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How Did Mammals Evolve?

Vertebrate Diversity in the Permian Age - 270 Million Years Ago (MYA)

Setting the stage for mammals. True mammals were not around yet, but the stage was set when a group of early animals that walked on four legs called synapsids eventually diversified into warm-blooded cynodonts. Their descendants included the first mammals.

Continental Drift - Beginning of the end for the super continent.
At the start of the Permian, 290 MYA, continents had joined to form a large supercontinent - Pangea. Without ocean barriers, early animals could move - or radiate - from the northern hemisphere. But no sooner did Pangea form than it begin to tear apart. By 270 MYA, Gondwanda, the part Pangea that would form the continents in the southern hemisphere would begin to break away.

Mammal Spotlight

Dimetrondon (Not a mammal)
Texas, USA. This early synapsid was a sail-backed, meat-eating pelycosaur that lived roughly 280 MYA, mainly in swampy areas. These animals were not warm-blooded, relying on the large sail-like flap of skin along their backs to absorb and release heat.

Procynosuchus (Not a mammal)
Russia and southern Africa. This terrier-sized cynodont from the late Permian is starting to show signs of mammalness; it has incisors, canine, and cheek teeth, just like modern mammals. Those teeth also tell us that it was a carnivore. Its legs seem to be adapted for running and swimming.

Cynodont (Not a mammal)
Russian and southern Africa. It's not a mmamal or reptile, but a special kind of primitive land vertebrate (synapsid) with some mammalian characteristics, including complex teeth. While reptile teeth are more uniform in shape, mammals have different kinds of teeth for different jobs. These and other factors tell us that cynodonts were closely related to the true ancestors of modern mammals.


Mammal Diversity in the Late Triassic - 210 MYA

Setting the stage for the first true mammals. At the end of the Permain, about 248 MYA, a mass extinction wiped out much of plant and animal life on earth. Most of the synapsids that had dominated the Permian disappeared, except for one group - the cynodonts - who spread and diversified, eventually giving rise to the first true mammals approximately 220 MYA.

Continental Drift - Early in the Triassic.
The vast supercontinent of Pangea straddled the equator and was surrounded by ocen. Much of the land was warm and arid. Toward the end of the period though, as Pangea continued to split apart, global climates had become cooler and wetter.

Mammal Spotlight

Thirnaxodon (Not a mammal)
Africa, Antarctica and China. This mammal-like carnivore bridged the gap between Triassic cynodonts, and the first mammals. Fossils of this house cat-sized animal had the mammalian trait of only two sets of teeth in a lifetime, instead of being replaced multiple times during life, as in reptiles.

Morganucadon (YES - a mammal)
One of earliest true mammalian ancestors, Morganucadon, roamed the earth about 210 MYA - way before the biggest dinosaurs! Warm-blooded, built for action, and bearing typical mammalian teeth, it probably feed on insects and worms at night when competing small predators, like cold-blooded lizards, were less active.

Hadrocodium (YES - a mammal)
The recent discovery of this 195 million year old fossil has added another member to the early mammal familiy during this time period. Hadrocodium weighed only two grams and grew to a length of 32 millimeters. It has the large brain and a middle ear of modern mammals, traits that suggest these two features may have evolved together.


Mammal Diversity in the Jurassic - 170 MYA

Mammals lived in the shadow of the dinosaurs. Small shrew-like mammals thrived, scampering about among the dinosaurs. Because they were warm-blooded, they could hunt at night to avoid being eaten by daytime carnivores, including dinosaurs. Over ten mammal lineages have evolved, the multituberculates, and predessors to placentals, monotremes and marsupials.

Continental Drift - Gondwanda breaks up.
By now, Gondwanda had separated from Pangea and drifted south, severing the land link between North and South America. This had a huge effect on the evolution of plants and animals in distinct northern and southern hemispheres. The Jurassic also saw the replenishment of plant and animal diversity after the major extinction event of the previous age.

Mammal Spotlight

Amphitherium
England and Wyoming. Mammal fossils from the Jurassic are few - this jaw of an Amphitherium tells us this belongs to a mammal lineage that is ancestor to marsupials and placentals.

Crusafontia
Portugal. A nearly complete fossil of this mammal was recently found in rich fossil deposits in Portugal. It was about eight inches long, shows adaptations for living in the trees, leaping from branch to branch, and feeding on insects and fruits.

Multituberculates
England and Wyoming. Named for their teeth, which do not from grinding molars as in other mammals, but instead grow in parallel tracks like the teeth of a comb, Multituberculates survived over 130 million years, and were the last of the primitive mammals groups to go extinct, around 35 MYA.


Mammal Diversity in the Cretaceous - 65 MYA

Mammals poised to take off. During much of the Cretaceous, dinosaurs dominated the land while small mammals become increasingly diverse. But mammals got their lucky break when a giant meteorite struck the earth 65 MYA, an event that most scientists think letd to the extinction of the dinosaurs. Mammals with characteristic adaptability, now had a whole world to fill.

Continental Drift - More continents are isolated.
The Atlantic Ocean widened, creating a greated distance between the Old World and the New World. South America, Africa and India were now separated. By the end of the Cretaceous, Australia split from Antarctica and was on its own. These continent shifts acted to shut down the exchange of animals and plants.

