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.)
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?