New information on the anatomically derived millerettid Milleretta rubidgei from the latest Permian based on µCT data

Abstract

The fossil reptile Milleretta holds a prominent role in phylogenetic analyses of early reptile relationships. It has often been used as the sole marker for the anatomically diverse middle to late Permian Millerettidae, a clade that has been hypothesized as the earliest diverging parareptiles and therefore only distantly related to the reptile crown group. However, the anatomy of Milleretta remains incompletely documented, presenting an obstacle to phylogenetic studies of early reptile evolution. We re-examine the cranial anatomy of Milleretta rubidgei using synchrotron micro-computed tomography of two specimens, representing a juvenile and a subadult. These immature individuals have clearly visible sutures, differing from osteologically mature individuals, in which cranial osteoderms obscure the cranial anatomy, particularly in the antorbital region. We demonstrate that Milleretta and other millerettids share many derived similarities with Neodiapsida (a derived clade that includes the reptile crown group), particularly of the neurocranium and palatoquadrate. Comparison with other millerettids reveals that some features seen in adult specimens of Milleretta are, in fact, derived features, resulting from secondary modification in osteologically mature individuals. These observations suggest that Milleretta is an anatomically derived millerettid and urge caution in using this taxon as the semaphorant of this disparate group of stem reptiles.