23-1 An Upper Cretaceous Vertebrate Assemblage from the Tooth Draw Quarry of the Hell Creek Formation (Upper Maastrichtian), South Dakota and its Implications for Coelurosaurian Theropod Phylogeny

Session: Paleontology of North America (Posters)

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Abstract:

The Upper Cretaceous Tooth Draw quarry is an extremely fossiliferous site within the underrepresented South Dakota portion of the Hell Creek Formation outcrop belt. The site represents a fluvial depositional environment of interbedded conglomerates and sandstones and yields a diverse range of vertebrate taxa. Dinosaurs are well represented, with many diagnostic specimens of coelurosaurian theropods, ceratopsians, and ornithopods previously reported, including Triceratops and the ornithopods, Edmontosaurus and Thescelosaurus. Non-dinosaur macrovertebrates are also abundant at the site as many fish and aquatic reptiles have been previously identified. Gars and bowfin fish are represented by abundant, well-preserved scales. In addition to scales, teeth and fragmentary cranial elements are also very common. There are also numerous, fragmentary, crocodylomorph teeth and osteoderms.

Perhaps the most significant aspect of the dinosaur assemblage is the abundance of tyrannosauroid and dromaeosaurid teeth. Teeth from these two theropod groups comprise the vast majority of the vertebrate assemblage of the site. Specimens provided for study include representatives of Tyrannosaurus, “Nanotyrannus,” Aublysodon, and Acheroraptor. Both the tyrannosauroids and dromaeosaurids have several different tooth morphs at the quarry which has implications for phylogenetic and ontogenetic studies. Specifically, this site has evidence supporting the distinction of Nanotyrannus as a separate taxon from Tyrannosaurus rex. The Nanotyrannus teeth appear laterally compressed compared to the known Tyrannosaurus teeth from this site as well as have a narrow, elongate resorption pit while the Tyrannosaurus teeth have a round, almost circular, resorption pit. Additionally there is a tooth consistent with the description of Aublysodon, which has previously only been described from the older (Campanian) Judith River Formation in Montana.This tooth exhibits a distinct lack of serrations on the carinae and an oddly shaped distal portion of the crown. The dromaeosaurid teeth also have implications for the phylogenetic placement of the genus Acheroraptor and the validity of the genus Dakotaraptor as well as on the faunal exchange between Asia and North America in the Late Cretaceous. Morphometrics can be utilized to further distinguish the different morphs of tyrannosauroid and dromaeosaurid teeth. Using multivariate analysis of tooth dimensions and other variable traits, these metrics can be used to group the teeth and obtain statistical evidence if individual teeth represent the same or a different taxa.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

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