Mammals

Mammals are four-legged backboned animals distinguished by a number of unique characteristics, including hair, constant warm body temperature, mammary glands, and specialized teeth that are replaced only once. Mammals first descended from a group of animals known as synapsids, or mammal-like reptiles, about 200 million years ago but remained small (mouse-sized) and unspecialized until the dinosaurs died off about 65 million years ago. They then underwent a huge evolutionary radiation—that is, they spread into different environments and diverged in structure.

The Age of Mammals

Mammals are a group of vertebrates (animals with backbones) that have been the dominant animals on land and sea since the dinosaurs died out about 65 million years ago. Indeed, the present period of geologic time, the Cenozoic era, is often called the age of mammals. About 5,500 species of mammals were alive in 2023, but at least five times that many were extinct.

Mammals have been very successful in occupying a great variety of terrestrial and aquatic ecological niches. They include terrestrial meat-eaters and plant-eaters, tree dwellers, burrowing forms, and even aquatic forms. The largest terrestrial mammals today are elephants, but the extinct hornless rhinoceros Paraceratherium was much larger, reaching 6 meters at the shoulder and weighing about 20,000 kilograms. The largest mammals, however, are whales; they can weigh up to 150,000 kilograms in the case of the blue whale, which is larger than even the largest dinosaurs.

Living mammals are easily distinguished from all other vertebrates by a number of unique evolutionary specializations. Unlike other vertebrates, mammals have hair, are equipped with mammary glands to nurse their young, and bear live young (except for the most primitive egg-laying species, the platypus and echidna). Mammals maintain a constant, relatively high body temperature. They have a four-chambered heart and a very efficient digestive and respiratory system, which includes a diaphragm in the chest cavity to aid in breathing. Mammals develop rapidly as juveniles and then stop growing when they reach adult size, unlike many other animals, which grow continuously through life.

Fossil mammals are known only from their fossilizable parts, which are mostly their teeth and bones. Their fossil record can be traced back to their ancestors, the synapsids, or mammal-like reptiles, which are known as far back as 300 million years ago. Fossil mammals are usually distinguished from their ancestors by a number of skeletal features. These features include specialized teeth that are replaced only once (comparable to our "baby teeth" and adult teeth), a jaw joint between the dentary and squamosal bones, and a middle ear that is composed of three bones (the hammer, anvil, and stirrup).

Major Mammal Groups

As the large synapsids were being replaced by the dinosaurs about 210 million years ago, mammals evolved as tiny, mouse-sized animals that fed on insects. Mammals remained in this state for almost 150 million years, living in the nooks and crannies of the world of the dinosaurs, which dominated the planet. A number of different "experimental" groups of insectivorous (insect-eating) mammals lived during this time, but all became extinct. About 100 million years ago, the three major groups of living mammals evolved. These include the monotremes, the marsupials, and the placentals.

The monotremes are the egg-laying mammals, the platypus and the echidna of Australia and New Guinea. The more advanced mammals bear live young. One group, the pouched mammals, or marsupials, give birth to a premature, partially developed embryo. The embryo then climbs to the mother's pouch and fastens to a nipple, where it finishes its development. The most familiar marsupials are the kangaroo, koala, Tasmanian devil, and opossum, although there have been many other types of marsupials in the past, and many are still alive today in Australia and South America. Where marsupials lived in isolation with no competition from placental mammals, they have evolved into many different body forms, which converge on the body forms of their ecological equivalents in the placentals. In Australia, there are marsupial equivalents of cats, wolves, mice, flying squirrels, rabbits, moles, tapirs, and monkeys. Among extinct marsupials of South America were the equivalents of lions and of saber-toothed cats. As similar as these animals look to their placental equivalents in their external body form, they are not related to true cats, wolves, or the rest, as they are all pouched mammals.

Unlike marsupials, placentals must carry the embryo in the womb through its full development. To allow this, the embryo is nourished by an extra membrane surrounding it in the womb. This membrane, the placenta, is shed when the baby is born and is part of the "afterbirth." This mode of reproduction makes the placental baby less vulnerable than a marsupial baby, but it means that the mother is more vulnerable, as she must carry a larger embryo for a longer time. A female marsupial can also reproduce faster, as she can carry one baby in a pouch and be pregnant with another.

