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Here's the scenario:

400 million years ago, a species of lobe-finned fish created tetrapod history as it gulped up air, not water. Fast-forward to 342 million years ago, and one species immediately split into immediate origins of two different groups--Reptilia and Therapsida (which would later become today's mammals.)

Back home...

Therapsids evolved 275 million years ago from a particular group of pelycosaurs (which consisted of Dimetrodon, a predator notoriously advertised to be a dinosaur) called the sphenacodonts.

But in an alternate evolution scenario, could the therapsids still exist if they immediately branched out from its reptilomorph ancestors instead of giving the pelycosaurs a chance to evolve first? Or is there some advance that worked in pelycosaurs that would not work in a reptilomorph?

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  • $\begingroup$ Dimetrodon is a member of the Sphenacodontidae, the sister clade of the Therapsida. Their common ancestors, the basal members of the Sphenacodontia, had the regular lizard-y aspect of basal synapsids. $\endgroup$ – AlexP Jan 4 '17 at 1:13
  • $\begingroup$ Since I can't write any less than 15 characters, I already have added enough before writing my original response: So? $\endgroup$ – JohnWDailey Jan 4 '17 at 1:26
  • $\begingroup$ So the pelycosaurs from which the Therapsids came were not at all like Dimetrodon, they were generic lizard-like synapsids. $\endgroup$ – AlexP Jan 4 '17 at 1:44
  • $\begingroup$ Like the title said, in this alternate scenario, there were no pelycosaurs. $\endgroup$ – JohnWDailey Jan 4 '17 at 2:21
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The correct answer is nobody knows. Considering the evolution of any group of organisms is dependent on their interactions, across time and space, with what can be described as a phase space of selection pressures. The selection pressures will be environmental and ecological factors.

Considering the therapsids emerged from the pelycosaurs, then it is easy to construct the evolutionary just-so story that for therapsids to evolve they needed the pelycosaurs to exist first. if there was an emergence of therapsids with out the benefit of pre-existing pelycosaurs, the phase space of selection pressures might need to be sufficiently different to 'allow' this happen.

Would the therapsids still exist in this alternative evolutionary scenario? Again nobody knows. There might be a group of organisms that sufficiently resembled the therapsids to be mistaken for them, but might not be exactly the same organisms.

The unknowns in achieving this alternative evolution are what the selection pressures would need to be. Perhaps therapsid evolution is highly contingent on the existence of the pelycosaurs. Ecological selection pressures due to the presence of pelycosaurs may have been among the drivers for therapsid evolution. Therefore, no pelycosaurs leads to no therapsids.

Basically this comes down to the choices you make as the creator of your alternative evolution. If you want therapsids without pelycosaurs, it's a simple matter of palaeonotological handwaving. Possibly only the rare specialist in therapsid and pelycosaur evolution will be upset by your rewriting of this chapter of evolutionary history

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