Reassessment of the Early Cretaceous North American "troodontid" Geminiraptor suarezarum as a therizinosaurian and mosaic evolution of coelurosaurian maxillae.

The only recorded member of the bird-like Troodontidae from Early Cretaceous North America is Geminiraptor suarezarum, known from an isolated maxilla recovered from the Suarez Site bonebed of the basal Cedar Mountain Formation (Utah, USA). Geminiraptor would be one of the geologically oldest and largest-bodied troodontids on the continent and a key taxon for understanding evolutionary, ecological, and biogeographic patterns in this clade. However, the association of the holotype specimen with a monodominant therizinosaurian bonebed, a poor understanding of early therizinosaurian crania, and a host of newly available comparative material since its original description, call for a reassessment of this important specimen. Here we redescribe the holotype maxilla of Geminiraptor and other cranial bones from the Suarez Site bonebed using updated anatomical, morphometric, and phylogenetic datasets. Several features of the holotype maxilla and other cranial bones (including a frontal, postorbitals, a quadrate, dentaries, a possible lacrimal, and a fragmentary braincase) are now known to characterize early diverging therizinosaurians or are more broadly distributed within Theropoda. Computed tomography data reveal previously unrecognized replacement teeth in the maxilla with a stout, lanceolate, labially convex morphology, a prominent lingual median ridge and basal swelling, and minute denticles. These features are consistent with the dentition of early therizinosaurians (e.g., Falcarius, Jianchangosaurus, Beipiaosaurus) instead of troodontids. Morphometric analyses of coelurosaurian maxillae demonstrate substantial overlap in morphospace between early therizinosaurians, troodontids, and Geminiraptor. Phylogenetic analyses recover the specimen as the sister taxon to Falcarius utahensis at the base of Therizinosauria. Accordingly, we reidentify "Geminiraptor" as a therizinosaurian and establish a novel combination, Falcarius suarezarum. This introduces a fourth therizinosaurian to the earliest Cretaceous of North America and restricts definitive, large-bodied troodontids to the Late Cretaceous. Shared maxillary proportions of early coelurosaurians may reflect morphological constraints and a non-carnivorous diet, converged upon by late-branching troodontids.