Late Cretaceous dinosaur assemblages of North America—characterized by gigantic tyrannosaurid predators, and large-bodied herbivorous ceratopsids and hadrosaurids—were highly successful from around 80 million years ago (Ma) until the end of the ‘Age of Dinosaurs’ 66 Ma. However, the origin of these iconic faunas remains poorly understood because of a large, global sampling gap in the mid-Cretaceous, associated with an extreme sea-level rise. We describe the most complete skeleton of a predatory dinosaur from this gap, which belongs to a new tyrannosauroid theropod from the Middle Turonian (~92 Ma) of southern Laramidia (western North America). This taxon, Suskityrannus hazelae gen. et sp. nov., is a small-bodied species phylogenetically intermediate between the oldest, smallest tyrannosauroids and the gigantic, last-surviving tyrannosaurids. The species already possesses many key features of the tyrannosaurid bauplan, including the phylogenetically earliest record of an arctometatarsalian foot in tyrannosauroids, indicating that the group developed enhanced cursorial abilities at a small body size. Suskityrannus is part of a transitional Moreno Hill (that is, Zuni) dinosaur assemblage that includes dinosaur groups that became rare or were completely absent in North America around the final 15 Myr of the North American Cretaceous before the end-Cretaceous mass extinction, as well as small-bodied forebears of the large-bodied clades that dominated at this time.
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The data that support the findings of this study are provided in the Supplementary Information and Supplementary Data 1–3. High-quality images of the histology sections are available on Morphobank (project number 3298; permalink: http://morphobank.org/permalink/?P3298). Reconstructed computed tomography slices (in .tiff stack format) are available for the holotype skull (MSM P4754) at https://doi.org/10.17602/M2/M68107. Data have been deposited in ZooBank under Life Science Identifier urn:lsid:zoobank.org:act:7A2A304F-66E9-4788-BAFC-1FBFD17ED47A (for the new genus and species).
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We thank M. Norell, T. Carr, N. Longrich, T. Holtz, D. Evans and P. Makovicky for discussion, and L. Zanno and G. McCullough for specimen numbers. We thank C. M. ‘Kay’ Molenaar for input on our stratigraphic interpretations. We thank C. Griffin and S. Werning for help imaging the histology slides, and M. Stocker for computed tomography scanning help. Field work was conducted under BLM permit MSM-8172-RS-1A (to D.G.W.). Preparation of the specimens was conducted by H. Bollan and P. Bollan (Grand Junction, Colorado). We thank B. Anderson for discovery and documentation of the holotype specimen. R. Gaston of Gaston Design provided study casts of the material to R.K.D., D.G.W. and J.I.K. This work was funded by the Discovery Channel (to J.I.K. and D.G.W.) and the Department of Geosciences at Virginia Tech (to S.J.N.). We specifically thank M. Harrington and the Pueblo of Zuni Tribal Council for discussion of the name Suskityrannus. The larger project—the ‘Zuni Basin Paleontology Project’, led by the Wolfe family—was supported by members of the Southwest Palaeontological Society, the Arizona Museum of Natural History and dozens of volunteers since 1996. We thank M. Hayden and S. Carney for UGS technical reviews.
The authors declare no competing interests.
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Supplementary Figures, differential diagnosis, geology, supplementary description of localities, ontogenetic age assessment, measurements, phylogenetic results, Late Cretaceous diversity data, dinosaur assemblage members and supplementary references
TNT data matrix used for the phylogenetic analysis. Loewen tyrannosauroid phylogeny dataset with Suskityrannus
TNT data matrix used for the phylogenetic analysis. Brusatte and Carr 2016 tyrannosauroid phylogeny dataset with Suskityrannus
TNT data matrix used for the phylogenetic analysis. Brusatte et al. 2014 theropod dataset with Suskityrannus
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Nesbitt, S.J., Denton, R.K., Loewen, M.A. et al. A mid-Cretaceous tyrannosauroid and the origin of North American end-Cretaceous dinosaur assemblages. Nat Ecol Evol 3, 892–899 (2019). https://doi.org/10.1038/s41559-019-0888-0