Ancestors Of Mammals

Scientists think that the ancestors of all mammals appeared more than 300 million years ago. Around that time, amniotes seem to have split into two groups. One group gave rise to dinosaurs, birds, and modern reptiles. The other group, known as synapsids, gave rise to mammals and their extinct relatives, as shown in Figure 43-2. Paleontologists recognize early synapsids by the structure of their skull, which has a single opening in a bone just behind the eye socket. This same type of skull is found in all later synapsids, including mammals, although often in a modified form.

The first synapsids were small and looked like modern lizards. By the early Permian period (272 million to 298 million years ago), various large synapsids had appeared. Some reached 4 m (13 ft) in length and weighed more than 200 kg (440 lb). Figure 43-3 shows an illustration of a carnivorous synapsid of the genus Dimetrodon (die-MET-ruh-DON). Unlike most other reptiles, which have uniformly shaped teeth, these early synapsids had specialized teeth—long bladelike front teeth and smaller back teeth.


A subset of synapsids, called therapsids, appeared later in the Permian period and gave rise to mammals. Therapsids were the most abundant terrestrial vertebrates during the late Permian period. They survived through the Triassic period (251 million to 203 million years ago) and into the Jurassic period (203 million to 144 million years ago).

A rich fossil record of transitional forms between therapsids and mammals exists. By studying these fossils, scientists can trace the anatomical changes that occurred during this transition and infer additional physiological, ecological, and behavioral changes. Several features we associate with mammals evolved first among early therapsids. For example, like the limbs of many early therapsids, mammals' limbs are directly beneath the body. Evidence suggests that some early therapsids were endothermic and may have had hair.

Early Mammals

Both the first mammals and the first dinosaurs appeared during the Triassic period. So, dinosaurs coexisted with mammals for more than 150 million years. Figure 43-4 shows a hypothesized image of an early mammal. Early mammals were about the size of mice. Fossil skulls with large eye sockets suggest that these mammals were active at night. Also, their teeth were adapted for feeding on insects. Hiding by day and specializing on insects allowed the mammals to avoid threats from dinosaurs or competition with them. Similarities in the mammary tissues of several kinds of mammals suggest that milk production had evolved by the end of the Triassic.

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