Re-examining ratio based premaxillary and maxillary characters in Eudromaeosauria (Dinosauria: Theropoda): Divergent trends in snout morphology between Asian and North American taxa

Abstract

The clade Eudromaeosauria predominantly comprises medium to large Late Cretaceous Laurasian dromaeosaurids. Because many specimens are fragmentary, the maxilla has frequently been used to infer the proportions of the entire snout. Given the strong relationship between snout shape and feeding behaviour, such inferences have potentially important palaeobiological implications, but they should ideally be justified by explicit analysis. Maxillae have also been hypothesized to have high interspecific variability and are used to erect new taxa. However, many maxillary characters are based on proportions that may vary ontogenetically, sexually, and randomly within species, as well as phylogenetically. Proportionate characters are often unjustifiably coded in terms of discrete states even when they represent a continuum of variation.In this study, eudromaeosaurian maxillae were examined using bivariate (least squares regression) and multivariate (Principal Component Analysis and cluster analysis) methods, to discern the extent to which this skull element can be used to draw ecological and phylogenetic inferences. The length/height ratio of the maxilla was found to be a strong predictor of that of the premaxilla, justifying inferences about snout morphology from the former element alone. PCA and cluster analysis show a strong morphological dichotomy between Asian and North American taxa, and consistently recover congeneric specimens as being most alike. Maxillary characters from previous analyses were found to be divisible into discrete states, but to nevertheless illustrate the potential problems of using such characters in large datasets. Parsimony analysis of the revised premaxillary and maxillary characters resulted in a different tree topology than the most recent phylogenetic analyses, emphasizing the importance of this element in dromaeosaurid phylogenetics. Evolution of divergent snout morphologies in Asian and North American eudromaeosaurians may have been driven by selective pressures resulting from differences in the range of available prey, or in environmental variables such as aridity, between the two continents.