The Evolution of Snout Shape in Eudromaeosaurians and its Ecological Significance

• Author / Creator

  Powers, Mark J

• Dromaeosaurids were small to medium sized theropod dinosaurs that diversified during the Late Cretaceous, reaching a near cosmopolitan distribution. They were diverse in morphology from the small four-winged gliders of Microraptorinae, to the bear-sized giant ‘raptors’ like Achillobator or Utahraptor from the Eudromaeosauria. Eudromaeosauria and its constituents that make up the general public’s view of ‘raptor’ dinosaurs. This group comprises the medium to large dromaeosaurids with pronounced teeth and claws to fill their respective predatory niches and were largely restricted to Laurasia. The known diversity of this clade has increased dramatically since the 1990s, new species described almost yearly. Except for a few nearly complete skeletons, eudromaeosaurians are represented by limited material used in diagnosing distinct species. The maxilla has been given a lot of phylogenetic and ecological weight due to its complexity and its relation to feeding behaviour. This creates an image of eudromaeosaurian systematics that is largely based on one element of the skeletal anatomy. It is important then to have the most complete understanding of the morphology of this one element and how it varies among specimens. This can be difficult as individual bones within a species can show ontogenetic, sexual and/or individual variation. Additionally, taphonomic processes can influence our understanding and interpretations of features affected by deformation. Once these factors are assessed, the level of phylogenetic and ecological inference can be better hypothesized.
  In this thesis project, I acquired computed tomography data of eudromaeosaurian maxillae to study these elements within this clade. This allowed me to perform a thorough examination of the morphology of internal features and suture patterns and attempt to retro-deform areas that were distorted during the fossilization process. Previous morphological descriptions were expanded on and modified as needed. Morphological variation of the maxilla was examined with an emphasis on the ingroup variation of Eudromaeosauria. Linear measurements were taken from a wide range of eudromaeosaurian maxillae for bivariate and principal component analyses to test for trends. Maxillary characters that are ratio-based were examined to look for natural breaks in the range of variation observed to assess the validity of character state thresholds. The characters were then either dropped or adjusted to fit the data. Using the same methods, the range of variation within Velociraptor was examined, coupled with a description of a previously undescribed specimen, to develop a baseline for intraspecific variation.
  Complex morphology of the maxillary sinus system is demonstrated to be shared among North American eudromaeosaurians from the Late Cretaceous. The sinus systems are noticeably different in related Asian taxa. Ratio-based maxillary characters pertaining to the anterior ramus overshadowed phylogenetically informative features previously not coded. Reinterpretation of the maxilla of Deinonychus revealed a morphology more in line with other North American dromaeosaurids, and thereby changed its close phylogenetic placement to Asian taxa. PCA analysis of the maxillae across eudromaeosaurian species shows clear distinction between Asian and North American morphologies. Asian taxa are shallow and elongate while North American taxa have either stout maxillary morphologies or intermediate between the extremes. The gradient of maxilla elongation across PC 1 possibly represents adaptations for prey selection. Maxillary variation within specimens previously identified as Velociraptor mongoliensis suggests one specimen falls outside a conservative range of variation and should be classified as a new species -- Velociraptor vadarostrum sp. nov. Supported by discrete autapomorphies of the frontal and pelvis, V. vadarostrum sp. nov. fits well in the trend in maxillary morphology in Asian velociraptorines, possessing the most elongate form compared to Linheraptor and Tsaagan, which possess the least elongate maxillae of the Asian velociraptorines. The consistency of elongate snout morphology in derived Asian eudromaeosaurians supports a monophyletic Velociraptorinae. Persistence of this trait may have been driven by environmental pressures of predominantly arid to sub arid environments which define the Djadokhta Formation of Mongolia and equivalent sediments in China. These arid environments would not support a high diversity of large animals and would cause pressure on Asian eudromaeosaurians to specialize in hunting and eating smaller, more abundant prey. The maxilla is an informative element due to its importance in theropods, for interacting with its lifestyle and environment. Therefore, ecological pressures and vicariance were likely driving eudromaeosaurian biodiversity and morphological disparity during the Late Cretaceous of Laurasia.

• Subjects / Keywords

  • Snout Shape
  • Biogeography
  • Eudromaeosauria
  • Morphometrics
  • Morphology
  • Evolution

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-hz8e-5n76

• License

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