Mammal Spotlight

Krypotobaatar
Mongolia. During the Cretaceous, multituberculates were still the most abundant type of mammal. Kryptobaatar was a tree-living herbivore with a keen sense of small. The entire Multituberculate lineage died out about 35 MYA.

Zalamdolestes
Mongolia. Insectivores are the earliest known placental mammals, with fossils found from the late Cretaceuos on. Resembling a small modern-day shrew, this one gave birth to live young, was nocturnal, and probably foraged for food with its long snout.

Purgatorius
Montana. The earliest primate found in the fossil record is Purgatorius it was roughly the size of a small rat and fed on insects and soft fruits. These early primates may have shared a common ancestor with rodents.


Mammal Diversity in the Miocene - 20 MYA

More grasslands mean more and larger grazing mammals. As climates cooled and became drier, forests shrank while grasslands - which are better adapted to survive seasonal extremes of moisture and temperature fluctuations - expanded. This led to increased diversity and the size of plant-eaters, which adapted to eating grass.

Continental Drift - The earth cools - seasons develop.
The continents and oceans are close to their present positions, but North and South America are still separate, and Australia, Africa and India are still moving north - their collisions with Europe and Asia create the Alps and the Himalayas.

Mammal Spotlight

Chalicotherium
Nebraska. WIth the spreading of grasslands, many Miocene mammals responded by becoming tough of tooth and fleet of foot, but others depended on size as a deterrent to predators. These giant North American hebivores were related to horses, rhinos, and tapirs, but had clawed feet instead of hooves.

Ekaltadeta (Killer Kangaroos)
Australia. Much of Australia during the Miocene was lush, and its long isolation allowed the marsupials (pouched mammals) to flourish. Unlike modern kangaroos, this killer kangaroo was a flesh-eater. It gripped its prey with its powerful arms and ripped the meat with its strong jaws and sharp teeth.

Kenyapithecus
Africa. As forests shrank and grasslands expanded, some primates began to live in more open habitats. Kenyapithecus still retained many of the skeletal features of an arboreal primate, but it's skull and teeth show that it is related to the early hominins that evolved to walk on the ground.


Mammal Diversity in the Pliocene - 5 MYA

By 5 MYA, the global diversity of large grazing mammals was still high but declining. There were massive extinctions of "megafauna" (large herbivores and carnivores) at the end of the Pleistocene on many continents.

Human evolution.In Africa, apes known as Australopithecines gave rise to the genus Homo between two and three MYA.

Continental Drift - The dynamic earth.
The continents are more or less in their present positions, but the dynamic activity of the Earth's crust caused frequent earthquakes, volcanic eruptions and plate movement. Antarctica was covered with ice and completely isolated from other continents. Australia continued its move northward. About three MYA, North and South America reconnected via the Isthmus of Panama.

Mammal Spotlight

Glyptodont
South America. These giant fortress-like mammals roamed the warm, moist areas of South America. Though awesome in appearance - there were the size of a small car and had spiked tails - these armadillo relatives were harmless vegetarians with simple grinding teeth.

Pliohippus
North America. Philohippus, the first one-toed grazing horse, became abundant during the Pliocene (5.3 - 1.8 MYA). It gave rise to the modern horse during the Pheistocene. North America was the center of horse evolution, but they mysteriously went extinct there about 8,000 years ago. Humans reintroduced horses from the Old World several hundred years ago.

Sivatherium
Africa. This extinct giraffe lived in Africa beginning five MYA and later migrated to Eurasia. Early artwork from 5,000 years ago suggests that they overlapped with humans before going extinct.

Agriotherium
Africa. Ancestors ofthis giant bear entered Africa from Eurasia about six MYA. They preyed on large savannah animals such as buffalo and the Sivatherium, but were never common and went extinct before the end of the Pilocene.


Mammal Diversity in the Quaternary Age - 18,000 YA

At the peak fo the last glaciation, so much water was frozen that sea levels dropped, connecting isolated continents and islands. These land bridges had a major impact on mammal distribution, including humans, who migrated from Asia into the Americas via the Bering Strait and from Europe to the British Isles.

Continental Drift - The most recent Ice Age.
The earth is almost the same as it is today, but parts of the surface were covered in huge ice sheets. During the last Ice Age, the cllimate swung between glacials - when ice sheets grew and which lasted for 100,000 years, and warmer interglacials - when ice sheets shrank. We are still experiencing an ice age, but fortunately, we are in a warm interglacial period.

Mammal Spotlight

Wooly Mammoth
Europe, northern Asia and Northern America. These massive elephants roamed in herds. They reached food by swinging their heads from side to side and using tusks to scrape snow from the ground. They went extinct around 10,000 years ago, when about 75% of all large land mammals died. out.

Sabre Tooth Cat
Noth and South America. About the size of a modern lion, these carnivores lived in the grassy plains and open woodlands. Their powerful front legs suggest that they used stealth and ambush rather than speed to cpature their prey, and their huge canines could bite open the soft belly or neck of prey animalas.

Dire Wolf
North and South America. About the height of modern gray wolves, dire wolves were more massive in build, but their brain cases were smaller. Like hyenas, their teeth and jaws were capable of crushing bone. They went extinct around 10,000 years ago.


Smithsonian Insitution