New craniodental materials of Falcarius utahensis (Theropoda: Therizinosauria) reveal patterns of intraspecific variation and cranial evolution in early coelurosaurians.

Despite documented ecomorphological shifts toward an herbivorous diet in several coelurosaurian lineages, the evolutionary tempo and mode of these changes remain poorly understood, hampered by sparse cranial materials for early representatives of major clades. This is particularly true for Therizinosauria, with representative crania best known for the late-diverging Erlikosaurus andrewsi and the early taxon Jianchangosaurus yixianensis. Here we describe a series of new cranial bones of Falcarius utahensis, the geologically oldest therizinosaurian from the Early Cretaceous Cedar Mountain Formation, Utah, United States. This new material provides the most complete understanding of the skull to date for Falcarius and frames the pattern and timing of cranial evolution in therizinosaurians and early coelurosaurians. Previously unknown elements include a well-preserved maxilla, jugal, parietals, squamosal, laterosphenoids, and pterygoid. Computed tomography data differentiate the incisiform rostral dentary dentition from possible premaxillary teeth, the first in a therizinosaurian. Notable features include a primitive morphology of the jugal and frontoparietal complex shared with other early diverging taxa (e.g., tyrannosauroids, Incisivosaurus, Ornitholestes, Fukuivenator), and a large maxillary fenestra, convergent with troodontids. Additional specimens of previously known elements confirm their taxonomic utility and provide insight into intraspecific variation. Following patterns of other archosaurs, variable traits relate to the prominence of ridges and contours (likely associated with musculature) and the proportions of pneumatic features, whereas invariant traits correspond to the topology of bony contacts and major cranial nerves. Early, integrated evolution of the rostrum and adductor complex characterized early therizinosaurians, which was further modified alongside reduced paranasal complexity in later therizinosaurids.