The Therapsids

Therapsid Ancestors
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First Therapsids — Descendants of the Pelycosaurs
Even before the last of the pelycosaurs died out, their successors, the therapsids, lived among them. These earliest  therapsids most likely arose from among those pelycosaurs called sphenacodonts. They were in good company. Dimetrodon, the famous fin-backed pelycosaur, sprang from this same group of aggressive predators.

Those first therapsids must have evolved away from the warm wet lowlands. So ... which habitat shaped them ?  They were obviously adapted to a marginal habitat  —  probably the cooler, drier upland environment.  Here, those early therapsids would have faced fewer competitors for their main food source —  crunchy invertebrates.*

The first therapsid, Tetraceratops insignis with Dimetrodon  —  above right
The earliest therapsid is Tetraceratops ('four-horned face' – skull  95mm)  from the very beginning of the Late Permian of Texas. This little animal shares features with the sphenacodont pelycosaurs  ( like Dimetrodon ) as well as with early therapsids.

The fossilized remains of Tetraceratops are very scrappy and what does remain has been badly distorted over time. It has been suggested that little Tetraceratops may simply be an odd sphenacodont. If this is true, it illustrates just how closely related the therapsids were to the sphenacodont pelycosaurs — it's hard to tell them apart.

*  Peripheral isolates  (animals voted “least likely to succeed” but who end up dominant anyway)  are usually the less specialized groups living in marginal habitats.  So it would have been for those bug-eating early therapsids.  Life in the uplands meant that they were already adapted to the climatic change since the once-marginal habitat had now become the norm. The therapsids diversified. New forms exploited the now-vacant niches available with the demise of the their pelycosaur ancestors.  The peripheral isolates were set to dominate the land.

Therapsid Ancestors
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