I have encountered a number of science-oriented weblogs in the past few months with posts on synapsids, in which it is emphasized that the term "mammal-like reptiles" is completely inaccurate, as synapsids are in a completely different group from true reptiles. Some of them also state that the skin of  gorgonopsids may have been neither hairy nor scaly, but smooth and dotted with pores (based on skin impressions from Estemmenosuchus). Does this mean that synapsids lost their scales at some point? Or, considering that they weren't reptiles, might they have never evolved scales at all? If this is the case, does this mean that all those reconstructions of scaly basal synapsids, such as Dimetrodon, are incorrect? And one more thing; if synapsids didn't have scales, what did mammalian hair evolve from?

Hi, Bryan.  It's more correct to describe synapsids as "reptile-like mammals" than "mammal-like reptiles", as they are part of the lineage that leads to mamals.  The great group of vertebrates that can lay eggs out of water is called Amniota, and that group split in two quite early in its history.  From an ancestor somewhat like a modern lizard, one subgroup -- Synapsida -- eventually led to mammals, while the other -- Sauropsida -- led to the reptiles we know today (turtles, lizards and snakes, crocs), plus dinosaurs and birds.  The early synapsids would have resembled their lizard-like ancestor (just as the early sauropsids did), but somewhere along the line to mammals, those scales were replaced by fur.  The most primitive synapsids are known informally as "pelycosaurs", and Dimetrodon and friends are part of this primitive grouping.  So it's reasonable to assume they had scales, despite being more closely related to mammals than to and reptile alive today.

Possibly even more correctly, the group Synapsida (=synapsids) include mammals, so when we talk about the early synapsids that had not yet attained 'mammalness' (and there is still considerable debate about where this particular line should be drawn), to be completely accurate the term should be 'non-mammalian synapsids'. This seems to be an almost universally hated name even among practicing scientists, but is the accepted scientific description as we cannot describe the non-mammalian members of Synapsida as a monophyletic (= single, natural) group. This is because Synapsida includes all taxa more closely related to mammals than to Reptilia, including the mammals, and we would be excluding the mammalian members of this natural group to make an unnatural group. A more easily understood example is the name 'fish'. Although everyone knows what this means, the group is unnatural since it excludes some of the descendents of the common ancestor of all fish - namely tetrapods, including us!

People often ask whether non-mammalian synapsids had scaly skin or not, and the answer that palaeontologists often provide is a 'theoretical' one based on phylogenetic inference. We can do better than that, however, as the answer can be found in the literature. We have ophiacodonts preserved with a dense covering of 'belly scales': these scales are narrow and elongate, and are the dermal structures homologous with the scales of more basal tetrapods, and with gastralia (important: they are NOT homologous with the scales of squamates - these are specialised >epidermal< structures). So basal non-mammalian synapsids - the animals informally called pelycosaurs - did have a scaly underside, but their upper surfaces weren't scaly so far as we know (Carroll 1969, p. 427). Exactly what sort of skin they had dorsally is still not sure so far as I know, though they must have had an impermeable epidermis. A skin impression of the basal therapsid _Estemmenosuchus_ has been stated to demonstrate a naked, glandular, mammal-like epidermis, but while this specimen is often mentioned in the popular literature I don't know if it's been re-evaluated.

Ref - -

Carroll, R. L. 1969. Problems of the origin of reptiles. _Biological Reviews_ 44, 393-432.

Last edited by Darren Naish (24th May 2008 15:47:10)