A remarkable sample of bird teeth from the palaeo-Arctic Prince Creek Formation of northern Alaska elucidates Late Cretaceous avialan richness.

Understanding the taxonomic affinity of putative avialan teeth is essential to assessing the geographic range and richness of Late Cretaceous toothed birds. Isolated teeth attributed to Avialae are known from numerous North American Late Cretaceous microfossil assemblages. However, because only a select few of these tooth types are known from teeth preserved in jaws, their taxonomic affinities remain uncertain. Several key morphologies used to diagnose avialan teeth are also present in non-avialan theropods and crocodyliforms, leading some to question whether many such isolated teeth belong to birds. Here, we provide detailed descriptions and high-resolution images of avialan teeth from the Late Cretaceous Prince Creek Formation (PCF) of northern Alaska. A total of 105 isolated teeth were identified as avialan and can be divided into seven distinct morphotypes. Crucially, decades of extensive microfossil sampling have failed to produce any evidence of crocodyliforms in the PCF, allowing us to rule out this possible identity. Further, the teeth described here can be distinguished from those of all non-avialan theropods known in the PCF. Our morphotypes thus provide an important framework for identifying bird teeth in other Late Cretaceous microfossil assemblages as well as understanding avialan diversity before the K-Pg extinction.