Nearly anywhere on Earth you'll find mammals.
There are more than 5,000 species. Each occupies a niche where it makes its living and raises its young.
Whales, dolphins and manatees live their entire lives in water. Seals and walruses live in coastal waters spending parts of their lives on land and sea. Great herds of grazing animals occupy savannahs and prairies.
Forests are home to large cats, bears, squirrels, monkeys and the like. Many rodents spend much of their lives in burrows underground, while some moles never see the light of day. Llamas, alpacas, sheep and goats live in mountainous areas.
There are about 1,110 species of bats, which accounts for about a fourth of all mammal species.
Often mammals compete with other animals such as reptiles or birds for food, water or space. However, in most locales on land, mammals are dominant among the larger animals. This has not always been the case.
The earliest mammals arose about 200 million years ago from one of the ancestral varieties of therapsids. These were mammal-like reptiles. Therapsids had been the dominate land animals after they arose from pelycosaurs in the middle Permian Period (300-250 million years ago).
Mammals have distinctive characteristics that tell of their common origin. They have spinal columns, which make them vertebrates, and they breathe air. But reptiles, birds and amphibians have spines and breathe. So what else distinguishes mammals?
Female mammals have mammary glands and nurse their young. All mammals have sweat glands and hair or fur. They also have three bones making up their middle ear and a neocortex region of the brain.
Some of these characteristics may seem odd to use to group animals in a class. However, the evidence is unequivocal; mammals share these characteristics because they have common ancestors. Mammals are more closely related to each other than they are related to other vertebrate animals such as reptiles.
Bones that make up the jaw and middle ear are of particular interest in tracing the ancestry of mammals. What had been part of the lower (articular) and upper (quadrate) jaw joint bones in the ancestors of mammals became bones in the middle ear of mammals. The process of co-opting an existing structure such as these jaw bones for a new function is called exaptation.
In reptiles, those bones were retained as structures of the jaw joint. Mammalian jaws developed from a different set of bones. More recent understanding of gene function during embryonic development demonstrates how exaptation occurs. This genetic evidence strongly parallels the fossil record.
Therapsid fossils show the transition from reptilian to ever more mammalian features. The earliest demonstrate them to have been very much like their pelycosaur ancestors. By the late Triassic era they were nearly indistinguishable from mammals.
The closest therapsid ancestors of mammals were creatures called cynodonts ("dog toothed" reptiles). Cynognathus (3 feet long) and Thrinaxodon (cat sized) had fur, possibly were warm-blooded and appear to have had wet, dog-like noses. Unlike reptiles, Cynognathus may have given birth to live young.
Cynodonts did well, dominating large animal niches until a catastrophic extinction event brought an end to the Permian period about 250 million years ago. Larger animals were particularly hard hit, with over two-thirds of land dwelling animals becoming extinct.
Only a few large therapsids survived. One was Lystrosaurus, a 200-pound creature whose fossils are found worldwide.
By the mid-Triassic large archosaurs had displaced the large therapsids. Archosours are the ancestors of crocodilians, pterosaurs, dinosaurs and birds. The first dinosaurs emerged by the end of the Triassic.
By the late Triassic period (250-208 million years ago) there were a number of intermediate creatures. These were mostly small shrew-like creatures measuring from a few inches to a few feet in length. Examples are: Eozostrodon (about 3 feet), Megazostrodon (4-5 inches) and Sinoconodon (6 inches).
Somewhat later, early in the Jurassic period (208-144 million years ago), a creature arose called Oligokyphus that had reptilian ears and jaw bones. However, its limbs were positioned under its body like a mammal, its teeth were rodent-like and it suckled its young.
Another Jurassic creature, Hadrocodium, had a jaw joint like that of a mammal, but the structure of its middle ear was intermediate between that of mammals and their ancestors.
Until dinosaurs met their demise in the mass extinction 65 million years ago, mammals flew under the radar. The lineage that would eventually yield mammals was relegated to making their livings as small nocturnal insect eaters.
As such, two distinctly mammalian characteristics became firmly established. Heat loss is a problem for small creatures. This favored development of warm bloodedness.
Being nocturnal, eyesight was of less importance, while acute hearing was paramount. This favored animals who would develop the middle ear and its distinctive structures as discussed above.
Once large dinosaurs had been wiped out, the many niches they had occupied were available to mammals.
Once again, the fossil record is replete with transitional animals. Without competition from other large carnivores and herbivores, mammals would come to inhabit nearly all the large animal niches on every continent.
Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.