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).

Keywords:

• chondrichthyans
• xenacanthids
• Orthacanthus
• Lower Permian
• Texas

Acknowledgements

Bill May (Norman, Oklahoma) helped collect the bulk sample at the Conner Ranch locality. He also identified many of the amphibian and reptile fossils that greatly enhance the significance of the Archer City and Conner Ranch collections. Jack Loftin (Archer County, Texas) arranged for permission to collect at the Archer City and Conner Ranch localities. Dale Winkler (Shuler Museum) entered all the catalogued identifications into the Shuler Museum database and provided assistance in figure preparation and valuable comments on an earlier draft of this paper. This earlier draft was also shown to Martha Richter (Natural History Museum, London) and to Oliver Hampe and Rodrigo Soler-Gijón (both Museum für Naturkunde, Berlin). Two anonymous reviewers provided comments that greatly improved the final version; their efforts are gratefully appreciated. Chuck Ciampaglio (Wright State University-Lake Campus) kindly provided information about his ongoing study of vertebrate microfossils from Ohio and Pennsylvania. Geb Bennett (Shenandoah Valley Discovery Museum) alerted me about the existence of the Benson's Pasture fauna, and Michael Brett-Surman (USNM) arranged to loan the xenacanth teeth to the Shuler Museum with catalogue numbers later provided by Mark Florence. Finally, thanks are extended to all the participants in the 12th International Early/Lower Vertebrates field trip to the Texas Permian in the June heat of 2011; preparation of the guide for that trip provided some of the background information for this paper.