Evolution of Modern Mammals and the Cenozoic

The Cenozoic is known as the age of mammals. It is divided into two main sub-divisions: the Tertiary and the Quaternary. Most of the Cenozoic is the Tertiary, from 65 million years ago to 1.8 million years ago. The Quaternary includes only the last 1.6 million years.


The Paleocene ("ancient recent life")

By the start of the Tertiary the present continental land masses were largely separate and independent evolutionary radiations of mammals took place in these relatively isolated areas.

On land, many new types of mammals appear, but it took several million years to evolve into even moderately large body sizes (No Paleocene mammal exceeded the size of a small modern bear, and most were a lot smaller). One reason may have been the dense forests - large animals would have difficulty moving about, whereas small tree-dwelling animals would have been favored.

They were all short-legged and plantigrade (walking on the soles of their feet), and they had five toes on each foot. Most or all have fourty-four low crowned teeth, another primitive feature (teeth become more specialized and fewer in number in more modern mammals). Almost all of them had slim heads with narrow muzzles and relatively small brain cavities. 

The Paleocene mammals had to share their world with giant flightless birds of prey, which appear during the Paleocene and continue through to the Middle Eocene.  Standing 2 meters or more in height and weighing in at around 200 kg, these large-beaked birds were the biggest and fiercest animals on land for some five or ten million years. 

The Eocene ("dawn of modern life")

Began 56 million years ago. Climate grew noticeably warmer - with the tropics reaching from Britain to the equator and temperate climates further north. South America, Africa, North America, Greenland, New Zealand and Eurasia were all distinct although at the very end India began to collide with Asia. Australia, and Antarctica were connected and Antartica had not yet moved so far south that it was completely frozen over.

The most marked feature of the Eocene is the origin and radiation of the 16 modern mammalian orders:

1. Pholidota (the pangolins)

Pangolins feed on burrowing social insects, such as ants and termites. Theys have stout, strong, clawed limbs, used for digging into ant and termite mounds. The tongue of a pangolin can be extended about 25 cm, and has muscular roots that attach to the animal's pelvis! Pangolins completely lack teeth, and the lower jaw is reduced to a small bladelike bone. Perhaps the most obvious feature of pangolins is their scaly epidermal armor, making them look a bit like pine cones with legs.

The relationship of pangolins to other mammals is uncertain.

2. Xenartha

Includes 31 living species of armadillos,


and tree sloths

The entire evolutionary history of the edentates is restricted to the Western Hemisphere and the majority of the living species occur today in South America. There are also eight extinct families of ground sloths and armadillo-like animals. Among extinct forms, the Pleistocene ground sloth, Megatherium americanum , was six metres (20 feet) long and was larger than a modern elephant; a Pleistocene glyptodon (Doedicurus clavicaudatus) was more than four metres (13 feet) long and 1.5 metres (five feet) high.

The majority of these animals have simple, peglike cheek teeth that lack enamel, a few lack teeth completely. They also possess other specialized traits, such as a long sticky tongue and powerful, clawed forefeet, and relatively small, uncomplicated brains (the latter suggests they may be relatively primitive mammals).

3. Insectivores (moles, hedgehogs, shrews)
All insectivores are relatively small animals with long narrow snouts. Most species in the order are nocturnal, solitary and feed mainly on invertebrates, especially insects. [NOTE: Insects are marvelously diverse in habit and morphology. They provide a rich resource for anything that eats them, and their diversity alone suggests that their predators might exhibit similar variety. And so it is not surprising that their are many different species of insectivores.]

Other characteristics of the order are relatively small brains and relatively unspecialized
teeth (incisors, canines and molars relatively simple). They are thought to be the ancestral stock from which all remaining mammal groups arise. Members of the order are found through much of the world. They are missing from Australia and all but the northernmost part of South America .

4. Chiroptera (bats)
Bats, the only true flying mammal The scientific name for this mammalian order is Chiroptera meaning "hand-wing". This name describes the fact that a bat's hand is modified into a wing. Each wing is composed of an elongated forearm and, except for the thumb, extremely long fingers sandwiched between two thin sheets of skin. The diminuative thumb is left free and is used to help the animal climb about on its perch. Elastic webbing connects the finges to one another and then to the body, form a broad wing surface. A similar membrane spreads between the legs and tail, completeing an air foil that surounds the entire body.

