318-300 million years ago



Synapsid reptiles and the mammals that evolved from them form one monophyletic clade. Early synapsids possessed both mammalian and reptilian traits.

From anapsid reptiles evolved two groups of higher reptiles: the synapsids (which would lead to mammals) and the diapsids (which would lead to crocodiles, pterosaurs, dinosaurs, and birds). Although the diapsids would produce many successful lineages which would reach enormous size during the Mesozoic, the earliest diapsid reptiles were small reptiles that probably fed on insects. In contrast, the earlier synapsids were the dominant carnivores and herbivores of their time, unlike the mouse- to opossum-sized mammalsof the Mesozoic. The first known pelycosaurs were large reptiles with large skulls and large canine teeth. These pelycosaurs were adapted to eating large prey and were the first carnivorous amniotes (Kemp, 1982; Carroll, 1988). Early synapsids possessed both mammalian and reptilian traits.


--the synapsid opening in the temporal region of the skull
--sloping and some consolidation of the occipital region of the skull (the back of the skull)
--when the occipital region solidified in later pelycosaurs, the stapes (a mammalian ear bone) was no longer needed for support of the skull
--in later pelycosaurs, the neural arches of vertebrae enlarged to support larger muscles of the limb girdles
--in later pelycosaurs, the lower jaw was modified and there was an increase in jaw musculature


-- retention all of the skull bones that had been present in the labyrinthodont amphibians (although the 2 postparietal bones fuse)
--sprawling gait
--neck (cervical) and lumbar ribs
--long reptilian tail
--lack of a secondary palate to separate the oral and nasal cavities
--reptilian brain size
--primitive pelycosaurs possessed the primitive features of 2 pairs of sacral ribs, 2 coracoid bones in shoulder girdle, and the shoulder girdle still contains an interclavicle and a splinter of a cleithrum
--intercentra in the vertebrae
--a pineal foramen
--an unexpanded iliac blade
--ribs with two heads