New sharks in a chondrichthyan fauna from the Upper Cretaceous Aguja Formation (lower Campanian) of West Texas support biogeographic segregation among chondrichthyans in the Western Interior

A new, oral tooth-based chondrichthyan fauna consisting of 16 species is reported, two of which, Lonchidion conrugis sp. nov. and Restesia corricki sp. nov. are described for the first time. The present work compliments previous descriptions of other vertebrate groups from the early Campanian, Lowerverse microfossil locality in the Abajo Shale Member of the Aguja Formation of West Texas. This local fauna accumulated in a brackish water intertidal depositional setting, and differs significantly from the paracontemporaneous Ten Bits chondrichthyan assemblage found in overlying nearshore marine deposits of the Aguja Formation. The Lowerverse fauna exhibits a comparatively greater diversity and greater relative abundance of hybodont and orectolobiform sharks, but comparatively lower diversity and lower abundance of lamniform sharks compared to the Ten Bits fauna. The most abundant sharks in the Ten Bits fauna (Scapanorhynchus, Squalicorax, and Serratolamna) are rare or absent in the Lowerverse fauna. Rays and sawfishes are also more diverse and more abundant in the Ten Bits fauna, but the most common taxa in the Lowerverse fauna (Cristomylus, Texatrygon) are absent or very rare at Ten Bits. The Lowerverse assemblage is unique among paracontemporaneous Santonian–early Campanian chondrichthyan faunas in the Western Interior of North America in the presence of Restesia. Several taxa (Scapanorhynchus, Rhombodus) are found only in the southernmost of these faunas, and others (Cantioscyllium, Columbusia, Cristomylus, Ptychotrygon, and Texatrygon) are found only as far north as Utah. The Lowerverse chondrichthyan fauna supports regional segregation of some chondrichthyan species in the Western Interior during this time.