The orientation of the hindlimb is also unique to bats. The hip joint is rotated 90° so that the legs project sideways and the knee faces almost backwards. This modification helps to support the wing in flight and allow the bat to roost hanging from its hindlimbs. Most bats have a tendon system in the toes that locks the claws in place so the bat can hang upside down even when asleep.

There are two kinds of bats: Megachiroptera (flying foxes)

and Microchiroptera (echolocating bats)

Bats are thought to be related most closely to the Dermoptera and are sometimes classified together in one large taxon, the Archonta

5. Dermoptera (colugos)
The order Dermoptera includes only two living species commonly known as the colugo. The colugo, was once called the flying lemur, but it is not a lemur and does not fly but glides on flaps of skin stretched from the side of the animals neck to the tip of the fingers an and toes and continues to the tip of the tail. Colugos can make controlled glides of 70 meters or more using this kite-like feature.

Colugos live in the multi-layered rain forest or rubber plantations where they glide in the canopy. Claws on the toes and fingers aid in tree climbing. They are so arboreal in habit that if they are on the ground colugos are almost helpless. Large eyes and stereoscopic vision gives good depth perception and night vision for these gliding nocturnal animals.

Colugos are herbivores; they have teeth unlike any other mammal - ll of the incisors are comb-like, with as many as 20 comb tines coming from one root. The exact function of the combs is not known. Colugo's diet consists mostly of leaves , shoots, buds, and some soft fruits.

6. Primates (lemurs, monkeys, apes)
The Primates are an ancient and diverse group, currently with around 233 living species. Most dwell in tropical forests. The smallest living primate is the pygmy marmoset, which weighs around 70 g; the largest is the gorilla, weighing up to around 175 kg.

Primates radiated in arboreal habitats, and many of the characteristics by which we recognize them today (shortened face - no snout - and forwardly directed eyes, associated with stereoscopic vision; opposable thumb and big toe; unfused and highly mobile radius and ulna in the forelimb and tibia and fibula in the hind) probably arose as adaptations for life in the trees. Several species, including our own, have left the trees for life on the ground; nevertheless, we retain many of these features.

Within primates, there is a tendency towards reduction of the olfactory region of the brain and expansion of the cerebrum (especially the cerebral cortex), correlated with an increasing reliance on sight and increasingly complex social behavior.

Living primates are divided into two great groups, the Lemurs and their relatives, and the monkeys and apes.

Lemurs and their relatives include mostly arboreal species with many primitive characteristics, but at the same time, some extreme specializations for particular modes of life:


Monkeys and apes include many more species, are more widely distributed, and in most areas play a more important ecological role. Monkeys are further divided into two major groups, the New World Monkeys and the Old World Monkeys.

New World Monkeys have flat noses, outwardly directed nasal openings, and 3 premolars.

Old World Monkeys have paired downwardly directed nasal openings, which are close together and usually 2 premolars in each jaw. (Apes - are a specialized type of old world monkey: )

7. Carnivores

There are two groups of carnivourous placental mammals the Creodonts and the modern carnivores. Although the Creodonts were the dominant group of carnivorous mammals in the early Tertiary and were quite diverse, they went extinct when the true carnivores appeared. The most primitive group of true carnivores are the dogs (wolves, dogs, coyotyes, etc):

Carnivores are distinguished by their enlarged canine teeth, and by the shape of their molar teeth. In humans and in many other mammals, the molars are flattened and are used for grinding food. In most carnivores, the last premolar of the upper jaw and first molar of the lower jaw are sharp and bladelike, and slide past each other like the blades of scissors when the animal chews. Molars farther back in the jaw are usually either missing or highly reduced.

8. Hoofed Mammals (Ungulates)
Hoofed mammals they walk on the terminal bones of the toes and have enlarged
toenails forming hoofs. This feature appeasrs to have evolved as an adaptation for life on open grasslands. There are two major types--

Artiodactyla, or even-toedPerissodactyla, or odd-toed

(a) Artiodactyla, or even-toed mammals, include such familiar animals as sheep, goats, camels, pigs, cows, deer, giraffes, and antelopes -- most of the world's species of large land mammals are artiodactyls.
More advanced artiodactyls, the ruminants, have evolved complex stomachs with three or four chambers. Food (typically grass or other plant material) is swallowed, partially digested and fermented, and then regurgitated for further chewing -- "chewing the cud." This allows symbiotic bacteria and protists that live in the stomach to break down tough plant material that would otherwise be indigestible. Correlated with this diet is the evolution of selenodont molar teeth -- teeth with crescent-shaped ridges -- for more efficient grinding of plants, as seen in this picture:

(b). Unlike artiodactyls, perissodactyls either walk on three toes (like rhinos, tapirs -see picture below, many extinct horses, and other extinct groups) or on a single toes (like recent horses).

