The Evolution and Phylogenetic Analysis of the Dinosaur Axial Skeleton
Open AccessThe dinosaur axial skeleton has received relatively little study compared to the cranial and appendicular skeletons. This study investigates how the evolution of the dinosaur axial skeleton is related to the evolution of posture and large body size, and how characters from the axial skeleton can be better incorporated into phylogenetic analyses.At least four groups of dinosaurs evolved a secondarily quadrupedal posture, but assessing the relative degree of bipedality or quadrupedality is difficult in many ambiguous taxa. The ratio of humerus to femur circumference shows distinct differences in allometric scaling between bipeds and quadrupeds, with a relatively more robust humerus in unambiguously quadrupedal dinosaurs. This ratio allows the evolution of posture to be compared across disparate clades of dinosaur, and suggests that hadrosaurs have a more bipedal posture than is suggested by other osteological features of the forelimb.Biomechanical models of bending predict that the dorsal region of the dinosaur axial skeleton is strongly affected by changes in posture. This study used geometric morphometrics to quantify variation in the dorsal vertebrae of ornithischian and theropod dinosaurs, and comparative phylogenetic methods to compare vertebral shape change to the evolution of posture and body size. Many features that have been shown to influence axial biomechanics in other vertebrates show similar changes in dinosaurs. These changes are consistent with predicted patterns of force distribution in the dorsal series, both craniocaudally within the series, and between taxa with different postures. However, removing phylogenetic non-independence from the dataset weakens these correlations although the extent that this is due to the undersampling of small bipedal taxa is unclear.The functional affect of these changes was further tested with finite element analysis. Ventral and torsional bending of theoretical models of vertebral morphologies were used to measure the changes in displacement and stiffness that result from individual variation in each feature. Features that affect the dorsoventral height of the vertebra, such as the size of the centrum, neural arch, and neural spine, all increased stiffness against ventral bending. This finding is consistent with their distribution in the dinosaur axial skeleton. However, transverse process shape had the opposite effect on axial stiffness to that predicted by its variation within the dinosaur dorsal series. This may be because the transverse processes influence axial biomechanics predominantly through their interaction with the ribs, but rib morphology was not incorporated into these finite element models.Phylogenetic analyses often have relatively few characters from the axial skeleton, but coding more axial characters is complicated by the ambiguous homologies of individual vertebrae in different taxa. This study compared the accuracy of phylogenetic analyses employing one of two approaches to coding and analyzing simulated characters from serially repeating structures such as vertebrae. The first approach summarizes serial variation into a single polymorphic character for the entire series which can be analyzed using regular parsimony analysis. The second codes characters from each structure individually and then aligns the structures during analysis using dynamic homology. This individual coding showed small improvements when the simulated characters were serially complex, but the range of character state values was similar in different taxa. However, as interspecific variation increased, and overlap in character states between taxa decreased, the summary approach became more accurate. In addition, individual coding became significantly less accurate than the summary approach as missing data was introduced into the dataset, reflecting the increased likelihood of misaligning structures when some are missing.
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