Jeffrey A. Wilson

Early Evolution and Higher-Level Phylogeny of Sauropod Dinosaurs

ABSTRACT Although sauropods played a major role in terrestrial ecosystems during much of the Mesozoic Era, little effort has been directed toward diagnosing Sauropoda and establishing higher-level interrelationships among sauropods. As a consequence, the origin and evolution of major skeletal adaptations in sauropods has remained largely speculative. The cladistic analysis presented here focuses on higher-level relationships among sauropods. Based on 109 characters (32 cranial, 24 axial, 53 appendicular) for 10 sauropod taxa, the most parsimonious arrangement places four genera (Vulcanodon, Shunosaurus, Barapasaurus, and Omeisaurus) as a sequence of sister-taxa to a group of advanced sauropods, defined here as Neosauropoda. Neosauropoda, in turn, is composed of the sister-clades Diplodocoidea and Macronaria; the latter is a new taxon that includes Haplocanthosaurus, Camarasaurus, and Titanosauriformes. Titanosauriformes includes Brachiosauridae and Somphospondyli, a new taxon uniting Euhelopus and Titanos...

Predatory Dinosaurs from the Sahara and Late Cretaceous Faunal Differentiation

Late Cretaceous (Cenomanian) fossils discovered in the Kem Kem region of Morocco include large predatory dinosaurs that inhabited Africa as it drifted into geographic isolation. One, represented by a skull approximately 1.6 meters in length, is an advanced allosauroid referable to the African genus Carcharodontosaurus. Another, represented by a partial skeleton with slender proportions, is a new basal coelurosaur closely resembling the Egyptian genus Bahariasaurus. Comparisons with Cretaceous theropods from other continents reveal a previously unrecognized global radiation of carcharodontosaurid predators. Substantial geographic differentiation of dinosaurian faunas in response to continental drift appears to have arisen abruptly at the beginning of the Late Cretaceous.

A nomenclature for vertebral laminae in sauropods and other saurischian dinosaurs

ABSTRACT The vertebrae of sauropods are characterized by numerous bony struts that connect the costovertebral and intervertebral articulations, centrum, and neural spine of the presacral, sacral, and anterior caudal vertebrae. A nomenclature for sauropod vertebral laminae is proposed that: 1) utilizes the morphological landmarks connected by the laminae (rather than their spatial orientation); and 2) provides the same name for serial homologues. This landmark-based nomenclature for vertebral laminae, which establishes the first criterion of homology (similarity), is the first step towards interpreting their phylogenetic significance. Nineteen different neural arch laminae are identified in sauropods, although all are never present in a single vertebra. Vertebral laminae can be divided into four regional categories, with each distinct lamina abbreviated with a simple four-letter acronym: diapophyseal laminae; parapophyseal laminae; zygapophyseal laminae; and spinal laminae. The distribution of neural arch ...

New dinosaurs link southern landmasses in the Mid–Cretaceous

Abelisauroid predators have been recorded almost exclusively from South America, India and Madagascar, a distribution thought to document persistent land connections exclusive of Africa. Here, we report fossils from three stratigraphic levels in the Cretaceous of Niger that provide definitive evidence that abelisauroid dinosaurs and their immediate antecedents were also present on Africa. The fossils include an immediate abelisauroid antecedent of Early Cretaceous age (ca. 130–110 Myr ago), early members of the two abelisauroid subgroups (Noasauridae, Abelisauridae) of Mid–Cretaceous age (ca. 110 Myr ago) and a hornless abelisaurid skull of early Late Cretaceous age (ca. 95 Myr ago). Together, these fossils fill in the early history of the abelisauroid radiation and provide key evidence for continued faunal exchange among Gondwanan landmasses until the end of the Early Cretaceous (ca. 100 Myr ago).

A revision of titanosaurus lydekker (dinosauria - sauropoda), the first dinosaur genus with a ‘gondwanan’ distribution

