Early Evolution of Titanosauiform Sauropod Dinosaurs.

Abstract

Titanosauriformes was a globally-distributed, long-lived clade of dinosaurs that contains both the largest and smallest known sauropods. In an effort to understand the phylogenetic relationships of their early (Late Jurassic–Early Cretaceous) members, this dissertation presents a taxonomic revision of Early Cretaceous North American titanosauriforms and a lower-level cladistic analysis of basal titanosauriforms worldwide. Taxonomic revision renders some Early Cretaceous North American sauropods nomina dubia, substantially augments the hypodigm of others, and recognizes a new genus and species. This revision reinforces the similarity among Early Cretaceous North American dinosaur faunas; this homogeneity stands in marked contrast to the latitudinal variation in dinosaur faunas that developed later in the Cretaceous. Reports of titanosaurs in the Early Cretaceous of North America are unsubstantiated. The latest register of Early Cretaceous North American sauropods (before the ‘sauropod hiatus’) occurs in the coastal units marking transgression of the Western Interior Seaway, whereas eight ecologically disparate dinosaur lineages are present just below and above this boundary in the same geologic units that sauropods are found in. I thus interpret the start of the sauropod hiatus as the result of a continent-wide extinction, coincident with and perhaps attributable to competition with ornithischian herbivores, rather than the result of preservation or sampling bias. Cladistic analysis of 25 taxa and 114 characters results in the recovery of three main titanosauriform clades: Brachiosauridae, Euhelopodidae, and Titanosauria. A re-evaluation of the phylogenetic affinities of fragmentary taxa and footprints based on synapomorphies recovered in this analysis finds no evidence for titanosaurs before the Early Cretaceous, in contrast to previous reports of Middle and Late Jurassic forms. The origin of Titanosauria is enigmatic because the geologically oldest titanosaurs are derived forms (lithostrotians) found penecontemporaneously on both Laurasian and Gondwanan continents. Finally, I explore the bone histology (osteocyte morphology) of recent birds in an attempt to discover proxies of bone growth rate that can be applied to Titanosauriformes. Characterization of osteocyte shape and size reveals substantial intra-bone, intra-individual, and intra-specific variation, and highlights the need for sampling control in studies of fossil bone histology.