The evolution and biogeography of ankylosaurid dinosaurs from the late cretaceous of western north america

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147 stegosaurid (e.g. Struthiosaurus) dinosaurs (Loewen and Kirkland, 2013). The sacral vertebrae are bowed to form a shallow, ventrally concave arch. Free Caudal Vertebrae At least eight proximal caudal vertebrae are preserved and represent the flexible portion of the tail (Fig. 3.18). All caudals experienced crushing and breakage to a certain degree, causing not one single vertebra to be complete. All vertebrae contain dorsoventrally tall haemal arches, which are tightly fused onto the ventral part of the centrum and possess a distinctly flattened keel that expands anteroposteriorly, forming a short but tall boot. Centra are laterally constricted and contain spool-shaped, slightly amphicoelous, articular anterior and posterior surfaces, similar to those observed in Nodocephalosaurus kirtlandensis, Talarurus plicatospineus, and Oohkotokia horneri. In Ankylosaurus and Anodontosaurus, the anterior and posterior faces of the centra are more deeply excavated, resulting in more pronounced amphicoelous conditions. The neural spines are tall on all caudal vertebrae and, including the neural arches, account for nearly 50% of the total vertebral height. Although there exists little variation in the proximal caudal series amongst different ankylosaurid taxa, significant morphological differences are expressed between the caudal vertebrae themselves. Proximal caudals 1-2 are the largest and most complete caudal vertebrae preserved in the series and are morphologically most similar to Ankylosaurus magniventris and Euoplocephalus tutus. The centra are ellipsoid in anterior and posterior view, a condition that is consistent across all eight proximal caudal vertebrae, however, the central facets do become more round in the more distal caudals of the proximal series. Additionally, the anteroposterior length of the first two caudals are strongly abbreviated