The Therapsids

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The 'Dog Teeth' Cynodonts  —  the Ancestors of All Modern Mammals

The earliest known direct-ancestor of you, me, and every other living mammal on the planet is a dog-sized animal from Russia called Dvinia prima. Only the skull and a few other remains of Dvinia ('from the Dvina River' - skull 10cm long) are known — for the drawing (right), we have assumed that the unpreserved parts of its body were similar to closely related Procynosuchus (see below).  Dvinia's few remains might be scappy but  they represent a critical evolutionary link. And that worn skull did preserved some wonderfully-complex, molar-like cynodont teeth.

At a glance, Dvinia's distinctive set of chompers may not be readily apparent.  More obvious is the big increase in the relative size of the temporal fenestra – contrast the shape and size of Dvinia's skull opening with that of Sauroctonus, the approaching gorgonopsid.  And Dvinia needed that expanded temporal fenestra to accommodate jaw muscles big enough to properly exploit those complex, grinding cheek teeth.  Something approaching chewing had arrived.

Sauroctonus (the 'reptile killer' - skull 17cm) may have been the ancestor of big Inostrancevia [previous page]. There were other Russian gorgonopsids of the time besides Inostrancevia and Sauroctonus including the marvelously complete fossil, Viatkogorgon ('Gorgon from Viatka') and Suchogorgon ('fierce crocodile').

Fossils of Procynosuchus ('before dog crocodile' - 65cm long) have been found in Africa and in Germany. Procynosuchus was adapted to a semi- aquatic lifestyle — in some ways, Procynosuchus was living rather like its ancient 'dawn therapsid' ancestors, the early eotitanosuchids.

Most reptiles swim using the same side-to-side undulations which had propelled their fishy ancestors. But therapsids had increased their stride by severely limiting that s-curve movement. Procynosuchus shared that stiffened therapsid backbone (if anything, the early cynodont spine was less flexible than those of  the other therapsid groups).  To compensate, the Procynosuchus had developed a new flexibility at either end!

Just forward of the pelvis, Procynosuchus's spine regained some of  its ancestoral movement. The side-to-side thrashing of its backside and tail would add propulsive force to Procynosuchus's dog-paddle  (although this may have made Procynosuchus a rather comic swimmer).  Still,  many small critters who were better swimmers probably fell prey to this little cynodont anyway. A quick sideways dart of the head, a snap of the jaws, and that good swimmer was now just a tasty lunch!

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The End Permian
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