Cranial morphology of recumbirostrans (Lepospondyli) from the Permian of Kansas and Nebraska, and early morphological evolution inferred by micro-computed tomography

ABSTRACT

‘Microsaurs’ (Lepospondyli) were a group of tetrapods whose fossil record spanned the Mississippian–Early Permian, and have sometimes been implicated in the origins of lissamphibians (especially the fossorial caecilians). Although common in the well-sampled Permian deposits of Texas and Oklahoma, little is known of conservative recumbirostran ‘microsaurs’ from more northerly localities (Kansas/Nebraska). Data on a new, well-preserved recumbirostran, Huskerpeton englehorni, gen. et sp. nov., from the Eskridge Formation, Nebraska, reveal affinities to the purported gymnarthrid ‘Euryodus’ bonneri (here moved to a new genus, Proxilodon) from the Speiser Formation, Kansas. High resolution micro-computed tomography reveals important endocranial data, including an ossified ‘supraoccipital’ that is usually absent in gymnarthrids, a dorsal sinus separating the synotic tectum from the dermal skull roof as in ostodolepids, ventral flanges of the frontals articulating directly with the ascending lamina of the sphenethmoid, and tall, ossified ‘pleurosphenoids,’ thereby providing useful comparison with morphologically derived recumbirostrans. A cladistic analysis of 60 taxa and 227 characters recovered a monophyletic clade of Huskerpeton, Proxilodon, and the recently described Tambaroter within a more inclusive clade that includes gymnarthrids and ostodolepids (both nested members of the Recumbirostra and putative outgroups to caecilians). Whereas other lepospondyls (i.e., lysorophians) are abundant in the region, the records of Huskerpeton and Proxilodon reveal that only two ‘microsaur’ genera are adequately known from the lowermost Permian of Kansas/Nebraska and demonstrate similarities between the Eskridge and Speiser faunas. Moreover, these forms may represent the ground plan from which more specialized fossorial forms (gymnarthrids and ostodolepids) evolved.

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ACKNOWLEDGMENTS

Field collections and support: J. and S. Bangert (for site access), J. Benca, W. May, and S. Tucker (field collections and support), and C. Sidor (sponsor for Burke Museum Dinosaur Fund). Preparation and laboratory assistance: J. Englehorn (preparation of UNSM 32144), and M. Colbert and J. Maisano (UTCT, University of Texas at Austin) for scanning the specimen of Huskerpeton. Specimen access: R. Corner and S. Tucker (UNSM), L. Martin and D. Miao (KUVP), and L. Ivy (DMNH). Insightful discussions and suggestions: J. Calede, D. DeMar, A. Henrici, H. Maddin, S. Nesbitt, B. Peecook, and L. Tsuji. We thank R. Holmes for handling the submission of the manuscript, and H. Maddin and an anonymous reviewer for their insightful reviews. This research was supported in part by grants from California State University Associated Students Incorporated and the Burke Museum Dinosaur Fund (to A.K.H.) and Natural Sciences and Engineering Council of Canada Discovery Grant (to J.S.A.).

Handling editor: Robert Holmes.