Mammals Mammals have the following characteristics:

1. Warm-blooded 2. Hair or fur (insulating body cover) 3. Mammary glands 4. Differentiated teeth (incisors, canines, molars)

Note the differentiated teeth in the modern coyote skull, Canis latrans.

Mammals5. Single bone on either side of jaw. (Reptiles and birds

have several jaw bones) 6. Ear bone-structure is derived from bones of the

ancestral reptilian jaw7. Seven neck vertebrae in most mammals, except for

manatee and sloth (low metabolic rates) 8. Large braincase compared to other vertebrates 9. Secondary palate separating mouth cavity from nasal

passages, allowing simultaneous breathing and feeding (needed for infants to nurse)

Origin of Mammals

• Mammals originated from an advanced group of synapsids called therapsids (sometimes called mammal-like reptiles) that lived during Permian and Triassic.

• Mammals appeared during Late Triassic.

• After the extinction of the dinosaurs, mammals expanded into habitats vacated by the dinosaurs, plus additional ones.

Early Mammals• The first mammals were small. • Insulation by hair aided survival by preventing heat

loss.• Mammary glands are modified sweat glands. The

young may have been nourished by secretions from glands that preceded the development of true mammary glands.

• Tooth patterns show early mammals ate insects.• Skulls show that smell and hearing were well

developed, suggesting they were nocturnal.

Types of Mammals1. Monotremes2. Marsupials3. Placentals

a. Insectivoresb. Edentatesc. Rodentsd. Rabbitse. Bats

f. Meat-eaters g. Primatesh. Ungulates

Monotremes

• Primitive egg-laying mammals, such as the platypus (living in Australia and Tasmania), and two species of spiny anteater or echidna (living in Australia and New Guinea).

• Milk is secreted from special glands onto hairs on the abdomen, where the young can lick it up.

Monotremes

Two monotremes living today: duck-billed platypus (left) and spiny anteater (right). Both are found in Australia.

Marsupials• Mammals with pouches in which they keep

their young. Many Australian forms.

– Opossum – Kangaroos– Wallabies– Wombats– Koalas– Others

Marsupials• Many marsupials developed in South

America, and resemble placental mammals found in North America (including a South American marsupial sabertooth cat).

• This is an example of convergent evolution.

Marsupials

Diverse Australian marsupial adaptations for various habitats. The Tasmanian wolf (upper left) is thought to be extinct.

Placentals• Placental mammals appeared during

Cretaceous as small insectivores.

Insectivores• Insect-eating mammals such as the moles.

The descendants of this group include:

– Edentates– Bats– Primates– Rodents – Carnivorous mammals– Herbivorous mammals– Marine mammal

– Paleocene rodent,Paramys

• Toothless mammals.

• This group includes the living armadillos, tree sloths, and South American anteaters.

• Extinct fossil edentates include the glyptodonts and giant ground sloths. A glyptodont under

attack by canines.

Edentates

• Toothless mammals.

• This group includes the living armadillos, tree sloths, and South American anteaters.

• Extinct fossil edentates include the glyptodonts and giant ground sloths. A giant ground sloth

skeleton, Megatherium.

Edentates

Rodents• The rodents probably outnumber all other mammals.

They have adapted to many habitats.• Includes partially aquatic mammals (beaver and

muskrat), desert-dwelling mammals (jerboas and kangaroo rats), and tree-dwelling mammals (squirrel).

• Also includes hamsters, gerbils, guinea pigs, chipmunks.

• Teeth are specialized for gnawing and nibbling. They lack canine teeth and have two upper and lower pairs of continuously-growing incisors.

Rabbits• Rabbits are not rodents. • Their teeth are similar to those of rodents,

but rabbits have two upper pairs of incisors, and only one lower pair.

• The tail is reduced. • The hind legs are strengthened for hopping.

Gomphus, a Paleocene relative of the rabbit

Bats• Flying mammals, the bats evolved during Cenozoic.

• Bat teeth have been discovered in Paleocene strata.

• The wings are developed on elongated fingerbones.

Meat-eaters• The earliest meat-eating placental mammals are Late

Cretaceous. • Creodonts - Extinct small-brained animals with short

limbs and claws. Dominant meat-eating mammals during Paleocene.

• Carnivores - Cats, hyenas, dogs, wolves, raccoons, bears, and weasels.Aquatic carnivores include the seals, sea lions, and walruses. Larger brains than the creodonts. They replaced the creodonts by Miocene.

Carnivores

Eocene creodont, PatriofelisSabertooth cat, Smilodon

Two examples of Cenozoic carnivores

CarnivoresGeneral relationships among meat-eating (carnivorous) animals.

Grazing Animals: Ungulates

• Mammals with hoofs, including horses, cattle, sheep, goats, deer, antelopes, camels, tapirs, rhinos, and other animals.

• Also includes the descendants of mammals with hoofs, including whales, manatees and dugongs, elephants and other animals with trunks (proboscideans), such as the extinct mastodons and wooly mammoths.

Ungulates

Odd-toed Ungulates

• Also called perissodactyls

• An odd number of toes (either 1 or 3 toes) on each foot. Reduction of lateral toes.

• Includes modern horses, rhinos, and tapirs, as well as extinct chalicotheres, brontotheres, and other groups.

