Osteology and phylogenetic position of the diminutive ‘microsaur’ Odonterpeton triangulare from the Pennsylvanian of Linton, Ohio, and major features of recumbirostran phylogeny

Abstract

Abstract The group of Permo-Carboniferous tetrapods known as Recumbirostra have recently been hypothesized to represent the earliest radiation of fossorial reptiles. Therefore, understanding the anatomy and diversity of this clade is essential to understanding the origin and early evolution of amniotes. Here, we redescribe the diminutive ‘microsaur’ Odonterpeton triangulare from the Moscovian-age deposit of Linton, Ohio, revealing new details on the structure of the cranium and palate, including the presence of a conspicuous transverse flange of the pterygoid, which is a feature traditionally associated with early amniotes. Phylogenetic analysis supports the placement of Odonterpeton triangulare within Recumbirostra as sister taxon to the recently described Joermungandr bolti from the slightly older Mazon Creek, Illinois, Lagerstätte. Together, these two diminutive recumbirostrans are placed in an Odonterpetidae clade. Furthermore, we provide a discussion of newly recognized anatomical features in recumbirostrans and discuss major features of recumbirostran phylogeny. We designate a new recumbirostran clade, Chthonosauria (Brachystelechidae + Molgophidae), which is supported by a number of shared derived cranial features.