Appearance of Mammals

After the extinction of the dinosaurs about 65 million years ago, the planet was ready for a new group of large animals to evolve and take over the vacant ecological niches. Placental mammals underwent a tremendous diversification until they occupied many ecological niches, and some reached the size of sheep. Most, however, were no larger than a cat. The placentals split into the edentates (anteaters and sloths, armadillos, and their relatives) and the rest of the mammalian orders (groups of genera). Like marsupials, edentates had their greatest success in isolation (in this case in South America), although the armadillo has successfully spread northward. The most successful placentals in the Northern Hemisphere in the early Cenozoic were the archaic ancestors of the insectivores (moles, shrews, hedgehogs, and their extinct relatives), primates (lemurs, monkeys, apes, humans, and their extinct relatives), the carnivores (meat-eating mammals), and a great number of extinct, archaic orders that have no living descendants.

About 54 million years ago, in the Eocene epoch, some of the modern orders of mammals began to appear. They were still small and unspecialized and would not be easy to recognize today. Some of these groups included the rodents (mice, rats, squirrels, guinea pigs); the even-toed hoofed mammals, or artiodactyls (pigs, camels, deer, cattle, antelopes, giraffes, and their relatives); and the odd-toed hoofed mammals, or perissodactyls (horses, rhinos, tapirs, and their extinct relatives). These groups lived along with many archaic groups that are now extinct, so the Eocene world had a strange mixture of modern and archaic mammals. During the Eocene, the first bats began to fly, and the first whales to swim.

About 38 million years ago, the world's climate got much cooler and more extreme. The tropical rain forests that had dominated the world in the Eocene were replaced by more mixed vegetation, including open grasslands. As a consequence, most of the archaic groups of mammals died out, and they were replaced by a great diversification of mammals from living groups. The archaic forms were mostly leaf eaters that could not survive the loss of the tropical forest or tree-dwelling forms, such as the ancient primates. At this time, the first true dogs, cats, elephants, rhinos, tapirs, camels, pocket gophers, rabbits, and shrews appeared. Although still very archaic representatives of their respective orders, the mammalian communities began to take on a more modern look. From this point on, environments (like the East African savanna today) were dominated by perissodactyls, artiodactyls, carnivores, rodents, rabbits, and insectivores.

Climate Changes

Since the end of the Eocene, the world's climate continued to get cooler and more extreme. More and more forests were replaced by open grasslands, and the mammals evolved in response to the changing environment. Most of the larger plant eaters (perissodactyls and artiodactyls) had to develop ever-growing molars to chew tougher grasses and long legs to escape their predators by running. This development can be seen not only in the evolution of the horse, but also in the rhinos, camels, and the many artiodactyls that chew their cud (including antelopes, deer, sheep, and cattle). Carnivores became more specialized into ambush hunters (cats), pack hunters (dogs), bone-crushing scavengers (hyenas), and a specialized type, the sabertooths, which evolved four different times independently. Rodents became the most common mammals of all, and they dominate the burrowing, ground-foraging, and tree-dwelling ecological niches.

During the last 5 million years, the earth's climate became increasingly severe as the ice ages developed. Many mammals became extinct, while others became adept at migrating away from ice sheets or developed thick coats of hair (such as the woolly mammoth and woolly rhino) in order to live in glaciated regions. The most severe extinction of large mammals happened only 10,000 years ago, when the ice sheets retreated and the present interglacial period began. Whether this extinction was attributable to the change in climate or to the severe overhunting by prehistoric humans is still disputed.

Since that time, many mammals have been wiped out by overhunting, by the destruction of habitat, or by competition from mammalian "weeds" (such as rats, rabbits, and goats) that accompany human habitation. The extinction of mammals reached its worst levels in the early twenty-first century, as human populations exploded to more than 7.8 billion in 2023. The first global assessment of the impact of human hunting on mammal diversity found in 2016 that approximately 25 percent of endangered mammals were primarily hunted for meat. Only a small remnant of the once-rich assemblage of mammals that dominated the Earth for 65 million years remains, and much of it is endangered in the wild.

Study of Mammal Remains

Because only the hard parts of the mammals commonly become fossils, mammalian paleontologists specialize in studying bones and teeth. Ideally, the paleontologist would study complete skeletons, but these are rarely found, as scavenging and stream erosion break up and scatter them. Most paleontologists thus make do with teeth, jaws, and skulls. Teeth, in particular, are valuable, as they are the hardest and most durable tissue in the body and thus are the most resistant to breakdown by erosion and stream abrasion. In addition, teeth are highly diagnostic of species and are influenced by the diet of the animal. Therefore, the paleontologist can not only identify a mammal by its teeth but also learn much about its ecology.

