Reevaluation of the holotypes of Koskinonodon princeps Branson and Mehl, 1929, and Borborophagus wyomingensis Branson and Mehl, 1929 (Temnospondyli, Metoposauridae)

ABSTRACT

Metoposaurids are some of the most commonly occurring tetrapods in non-marine Upper Triassic sediments in the northern hemisphere of Pangea. Since the first description of a metoposaurid in 1842, nearly two dozen species have been named, but many of these have been regarded with increasing skepticism by modern workers because of minor differences used to validate novel species and sometimes novel genera. More recent comprehensive descriptions and evaluations of intraspecific variation from several presumed monospecific bonebeds of metoposaurids have prompted reevaluation of holotypes due to variation in proposed apomorphies. Four metoposaurid species were named from the Popo Agie Formation exposures of Wyoming, U.S.A., but at present, only a single species, Anaschisma browni, is considered valid following a recent redescription of two of these taxa (An. browni and An. brachygnatha). The other two taxa, Borborophagus wyomingensis and Koskinonodon princeps, have not been redescribed since their original description in 1929. A redescription of the holotypes of these two taxa is presented here to assess their historic synonymy with An. browni and to provide a detailed, updated record of the Popo Agie Formation metoposaurids in light of a historic relative lack of attention compared with other North American deposits. Our confirmation of the conspecificity of all four Popo Agie Formation metoposaurids permits a detailed discussion of potential ontogenetic variation in the Popo Agie Formation metoposaurids and latitudinal variability in An. browni.

ACKNOWLEDGMENTS

We thank J. D. Schiffbauer, J. W. Huntley, and T. Selly for access to and assistance with the University of Missouri vertebrate paleontology collections. We thank D. M. Lovelace for helpful comments on the stratigraphy of the Chugwater Group. We recognize the Eastern Shoshone and Northern Arapahoe people who occupy the Wind River Reservation and from whose land these fossils were collected. Lastly, we thank A. D. Marsh and an anonymous reviewer for comments that greatly improved the manuscript. This work was partially funded by a Geological Society of America Graduate Student Research Grant awarded to A. M. Kufner (grant no. 12497-19).