A new hypothesis of eudromaeosaurian evolution: CT scans assist in testing and constructing morphological characters

Eudromaeosauria is a clade of derived dromaeosaurids that typifies the common perception of 'raptor' dinosaurs. The evolutionary history of this clade has been controversial due to conflicting views of taxonomic identity, and because, due to taphonomic bias, several species were diagnosed primarily or solely by the maxilla. The maxilla is therefore crucial in understanding the phylogenetic relationships within the clade. Morphometric characterization has been commonly applied to recognize and distinguish major dromaeosaurid clades. However, morphometrics mainly showed morphological convergence rather than phylogenetic relationships. This approach has made it difficult to get resolution of phylogenetic relationships among eudromaeosaurian taxa, often resulting in large polytomies or inconsistent placement of key species. To test previous character statements, computed tomography was used to analyze the maxillae of Acheroraptor, Atrociraptor, and Deinonychus, and compare them with other eudromaeosaurians from Asia and North America. Morphometric characters were examined, and regressions were used to look for allometric trends in maxillary dimensions and the relationship to topological landmarks within Eudromaeosauria and its outgroups. Characters were improved and implemented to better capture eudromaeosaurian morphological variation and better resolve their phylogenetic relationships. Phylogenetic analysis recovered three well-defined clades within Eudromaeosauria and corroborated occurrence data within the fossil record. Acheroraptor and Atrociraptor were recovered as derived members of Saurornitholestinae. Deinonychus is recovered as a basal eudromaeosaurian, sharing features with dromaeosaurines and saurornitholestines. These results challenge previous biogeographic hypotheses suggesting Asian and North American faunal interchange during the Late Cretaceous and support convergence of traits relating to snout dimensions and proportions.