359-299 million years ago


The first amphibians evolved in the Devonian Period. During the Early to Mid-Carboniferous Period, the first reptiles evolved which possessed a mix of primitive ancestral features and derived reptilian characteristics.

The first reptiles are known from the Carboniferous Period: Westlothiana and another fossil are known from the early Carboniferous (Gee, 1988) and Hylonomus and Paleothyris are known from the Middle Carboniferous. These earliest reptiles possessed a number of primitive features in common with anthracosaur amphibians which were not present in later reptiles.

The bones in the back of their braincases (in the occipital region) were still not firmly attached to each other and the stapes was still a large bone, rather than the small structure in modern reptiles which is modified for hearing. The first two vertebrae (the atlas and axis) retained the primitive amphibian condition, the vertebrae retained intercentra components which would be lost in later reptiles, and the notochord still passed unrestricted through the vertebral column. There were still small dermal scales on their undersides between their arms and legs and the number of bones in the digits were the same as those of anthracosaurs.
Although these first reptiles possessed a number of traits which linked them to anthracosaur amphibians, they also possessed derived features which identify them as reptiles. There was reduction of some of the skull bones (the tabular, postparietal, supratemporal), the process from the pterygoid bone allowed for jaw muscle attachment, the braincase structure was more advanced, the primitive palatoquadrate was reduced to small epipterygoid and quadrate bones, the clavicle and cleithrum bones in the shoulder girdle were reduced, and the dorsal dermal scales were lost. Instead of the ancestral condition of many small bones in the foot (the tarsus), some of these small bones fused to form larger tarsal bones. Three small tarsal bones fused to form the larger astragalus (which is called the talus in humans and forms the ankle joint) while one tarsal bone (the fibulare) was enlarged and is referred to as the calcaneus (the heel bone in humans). The teeth are small compared to those of early amphibians and they probably fed on arthropods as opposed to vertebrates (Carroll, 1988).