: Evolution of tetrapods from fish that crawl onto land. These first tetrapods were amphibians, which still needed to return to the water to lay their eggs (Mammals by Brusatte).

• 540-520 Ma: The Cambrian Explosion; fishes evolve from the first vertebrates as minnowy swimmers that start reinforcing their bodies with bone (Mammals by Brusatte).

• 635-541 Ma: Ediacaran period; Earth changes from a planet dominated by microscopic organisms to a Cambrian world with animals (Ediacaran.org). 

• ~2 Ga: Evolution of the first animals from bacteria that clump together into larger more complex multi-cellular forms (Mammals by Brusatte).

• ~4 Ga: Evolution of the first single-celled bacteria (Mammals by Brusatte).

___________________________________________________________________________