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Basal Therapsids



Here are some of the most basal (that is, closer to the 'trunk' of the family tree) known therapsids. For those not so scientifically inclined: therapsids are the 'first step' the synapsids, the group to which mammals belongs, took in their evolution from their very reptile-like pelycosaur ancestors. In the therapsids some sort-of mammalian features emerge. For instance you can see in all except Nikkasaurus, here, the presence of a single prominent canine tooth, like many mammals such as dogs today also possess. Nevertheless, these therapsids were still at a very, very early stage of evolution. It is very unlikely that they had hair or were warmblooded.

Hipposaurus is known from South Africa in the late Permian, about 260 million years ago. It used to be considered a member of the Gorgonopsia (a more advanced therapsid group I'll also feature soon) but is now considered to be more basal. It had a skull of about 21 cm, and a total length of about 1.2 m.

Biarmosuchus was similar to Hipposaurus, but was found in Russia, and lived about 270 million years ago. It reached about 1.5 m in length. It, or something very similar to it, was a likely ancestral form for all the more advanced therapsids that followed.

Eotitanosuchus was also found in Russia. It was a large form reaching 2.5 m (or likely more) in length.

Ivantosaurus was also a Russian species, also of about 270-265 million years ago. Based on the skull fragment found it might have been one of the largest therapsids ever, with a skull of about a metre in length, and an overall length of about 6 m. Some people think that Ivantosaurus, Eotitanosuchus and Biarmosuchus represent growth stages of a single species, from juvenile to adult.

Nikkasaurus was a form from the Middle Permian that was very basal, with just a few differences from the pelycosaurs. Unlike the other species shown here it didn't have prominent canine teeth, all its teeth being similar in size and shape. It was probably insectivorous. It had huge eyes and might have been nocturnal. Or it might have been the juvenile form of a species that did grow larger. With a skull only 4.5 cm long, it was about the size of a rat.

Ictidorhinus was another very early therapsid from South Africa, about 260 million years ago. Another small species, it had a skull 5 cm in length. Once again it might have been a juvenile.

These things are not at all well known, but still, they do give us some indications of how mammalian features started to arise.

I will soon also cover the pelycosaurs, the very earliest of the synapsids, from which all these other therapsids and things evolved.
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darklord86's avatar
I love your work!