299-252 million years ago




Early therapsids were a mix of primitive and derived traits.


Eotitanosuchians were the earliest and most primitive therapsids. The pterygoid bone in the roof of the mouth still included teeth (unlike mammals which only have teeth in the maxillary and premaxillary) and the 2 vomer bones of the nasal septum were only partly fused (in mammals, there is only one vomer). The stapes had become a smaller bone that was no longer has to support the back of the skull. It contacted the quadrate bone of the upper jaw and both these bones would be middle ear bones in mammals. Phtinosaurus and Pthinosuchus were the most primitive eotitanosuchians. They retained primitive pelycosaur features including a curved top of the skull and more post-canine teeth. Biarmosuchus had slender and more agile limb bones. The pelvis still retained a large pubis and the ilium was only slightly expanded. The femur included modifications seen in later therapsids such as a more medial head (which would allow therapsids to walk more upright) and 2 bumps called trochanters. Biarmosuchus was related to the gorgonopsids given that they both had a preparietal bone between frontal and parietal bones of the skull (Kemp, 1982; Carroll, 1988).