For the most part the fossil record of the pre-mammalian synapsids is limited to bones, and very little soft tissue evidence. This record is, however, fairly extensive, the largest gap being between the Permo-Carboniferous pelycosaurs, and the Therapsida.
The oldest known synapsid is the ophiacodontid Protoclepsydrops from Joggins, Nova Scotia. The enclosing rocks are dated at around 314 million years ago in the mid Carboniferous, but the bones are fragmentary, so it is not clear whether it is a pelycosaur or not.
Archaeothyris is the oldest known synapsid from good remains, and comes from slightly younger sediments in the same area of Nova Scotia. Although the teeth are all sharp, and have the same pointed shape, they are different sizes, demonstrating the first step in specialisation to the carnivorous diet, different teeth having different functions.
Further discussion on the evolution of the Synapsida through examination of specimens in the fossil record can be found on the characters and anatomy page.
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