Simplified evolutionary relationship among the perissodactyls

Perissodactyls

Evolution of the Horse

• The modern horse is an odd-toed ungulate that evolved from small (about 40 cm tall) Eocene browsing horses with 4 toes on the front feet and 3 toes on the rear feet.

• The horse changed from a small animal with a short skull and low-crowned teeth to a larger animal with fewer toes, longer skull, larger brain, and complexly-ridged high-crowned teeth for chewing grasses.

Evolution of the Horse

Evolution of the lower foreleg of the horse. Fossils start with Eocene horse on the far left and a modern horse on the far right.

Evolution of the Horse

• Mesohippus, a greyhound-sized ancestral horse that lived during Oligocene, about 40 m.y. ago. It ate leaves rather than grasses.

Evolution of the Horse

• Merychippus, an ancestral horse that lived during Miocene to Pliocene, about 25 m.y. ago. It had three toes, but the central toe carried most of the weight. It was about 1 m high at the shoulder, and was well adapted for life on the grasslands.

Even-toed Ungulates • Also called artiodactyls• An even number of toes (2 or 4 toes) on each foot. • Those with two toes have cloven hoofs.• Includes cattle, pigs, deer, hippos, goats, sheep,

camels, llamas, giraffes, and antelope. • This group of animals is important to humans because

it provides meat, milk, and wool.

Even-toed Ungulates

Dinohyus, giant pig-like Miocene entelodont(3 meters (9 feet) long)

Oxydactylus, a Miocene camel of the North American grasslands

Examples of early even-toed unglates (artiodactyls)

Even-toed Ungulates

• Some even-toed ungulates are ruminants that have multichambered stomachs and chew their cuds, for digesting coarse vegetation.

• Ruminants include sheep, cattle, giraffes, and deer.

• Extinct even-toed ungulates include the oreodonts and entelodonts.

Even-toed UngulatesCladogram of major artiodactyl groups. This diagram shows the close evolutionary relationship between artiodactyls and whales (cetaceans).

Cetaceans (Whales) • Mammals that have adapted fully to life in the sea,

such as the whales, porpoises, and dolphins.

• They are descended from hoof-bearing land dwellers related to the hippo.

Cetaceans• Early whales had tiny, vestigial hind legs that were too

small to be of use in swimming and too small to hold the animal up on land.

• The Eocene whale, Pakicetus, is found in non-marine shales indicating that they lived in lakes, streams, and estuaries

• Later, whales made the transition to the sea.

Ancestral whale Pakicetus, Eocene of Pakistan. Because the remains of Pakicetus were found in stream-deposited sediments, it is doubtful that this primitive whale ever ventured into the open ocean. Except for whale-like characteristics of its teeth and the ear region in the skull, Pakicetus did not resemble a whale at all, as you can see.

Ancestral Whale

Whales with teeth

Eocene toothed whale, Basilosaurus

Whales with Legs

Fossil whale with vestigial legs (note femur and pelvis).

Eocene Ambulocetus from Pakistan.

Proboscideans • Mammals with a proboscis or trunk, including

elephants and the extinct mastodons and wooly mammoths.

Dinotherium (Miocene, left) and four-tusked mastadon (Miocene-Pliocene, right)

Late Pleistocene (Ice Age), wooly mammoths walking the borderlands surrounding the continental ice sheets

Proboscideans

Proboscideans

Comparison of teeth: mastodon (above) and mammoth (below)

Proboscideans

Baby mammoth dug from permanently frozen soil in northeastern Siberia. The mammoth stood about 104 cm (3 feet) tall at the shoulders, was covered with reddish hair, and was probably only several months old. Radiocarbon dating indicates it died 44,000 years ago.

Cenozoic Migrations

• The southern continents (South America, Australia, and Antarctica) were separated from North America and Eurasia during most of Cenozoic.

• As a result, distinctive assemblages of mammals developed on the southern continents, showing convergent evolution with northern hemisphere species.

Panamanian Land Bridge• Marsupials moved northward • Placentals moved southward • Eventually, the marsupials began to decline.• All of the hoofed marsupials became extinct. • Ground sloths and glyptodonts also became extinct. • The land bridge caused many species of South

American marsupial mammals to go extinct, because of migrants from the north.

The development of the Panamanian land bridge about 3 m.y. ago (during late Pliocene) led to the migration of mammals between North and South America.

Bering Land Bridge • The Bering land bridge existed between North

America and Eurasia during Pleistocene (now occupied by the Bering Sea).

• Camels, horses, mammoths, and a wide variety of other land mammals migrated across the Bering land bridge during Pleistocene.

• The land bridge was also used by early humans to enter North America at least 14,000 years ago.

Extinction of the Large Pleistocene Mammals

About 17,000 years ago, during the last glaciation, North America supported large numbers of many types of large mammals:

– Odd-toed ungulates– Even-toed ungulates – Giant beavers – Mammoths – Mastodons – Elks – Huge ground sloths

Extinct Irish elk, Megaloceros

Extinction of the Large Pleistocene Mammals

• Most of these large land mammals began to become extinct around 8000 years ago.

• Why? There are two hypotheses:

– Human hunting and predation – Climate change associated with global warming at

the end of the last Ice Age.