Were the first synapsids scaly?

[Tristan L]

I’ve read some claims that the synapsids (theropsids) had smooth, glandular skin rather than scaly skin like modern reptiles – Estemmenosuchus is often cited in this regard –, and that the scale-like structures that they did have, their belly-scales, aren’t homologous to lepidosaur scales. While the latter claim may be the case, I do have qualms with the former claim. After all, this study (see https://www.sciencedaily.com/releases/2016/06/160624154658.htm for an overview) has proven that modern reptiles have anatomical placodes just as mammals and birds do, and that mammalian hair, modern reptile scales, and bird feathers are all homologous to each other and come from the skin appendages of the last shared forebear of mammals, modern reptiles, and birds.

Estemmenosuchus is a derived synapsid (namley a therapsid), so its lack of scales may very well be secondary, right?

Moreover, the lepidosaurian-like scales of the varanopid Ascendonanus (see https://www.researchgate.net/publication/323782950_First_arboreal_%27pelycosaurs%27_Synapsida_Varanopidae_from_the_early_Permian_Chemnitz_Fossil_Lagerstatte_SE_Germany_with_a_review_of_varanopid_phylogeny) show that varanopids had reptilian scales, and even lepidosaur-like ones. Of course, this isn’t necessarily evidence for synapsid scales since varanopids might be bird-line amniotes rather than mammal-line amniotes (though this suggestion has been contested and argued for and against).

Still, doesn’t the homology of hair, scales, and feathers strongly indicate that the last common forebear of mammals, modern reptiles, and birds had scaly skin, and that early synapsids thus were scaly, too, in a way homologous to modern reptiles?

If so, am I right in rejecting such claims as the one that early synapsids looked like naked lizards, or that the skin of our early ancestors was frog-like rather than reptile-like?

The latter claim is false anyway since even if they didn’t have scales, their skin would still have been suited to a dry environment like reptiles’ and unlike frogs’, right?

Is there other evidence for (or against) scaly skin being the plesiomorphic condition of all crown-group amniotes?

Edited October 29, 2020 by Tristan L

[Tristan L]

However, the shared forebear of epidermal scales, feathers and hair need not itself have been a scale.

Edited February 6, 2021 by Tristan L