On the earliest record of Cretaceous tyrannosauroids in western North America: implications for an Early Cretaceous Laurasian interchange event

The sudden appearance of Asian dinosaur clades within Lower Cretaceous strata of western North America has long been recognised as a biotic dispersion event related to initial establishment of a Beringian land bridge. To date, uncertainty exists regarding the timing of the Early Cretaceous Laurasian interchange event (EKLInE) and the pattern of associated biotic dispersal. Here, we report a tyrannosauroid premaxillary tooth (FMNH PR 2750) from the Cloverly Formation, Wyoming, USA, that pushes back the earliest Cretaceous record of the clade in North America. Although fragmentary, the tooth is consistent with mounting evidence for a pre-108 Ma initiation of EKLInE and earliest Albian emplacement of Beringia. Previous authors have considered the Aptian/Albian of western North America a depauperate dinosaur fauna, characterised by regional extinction and diversity decline. Documentation of Albian tyrannosauroids in the region indicates a more dynamic ecosystem than previously appreciated and marks an early start to faunal mixing between immigrant and endemic dinosaur clades. Finally, we find that the enamel microstructure of FMNH PR 2750 conforms to the morphotype of tyrannosaurids, yet exhibits poor columnar differentiation. This morphology bolsters prior interpretations on the phylogenetic utility of enamel microstructure and suggests a trend of increasing enamel complexity within Tyrannosauroidea.