9. Whales, Dolphins, and Porpoises
Cetaceans live, breed, rest, and carry out all of their life functions in the water.

All cetaceans share a number of similarities: they have a fusiform, or streamlined body shape; paddle shaped front limbs for steering; vestigial hind limbs (which are within the body wall); no external digits or claws; tail flattened laterally and bearing horizontal flukes constructed of cartilage; basically hairless body (a few hairs arround the blow hole and on the chin); thick subcutaneous blubber layer filled with fat and oil; nostrils (blowhole) on the top of the head; and an airway reinforced with cartilage down to the alveoli (small passageways in the lungs). Many of these characteristics are adaptations to reduce drag for fast swimming in an aquatic environment.

There are two main types of cetaceans: The toothed cetaceans (include dolphins, porpoises, killer whales, and narwhales) and the baleen cetaceans (grey whales, sperm whales, humpback whales).

10. Aardvaks
Aardvarks are pig-sized mammals (up to 82 kg) that specialize in insectivory, especially in capturing and consuming termites. Their limbs are modified for digging into the very hard termite mounds found in African savannahs. The nails are actually somewhere between true nail and hoof in form. They are strongly constructed, shovel-like, and obviously adapted for digging. Aardvark skin is thick and sparsely haired. The thickness of the skin protects these animals from biting ants, and aardvarks may sleep in the ant nests they have recently excavated for feeding.

11. Elephants and Hyraxes
There are only two species of elephants alive today: the Indian elephant (Elephas maximus) and the African elephant (Loxodonta africana). In the past, however, a diversity of unusual elephant relatives traversed areas around the world. The most famous of these extinct relatives are the mammoth and mastodon. All have highly modified grinding teeth and large body sizes.

In addition to their unique morphology, modern elephants are unusual among mammals in two key respects of their social structure. Firstly, they have a matriarchal social structure, in which herds consist of related females led by the oldest among them. Males tend to be solitary, but may come together in small herds.
Secondly, baby elephants are given care and guidance for several years -- longer than most other mammals. During this time members of the herd may care for the babies, regardless of which is the mother.

The hyraxes and manatees are the elephant's closest relatives.

Hyraxes, also known as dassies or conies, are jack-rabbit sized animals with short tails and peculiar, 3-toed hind feet with almost hoof-like nails on two of the toes (the inner toe has a claw). The forefeet have 5 toes. The soles of their fore- and hindfeet, which are moistened by special sweat glands, are remarkably soft and elastic, which works to increase their friction against the substrate. They have specialized muscles in the soles of the feet that help them to work almost like a suction cup.

Manatees or Sirenians, which are sometimes called sea cows, are large mammals that spend their entire lives in water. Their forelimbs are modified to form flippers, their hindlimbs are reduced to nothing more than a vestigial pelvis, and their tail is enlarged and flattended horizontally to form a fluke or paddle.

13. Rodents
Rodents are one of the largest groups of mammals. Most people are familiar with mice, rats, squirrels, and guinea pigs, but there are many other interesting rodents as well. Rodents are often quite small (in the mouse to rat size range), but others are quite big. The capybarais the largest living rodent. It is about the size of a pig and lives along rivers in the llanos of South America.

Despite their morphological and ecological diversity, all rodents share one characteristic: their dentition is highly specialized for gnawing. All rodents have a single pair of upper and a single pair of lower incisors which are rootless, growing continuously.

During gnawing, as the incisors grind against each other, they wear away the softer dentine, leaving the enamel edge as the blade of a chisel. This "self sharpening" system is very effective and is one of the keys to the enormous success of rodents.

14. Lagomorphs (hares, rabbits, and pikas)

Currently, we recognize 80 living species of lagomorphs, placed in 13 genera distributed among 2 families. Native populations are found on all continents except Australia, southern South America, and Antarctica; they are absent from most islands. Humans have introduced them, however, to many areas where they were originally not part of the fauna. They occupy a wide diversity of habitats, ranging from tropical forest to arctic tundra. All are herbivores that feed on grasses and other small plants.