A paleontologist starts searching in sedimentary beds of the appropriate age and environment (river-channel sandstones and floodplain mudstones), particularly those that have produced fossil mammals in the past. The best results occur in areas with desert climates and badlands exposures, as there are few plants to cover the rocks, and the erosion is fast enough that new fossils are exposed each year. The most successful method is to prospect, or to walk head-down with the eyes "fastened" to the ground, for as many hours and days as it takes until bones or teeth are seen. When fossils are found, they may not be worth collecting if they are too fragmentary or are unidentifiable. If the fossil is worthwhile, the paleontologist collects not only the fossil itself but also exact data about where it occurred and at which stratigraphic level (so that it can be used for dating). The most common mistake made by amateurs is to collect a fossil without carefully noting this information.

Occasionally, mammal fossils are large enough or fragile enough that special methods are necessary. In this case, the paleontologist digs around the exposed fossil and clears the rock around it until it is almost free, resting on a thin pedestal of rock. Then it is encased in a "jacket" of burlap and plaster of Paris, much like the plaster cast used to set broken limbs. When the plaster cast is dry and hard, the fossil is removed from the pedestal, and more plaster and burlap seal it in the cast. When it reaches the lab, technicians can open the cast and carefully clean away the excess rock, treating the fossil with hardener and preservative as they expose it.

Fossils of small mammals (particularly rodents, rabbits, primates, and insectivores) are often too tiny to see while prospecting. These fossils are more often collected by finding a rich, bone-producing level from which bags of fossil-rich material are then filled. This material is later placed in a wooden box with a window-screen bottom and allowed to soak in a stream. The water washes out all the silt and clay, leaving a concentration of pebbles and fossils. This concentrate can then be dried and spread out on a table to be sorted through by hand.

Principal Terms

Cenozoic era: the period of geologic time from about 65 million years ago to the present

genus (plural, genera): a group of closely related species; for example, Homo is the genus of humans, and it includes the species Homo sapiens (modern humans) and Homo erectus (Peking Man, Java Man)

mammary glands: the glands that female mammals use to nurse their young

marsupials: mammals that give birth to a premature embryo that then finishes its development in a pouch

monotremes: primitive mammals, such as the platypus and echidna, that lay eggs and have other archaic features

order: a group of closely related genera; in mammals, orders are the well-recognized major groups, such as the rodents, bats, whales, and carnivores

placentals: mammals that carry the embryo in the mother until it is born in a well-developed state; it is nourished in the womb by a membrane (the placenta)

synapsids: the mammal-like reptiles that lived from about 300 to 200 million years ago and evolved into mammals

Bibliography

Benton, Michael J. The Rise of the Mammals. Crescent, 1991.

Benton, Michael J. Vertebrate Paleontology. 3rd ed. Blackwell Science, 2005.

Carrington, Damian. "World's Mammals Being Eaten into Extinction, Report Warns." The Guardian, 19 Oct. 2016, www.theguardian.com/environment/2016/oct/19/worlds-mammals-being-eaten-into-extinction-report-warns. Accessed 15 Apr. 2023.

Carroll, Robert L. Vertebrate Paleontology and Evolution. W. H. Freeman, 1988.

Feldhamer, George A. et al. Mammalogy. 3rd ed. Johns Hopkins UP, 2007.

Kemp, T. S. The Origin and Evolution of Mammals. Oxford UP, 2005.

MacDonald, David W., ed. The Encyclopedia of Mammals. 3rd ed. Facts on File, 2006.

"Mammals." National Geographic, 2017, www.nationalgeographic.com/animals/mammals. Accessed 15 Apr. 2023.

Savage, R. J., and M. R. Long. Mammal Evolution: An Illustrated Guide. Facts on File, 1986.

Wicander, Reed, and James S. Monroe. Historical Geology. 8th ed. Brooks/Cole, Cengage Learning, 2019.

Woodburne, Michael O., ed. Late Cretaceous and Cenozoic Mammals of North America: Biostratigraphy and Geochronology. Columbia UP, 2004.