A ceratopsian bone bed from the Aguja Formation (Upper Cretaceous) Big Bend National Park, Texas



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The Upper Cretaceous Aguja Formation, exposed in the Big Bend region of southwest Texas, consists of prodeltaic, deltaic, and fluvial sediments, which were deposited during the final regression of the western interior epeiric seaway. Interdistributary marsh deposits within the deltaic part of the Aguja have yielded a vertebrate fauna of Campanian aspect, which includes a new species of ceratopsian dinosaur. Chasmosaurus mariscalensis n. sp. is represented by remains of at least ten individuals obtained from a single quarry in 1938. This "bone bed" deposit represents a winnowed assemblage of disarticulated but non-transported remains of juvenile and adult animals, accumulated within an abandoned crevasse channel. C. mariscalensis is distinguishable from other species of Chasmosaurus by its short squamosal, large laterally-compressed maxilla, abbreviated premaxilla, and large supraorbital horncores. A full description of the postcranial skeleton of C. mariscalensis is the first given for Chasmosaurus. C. mariscalensis is small and gracile, but similar in proportions to Pentaceratops. Ontogenetic changes in the post-cranial skeleton of C. mariscalensis reflect a greater weight and body girth in the adult. In the skull, there is ontogenetic elongation of the facial region, supraorbital horncores, and frill. These observations suggest that Eoceratops should be invalidated as a juvenile variant of Chasmosaurus. C. mariscalensis exhibits sexual dimorphism in its supraorbital horncores. Females have upwardly-curved, antero-laterally directed horncores. Males have erect, dorsally-directed horncores. It is possible, based on horncore orientation, to distinguish the sexes in other long-frilled ceratopsids as well. A division of the Ceratopsidae at the subfamily level is justified, and the names Chasmosaurinae and Centrosaurinae, originally proposed by Lawrence M. Lambe, are revived for the long-frilled and short-frilled ceratopsids, repectively