The origin of Amniota in phylogenetic context

Abstract

The membership, phylogeny and external relationships of Amniota quickly achieved “textbook wisdom” status in the mid-1990s, but were all called into question by phylogenetic analyses in the last few years. However, all these analyses and all their predecessors have either focused on early limbed vertebrates and barely extended into Amniota, or focused on amniotes and included very few non-amniotes as outgroups. Here I take an analysis of the first type, perform corrections and updates, and enlarge its sample of amniotes and amniote-related characters (among other things) in order to test these matters. Despite its very high rates of homoplasy, my matrix supports all “microsaurs” as amphibians, perhaps even less close to Amniota than Seymouriamorpha. For Diadectomorpha its “classical” position on the stem of Pan-Amniota is weakly supported; this stem has, however, greatly expanded to include many supposed amniotes, among others the fish-scaled Brouffia. The recent finding of Petrolacosaurus outside Diapsida, indeed outside Amniota along with Captorhinidae, is—weakly—corroborated despite my quite different taxon and character samples. Caseasauria, normally considered part of Pan-Mammalia, is instead nested among the “parareptiles” within Sauropsida; Varanopidae, similarly traditionally placed among the pan-mammals, instead emerges in three separate sauropsid positions. A period of very small body size around the origin of the amniotic egg is not supported by the optimal trees, but trees that seem compatible with it are nearly optimal. Slightly worse trees allow for aïstopods as limbless anthracosaurs. The generally low support for the trees highlights specific needs for future research. Still, it appears that the recently proposed membership of many traditional “microsaurs” in Amniota can be excluded with reasonable confidence, as can a position of Anthracosauria crownward of Temnospondyli or a temnospondyl origin for any extant amphibians. Coding the temporal bone of “microsaurs” and other taxa as the tabular or the supratemporal has practically no effect.