Lagomorphs are small to medium-sized animals that appear to be closely related to rodents. They have a rudimentary or short tail. Lagomorphs have a pair of
incisors in each quadrant of the upper jaw, one large and rodent-like, and the other a small peg located immediately behind the larger tooth. These teeth grow throughout the animal's life and have a layer of enamelthat extends around to the posterior surface of the tooth (in contrast to rodent incisors, which have enamel on
one face only.

The Oligocene

A relativly short span of time, though a number of modern families took the form they have today during this period. These include the appearance of the first elephants with trunks, rhinocerus, and early horses. Some of these changes may be tied to the decline of evergreen forests and the appearance of grasses -- plants that would produce vast tracts of grasslands in the following epoch, the Miocene

In fact there was a great expansion and diversification of grazing mammals including the appearance of some forms that have since gone extinct such as titanotheres:

and oreodonts:

This period was also marked by a relative free change of hoofed animals among North America, Europe, and Asia, as evidenced by the similarity in vertebrate faunas.

The first primates appeared in Africa (explain this mystery to them)

The late Oligocene, the caviomorph rodents (e.g. Porcupines, capybaras, chinchillas, guine pigs, etc), invaded South America from the north.

The Miocene

The Miocene was a time of warmer global climates than those in the preceeding Oligocene, or the following Pliocene. The expansion of grasslands is correlated to a drying of continental interiors as the global climate warmed and it is not surprising that the hoofed animals continued their rapid evolution during the Miocene.

In mammals we begin to see an evolutionary arms race in brain size. First the cerebral brain size increased in predators (a mark of the ability to integrate sensory and motor information), then in prey, then again in predators, then again in prey .... Such co-evolutionary changes led to the replacement and extinction of many primitive mammals.

The Pliocene

The Pliocene was a time of global cooling after the warmer Miocene . This was due in part to antartica moving to the South pole and the connection of Africa with Asia, blocking ocean currents.

Additionally, the Panamanian land-bridge between North and South America appeared during the Pliocene, allowing migrations of plants and animals into new habitats.



The Pleistocene

This period is dominated by the ice ages. Much of North America and Eurasia covered with glaciers but its important to realize that these periods of glaciation came periodically. During periods of Bering land bridge, man crossed into North America .

Some of the major giant placental mammals by end of the Pleistocene:

1. Wooly Mammoth (12 feet at shoulder)

2. Wooly Rhinoceros (7 feet at shoulder)

3. Saber toothed cats

The Pleistocene epoch is marked by five or more major ice ages, the last of which ended at about 11,000 years ago. At this time, somewhere between 12,000 and 10,000 years ago, the mammalian faunas of all continents underwent major changes:

North America = 73% of all large mammals (33 genera) died out, including all proboscidians (mammoths and mastodons), the horses, tapirs, camels, ground sloths, glyptodonts, saber toothed cats and other predators, and some types of deer. Large species show more dramatic extinctions than small ones.

South America - 80% of mammals died out including endentates, rodents, carnivores, peccaries, camels, deer, litopterns, notoungulates, horses, and mastodons.

Australia - 55% of the species vanished including echidnas, marsupial carnivores, wombats, diprotodonts, kangaroos, and wallabies.

In Europe, the losses were less severe. True the wooly rhino, mammoth and giant deer (Irish Elk) died out, but others, such as the horse, hippopotamus, musk ox, hyena, and saiga antelope. imply contracted their range to other parts of the world.

Extinctions in Africa and Asia were seemingly modest.

Paleontologists blame these extinctions on a number of causes and these have polarized into 2 camps:


1. Climate Change - climates and environments changed as ice sheets retreated

2. Spreading human population exerted pressure (= Overkill hypothesis).


Evidence for Climate Change:

1. Climate change correlates with extinctions

a. Global warming

b. Environment simplified - What is today is grassland or forest was more mixed and patchy.


Evidence Against Climate Change:

1. There are no major extinctions of small mammals or plants

2. No major extinctions in Asia (esp Siberia), Africa and Europe but they experienced same climatic changes.

3. Earlier climatic changes in the Pleistocene had no effect on the large mammals.


Evidence for Overkill:

1. Spread of human populations correlates with extinction

2. Victims of extinction are all large mammals - the type to be affected by hunters.


Evidence Against Overkill:

1. There is a lack of archeological evidence of kill sites.

2. Taxa that were not human prey went extinct.