Abstract

We redescribe the holotype and only known specimen of the early eureptile Coelostegus
prothales from the Upper Carboniferous of the Czech Republic using photogrammetric
scanning and a virtual 3D rendition of its skull. New information is available on several skull
and lower jaw bones, including the postorbital, supratemporal, tabular, postparietal, angular,
and prearticular. The new data also permit the correct identification of previously undetected
or mis-identified elements (e.g., supratemporal; quadratojugal; angular). We provide an
amended diagnosis of Coelostegus and a new reconstruction of the skull in dorsal and lateral
views. To evaluate the affinities of Coelostegus, we code this taxon in two recently published
taxon-character matrices. Parsimony and Bayesian analyses do not permit firm
conclusions on the phylogenetic position of Coelostegus or, indeed, the status and extrinsic
relationships of protorothyridid amniotes. Coelostegus emerges either as the sister taxon to
the recently redefined Diapsida (Araeoscelidia; Varanopidae; Parareptilia; Neodiapsida), as
one of the most basal protorothyridids, or as a derived stem-group amniote in various parsimony-
based analyses, or as the basalmost protorothyridid in one Bayesian analysis, with
protorothyridids forming a paraphyletic array relative to Diapsida. We review the cranial similarities
and differences between Coelostegus and other protorothyridid genera and discuss
the implications that various phylogenetic results have for our understanding of early amniote
relationships.