Elasmobranch and Osteichthyan Fauna of the Rattlesnake Mountain Sandstone, Aguja Formation (Upper Cretaceous; Campanian), West Texas



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A thin granular conglomerate within the Rattlesnake Mountain sandstone member of the Upper Cretaceous Aguja Formation on Ten Bits Ranch in Brewster County, Texas preserves a diverse assemblage of small teeth, denticles, vertebrae, and other bones of chondrichthyan and osteichthyan fishes. This thin layer of teeth and bones probably represents a winnowed lag deposit, concentrated by wave action in a coastal marine environment. The deposit, referred to herein as the "Ten Bits Microsite" is highly fossiliferous; screen-washing methods yielded about 5000 specimens. Chondrichthyan fishes are represented by 23 species,while identifiable osteichthyan fishes represent four species. Two of the three most abundantly occurring chondricthyan species (Scapanorhynchus texanus and Ischyrhiza mira) are also the most common species in other middle to Late Campanian marine vertebrate faunas along the Gulf of Mexico, and Atlantic Coastal Plain. Ptychotrygon agujaenisis is abundant at Ten Bits, but unknown in correlative marine faunas elsewhere. The abundance and diversity of ptychotrigonid rays may be a unique feature of the Ten Bits fauna. The most common osteichthyan fishes found in the Ten Bits fauna (Paralbula casei and Albula sp.) are also reported elsewhere in Gulf and Atlantic Coastal Plain faunas, but they are rare there and subordinate to bony fishes not recovered at Ten Bits. Paralbula casei is a common bony fish found in marine vertebrate faunas of the Western Interior.The most common chondrichthyan fishes found in Western Interior faunas are either unknown or rare in the Ten Bits fauna, and the common Western Interior ray Myledaphus bipartitis does not occur at Ten Bits or any other Gulf or Atlantic Coast fauna. These differences probably reflect latitudinal variation, oceanic water circulation pattern, or variation in other environmental conditions between the Western Interior Seaway and the Gulf or Atlantic Coast that restricted the distributions of some marine fish species. The similarities between the Ten Bits fauna and those of the Atlantic and Gulf Coast indicate that western Texas was more closely allied biogeographically with that province than with the Western Interior of North America. One species tentatively identified in the Ten Bits fauna on the basis of a single tooth, Igdabatus indicus, is otherwise known only from Africa and Asia. If this identification is correct, it would represent the only known occurrence of the species in North America. This could reflect the chance preservation of a single individual outside of its normal range. Western Texas may have been near the northern limits of the range for tropical marine vertebrate species.