During the Late Carboniferous geological period, just over 300 million years ago, a group of animals called amniotes evolved from amphibians. These animals differed from amphibians in that they laid a different kind of egg, called the amniotic or cleidoic egg. This special egg allowed amniotes to colonize the land as it could survive out of water, allowing these animals to expand into drier areas. The amniotic egg contains three membranes that surround the embryo, and it is also semi-permeable, allowing oxygen in and waste products such as carbon dioxide out. The amniotes diverged fairly early into two main groups. One group evolved into the Reptilia and the other evolved into the Synapsida. Synapsids include all living mammals and their ancestors. The word Synapsida refers to a single opening on each side of the skull (technically called a temporal fenestra), and amongst other features is found in all members of this group (including humans). Synapsida include basal synapsids, also known as pelycosaurs, which looked similar to reptiles, and a more derived group known as the Therapsida, which is more closely related to modern mammals (Figure 1).
Therapsida include six major groups, namely the Biarmosuchia, Dinocephalia, Anomodontia, Gorgonopsia, Therocephalia and Cynodontia (Figure 2). The Biarmosuchia are the most primitive therapsids and arose during the Permian Period between 265 and 270 million years ago (Figure 3). They were similar in size to a German Shepherd dog and all were carnivorous. Very little is known about them because they are exceptionally rare in the fossil record. Many species are known from only one specimen. Many are characterized by peculiar bosses (knobs) on the skull and researchers are still uncertain as to what they were for. The second most primitive group is the Dinocephalia (Figure 4). They are distinguished by their large size (up to the size of a rhinoceros) and began as carnivores. However, later forms developed herbivory and they became the dominant herbivores during the Middle Permian. They also developed pachyostosis in the skull, which is a thickening of the bones and may have been used in head butting during fighting.
By far the most successful and diverse group of therapsids were the Anomodontia. This group consists of a few species of non-dicynodont anomodonts, but mostly comprises the very diverse and abundant Dicynodontia (Figure 5). These animals took over from the dinocephalians, which went extinct during the Middle Permian end-Guadalupian extinction some 259 million years ago. The dicynodonts became the dominant herbivores of the Late Permian and were the most successful therapsids in terms of phylogenetic longevity, numbers of individuals and the extent of distribution over continental area. Many different species existed side by side, from small mole-like burrowers to giant ox-sized browsers. They had specialized skulls, characterized by a hard bony beak, similar to turtles, and most had tusks, which were probably used to dig up roots and vegetation. The body was short and broad, and supported by strong thick limbs, with the hind limb held straighter beneath the body than the forelimb.
The Gorgonopsia, Therocephalia and Cynodontia all fall into a single major group known as the Theriodontia. The Gorgonopsia are an ancient group that appeared at the same time as the biarmosuchians and dinocephalians (Figure 6). They developed into giant carnivores and are called the “sabre-toothed cats” of the Permian because they had huge, elongated canine teeth that stuck out of their mouths. They were efficient, capable hunters and the dominant predators of the Late Permian terrestrial ecosystems, feeding mainly on the herbivorous dicynodonts. The therocephalians (Figure 7) are superficially similar to the gorgonopsians and were mainly carnivorous, although some herbivorous forms developed towards the end of their lineage. They were more mammal-like than the gorgonopsians and had an enlarged temporal opening, making the skull lighter and better for muscle attachment.
The Cynodontia (Figure 8) were the latest therapsid group to appear and may have arisen from the therocephalians, although researchers are uncertain about this. They were a very successful and diverse group ranging from large, carnivorous and herbivorous forms to small and extremely mammal-like insectivores and herbivores. The Tritylodontia are the most likely ancestors of modern mammals. The extremely mammal-like structure of cynodonts has been known for nearly a century, but only within recent years have researchers learned enough to say with confidence that all mammals are indeed descended from a single group of cynodonts. During the Late Triassic, around 220 million years ago, the dinosaurs became larger and more abundant and the cynodonts decreased in size, possibly becoming nocturnal. They depended less on sight and more on hearing and smell, leaving the day to the visually-oriented dinosaurs. It was only after the extinction of the dinosaurs during the end-Cretaceous mass extinction, some 66 million years ago, that mammals were free again to diverge into the open niches left by the dinosaurs, to become the most successful and diverse group of amniotes today.
Figure 1. A simplified synapsid family tree.
Figure 2. The therapsid family tree.
Figure 3. The most primitive therapsid, the biarmosuchian (courtesy of Wikipedia).
Figure 4. A primitive, rhino-sized dinocephalian (Reconstruction by Liz Granger).
Figure 5. The dicynodont Lystrosaurus (Reconstruction by Liz Granger).
Figure 6. A member of the sabre-toothed gorgonopsian group (Reconstruction by Liz Granger).
Figure 7. The therocephalian Moschorhinus (Reconstruction by Maggie Lambert-Newman).
Figure 8. The cynodont Thrinaxodon (Reconstruction by Maggie Lambert-Newman).