Michael D. D'Emic

A Nomenclature for Vertebral Fossae in Sauropods and Other Saurischian Dinosaurs

Background The axial skeleton of extinct saurischian dinosaurs (i.e., theropods, sauropodomorphs), like living birds, was pneumatized by epithelial outpocketings of the respiratory system. Pneumatic signatures in the vertebral column of fossil saurischians include complex branching chambers within the bone (internal pneumaticity) and large chambers visible externally that are bounded by neural arch laminae (external pneumaticity). Although general aspects of internal pneumaticity are synapomorphic for saurischian subgroups, the individual internal pneumatic spaces cannot be homologized across species or even along the vertebral column, due to their variability and absence of topographical landmarks. External pneumatic structures, in contrast, are defined by ready topological landmarks (vertebral laminae), but no consistent nomenclatural system exists. This deficiency has fostered confusion and limited their use as character data in phylogenetic analysis. Methodology/Principal Findings We present a simple system for naming external neural arch fossae that parallels the one developed for the vertebral laminae that bound them. The nomenclatural system identifies fossae by pointing to reference landmarks (e.g., neural spine, centrum, costal articulations, zygapophyses). We standardize the naming process by creating tripartite names from “primary landmarks,” which form the zygodiapophyseal table, “secondary landmarks,” which orient with respect to that table, and “tertiary landmarks,” which further delineate a given fossa. Conclusions/Significance The proposed nomenclatural system for lamina-bounded fossae adds clarity to descriptions of complex vertebrae and allows these structures to be sourced as character data for phylogenetic analyses. These anatomical terms denote potentially homologous pneumatic structures within Saurischia, but they could be applied to any vertebrate with vertebral laminae that enclose spaces, regardless of their developmental origin or phylogenetic distribution.

The titanosaur (Dinosauria: Sauropoda) osteoderm record: review and first definitive specimen from India

ABSTRACT Titanosaurs are the only sauropods that possessed osteoderms, although the phylogenetic distribution of this feature within Titanosauria is not yet resolved. Whereas the majority of titanosaur osteoderms are from South America, several have been found in Cretaceous rocks in Africa, Madagascar, and Europe. Here we describe a titanosaur osteoderm from the Maastrichtian of India that extends the geographic range of these armored sauropods. The known spatiotemporal distribution of titanosaurs is broader than the distribution of titanosaur osteoderms. This discrepancy can be explained by one or more of several factors: (1) few titanosaur genera had osteoderms; (2) titanosaurs were not heavily armored; (3) there are strong collection and/or (4) taphonomic biases against these elements. Almost 90 individual titanosaur osteoderms have been reported and can be assigned to ten of the 40+ currently recognized titanosaur genera. Although they are not divisible into discrete size classes, titanosaur osteoderms are here shown to fall into four morphotypes: “ellipsoid,” “keeled,” “cylindrical,” and “mosaic.” No morphotype is unique to any one taxonomic group, geographic area, or time period. Despite the relative scarcity of osteoderms, it appears that armored titanosaurs are relatively diminutive in body size, with average femoral and humeral lengths of about 60 and 70% those of unarmored taxa, respectively. An apparent exception to this relationship is the titanosaur osteoderm from India, which likely pertains to one of the two relatively large-bodied titanosaur species that are known from the Lameta Formation.

New remains attributable to the holotype of the sauropod dinosaur Neuquensaurus australis, with implications for saltasaurine systematics

The Late Cretaceous South American sauropods Neuquensaurus australis and Saltasaurus loricatus are represented by well-preserved and abundant material that has been integral to our understanding of titanosaur anatomy for decades. Although the hypodigms for these species span most of the skeleton, holotypic materials are limited to a few bones that do not overlap between the two taxa. In this contribution, we augment the holotype of Neuquensaurus australis with a partial sacrum that was preserved in articulation with one of the caudal vertebrae from its original description, but not recognised as such at the time. We document this field association via the presence of a broken piece of matrix on the sixth sacral vertebral centrum that has a snap-fit to matrix on the rim of the anterior condyle of the holotypic biconvex vertebra. Based on comparisons with a more complete sacrum and ilium of a referred specimen of Neuquensaurus australis, we interpret this biconvex vertebra to be the seventh sacral vertebra. This raises the possibility that the biconvex “first caudal” vertebra of some other titanosaurs may be part of the sacrum as well. Augmentation of the Neuquensaurus australis holotype to include a sacrum makes it directly comparable to the holotype of Saltasaurus loricatus. Morphological differences in the number, shape, and proportion of sacral vertebrae allow discrimination between Neuquensaurus and Saltasaurus, confirming their generic separation. The El Brete quarry, which preserves the holotypic sacrum and abundant referred specimens of Saltasaurus loricatus, also preserves a sacrum consisting of seven vertebrae that bears autapomorphies of Neuquensaurus australis, indicating that these two saltasaurines coexisted.

A Sauropod Dinosaur Pes from the Latest Cretaceous of North America and the Validity of Alamosaurus sanjuanensis (Sauropoda, Titanosauria)

ABSTRACT Complete sauropod pedes are rare in the fossil record, which has limited their use in systematics. We describe a nearly complete, large sauropod pes from the Maastrichtian-age Naashoibito Member of the Kirtland Formation of New Mexico, U.S.A., that bears synapomorphies of some eusauropod clades, such as the presence of metatarsal I with a wide shaft and laterally deflected pedal unguals. Novel pedal characters presented herein, such as the presence of an embayment on the proximomedial corner of metatarsal IV, suggest that the Naashoibito specimen likely belongs to a titanosauriform. Based on its provenance, the Naashoibito specimen likely belongs to the derived titanosaur Alamosaurus sanjuanensis, which is the only recognized Late Cretaceous titanosaur in North America. However, formal referral to Alamosaurus awaits discovery of overlapping materials with the holotype or definitively referred remains. The holotypic scapula of Alamosaurus sanjuanensis is diagnostic, providing a basis for referral of some other Maastrichtian North American titanosaur specimens to the genus. Confirmation of these referrals and the description of the pes presented herein augment the data relevant to the systematic problems that have historically surrounded Alamosaurus.