Possible origin of the xenacanth sharks Orthacanthus texensis and Orthacanthus platypternus in the Lower Permian of Texas, USA

Abstract

Two species of Orthacanthus (Chondrichthyes: Xenacanthiformes), Orthacanthus texensis and Orthacanthus platypternus, occur in the Lower Permian of Texas, where O. texensis is restricted to the Wichita Group and O. platypternus to the Wichita and overlying Clear Fork groups. A third species, Orthacanthus compressus, occurs in the Pennsylvanian and questionably in the lowermost Permian of North America. The teeth of O. ?compressus, including what are interpreted as juvenile teeth, occur in the Archer City Bonebed 3 in the Archer City Formation, Bowie Group (Asselian). Larger teeth of this species are distinguished from O. texensis teeth by the presence of a more pronounced basal tubercle; otherwise the two species appear to be equivocal. However, the juvenile teeth of O. texensis have never been recovered in Permian localities; there is no evidence of a distinct juvenile dentition in this species. The larger Archer City adult teeth tend to have serrations on both carinae of each cusp, whereas the medial carinae are not serrated in the smaller adult teeth. The base tends to be wider than long, similar to O. texensis, whereas the juvenile teeth tend to be longer than wide, have a thinner base and lack serrations, similar to O. platypternus teeth. The distribution of dental features suggests that both O. texensis and O. platypternus could be derived from O. ?compressus. It is therefore proposed that the juvenile dental features of O. ?compressus are retained in the adult teeth of O. platypternus via paedomorphosis, whereas the juvenile features of the ancestor are truncated in O. texensis (i.e. its dentition retains only the characters of O. ?compressus adult teeth).