Therapsids are one of the two great clades within Synapsida. The other clade is Pelycosauria. The whole “Mammal-like reptile” classifications are up for intense debate and reviews. So far, Synapsids, which are mammals and their shared relatives all the way back to the last common ancestor of Synapsids and Sauropsids (reptiles, birds, etc.) are regarded as part of Reptilimorpha.

Many workers in the field privately think the old concept of reptiles and mammal independently evolving from amphibians may not have been far off the mark. They’re still finding earlier and earlier basal specimens of the two classes.

Skin characteristics alone show marked differences between the two classes. Mammals have very primitive amphibian-like skin while reptiles have a very advanced scaly skin.

Yes that’s a great post! I for one am not convinced that all of the early mammals (or mammal-like creatures) were sprawlers. Nor do I believe that they were all ectotherms. Individuals of some small species have been found in what appears to be a sleeping position with their very long tails curled around over their muzzles, much as modern mammals with furry tails sleep, and if they had fur it’s likely that at least some of them were endotherms. Their is other strong evidence that some of the synapsids not only had fur but had whiskers as well. Vibrissae are usually an indication that the animal is nocturnal at least part of the time. And most nocturnal animals are endothermic.

We have had the image of synapsids as big, lumbering, mindless, cold-blooded lunks drummed into our heads for so long that it comes as something of a shock to think they may have been furry, active creatures somewhat like modern mammals. And if they were more like modern mammals than they were like “reptiles”, and they were wiped out by widespread climate change, that means that we could be too, despite our big brains and active metabolisms.
The fossil record for synapsids is much more fragmentary than even that for dinosaurs. What has already been discovered and described is bound to be just the tip of the iceberg.

There are two traits that certainly would shock us if we went back in time to say the Jurassic period. Every species of mammal at the time would probably have all the traits you listed, but they would also have two traits known only among modern-day monotremes. They would be egg-layers and every last one of them would look like furry sprawling salamanders.

No one knows why warm-blooded mammals remained sprawlers for more than 140 million years (if you count the likely warm-blooded permian synapsids).

It’s not until the eutherian (placental) eomaia and the metatherian (marsupial) sinodelphys that erect gaits evolve among mammals.