Synopsis Titanosaurs represent approximately one‐third of sauropod diversity and were geographically widespread throughout the Cretaceous, especially on southern continents. Titanosaurs evolved numerous appendicular synapomorphies that account for their specialised ‘wide‐gauge’ limb posture, which can be recognised in their trackways. The macronarian origin of titanosaurs is only recently agreed upon and aspects of their inter‐relationships remain poorly understood. Titanosauria is named for the poorly known genus Titanosaurus, which was coined by Lydekker in 1877 on the basis of a partial femur and two incomplete caudal vertebrae. Fourteen species have since been referred to Titanosaurus, which distribute the genus across Argentina, Europe, Madagascar, India and Laos, and throughout 60 million years of the Cretaceous. Despite its centrality to titanosaur systematics and biogeography, the genus Titanosaurus has never been revised. A re‐evaluation of all Titanosaurus species recognises as diagnostic only five. The type species T. indicus is invalid because it is based on ‘obsolescent’ characters ‐ once diagnostic features that have gained a broadertaxonomic distribution over time. Consequently, the genus Titanosaurus and its co‐ordinated rank‐taxa (e.g. Titanosaurinae, Titanosauridae, Titanosauroidea) must be abandoned. The unranked taxon Titanosauria, however, remains valid. A new phylogenetic taxonomy is proposed for Titanosauria that utilises nodes that have been judged stable by the most recent cladistic analyses. The early appearance of titanosaur ichnofossils (Middle Jurassic) and body fossils (Late Jurassic) precludes a vicariant origin for the group, but such a pattern cannot yet be ruled out for lower‐level taxa within Titanosauria.

Early Cretaceous Dinosaurs from the Sahara

A major question in Mesozoic biogeography is how the land-based dinosaurian radiation responded to fragmentation of Pangaea. A rich fossil record has been uncovered on northern continents that spans the Cretaceous, when continental isolation reached its peak. In contrast, dinosaur remains on southern continents are scarce. The discovery of dinosaurian skeletons from Lower Cretaceous beds in the southern Sahara shows that several lineages of tetanuran theropods and broad-toothed sauropods had a cosmopolitan distribution across Pangaea before the onset of continental fragmentation. The distinct dinosaurian faunas of Africa, South America, and Asiamerica arose during the Cretaceous by differential survival of once widespread lineages on land masses that were becoming increasingly isolated from one another.

Redescription and reassessment of the phylogenetic affinities of euhelopus zdanskyi (Dinosauria: Sauropoda) from the early cretaceous of China

Synopsis Euhelopus zdanskyi was the first dinosaur described from China. Both traditional and modern cladistic assessments have found support for an endemic clade of Chinese sauropods (Eu‐helopodidae) that originated during an interval of geographic isolation, but the monophyly of this clade has remained controversial. The phylogenetic affinity of the eponymous genus Euhelopus is central to this controversy, yet its anatomy has not been completely restudied since the original German‐language monograph in 1929. We jointly re‐examined the cranial and postcranial anatomy of the holotypic and referred materials of Euhelopus to provide a new diagnosis for the genus and to explore its phylogenetic affinities. Diagnostic features of Euhelopus include: postaxial cervical vertebrae that have variably developed epipophyses and more subtle “pre‐epipopophyses” below the prezygapophyses; cervical neural arches with an epipophyseal‐prezygapophyseal lamina separating two pneumatocoels; anterior cervical vertebrae with three costal spurs on the tuberculum and capitulum; divided middle presacral neural spines, which in anterior dorsal vertebrae bear a median tubercle that is as large or larger than the metapophyses; middle and posterior dorsal parapophyseal and diapophyseal laminae arranged in a “K” configuration; and presacral pneumaticity that extends into the ilium. Following this morphological study, we rescored Euhelopus for the two most comprehensive sauropod data matrices (Wilson 2002; Upchurch etal. 2004a), which previously yielded vastly different hypotheses for its relationships. Both matrices decisively demonstrate that Euhelopus is closely related to Titanosauria; traditional and cladistic claims that Euhelopus, Omeisaurus, Mamenchisaurus and Shunosaurus formed a monophyletic “Euhelopodidae” endemic to East Asia are not supported. These results suggest that there were at least two clades of very long‐necked sauropods in East Asia, occurring in the Middle Jurassic (i.e. Omeisaurus + Mamenchisaurus) and Early Cretaceous (e.g. Euhelopus, Erketu), with the latter group perhaps also occurring in Europe (Canudo et al. 2002). It is probable that the Euhelopus + Erketu lineage invaded East Asia from another part of Pangaea when isolation ended in the Early Cretaceous. The large number of basal titanosauriforms from East Asia has been interpreted to mean that this area may represent their centre of origin (You et al. 2003), but the titanosaur fossil record and phylogenetic studies indicate that the group probably originated prior to the Middle Jurassic and acquired a virtually global distribution before Pangaean fragmentation.

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.

Evidence for Avian Intrathoracic Air Sacs in a New Predatory Dinosaur from Argentina

Background Living birds possess a unique heterogeneous pulmonary system composed of a rigid, dorsally-anchored lung and several compliant air sacs that operate as bellows, driving inspired air through the lung. Evidence from the fossil record for the origin and evolution of this system is extremely limited, because lungs do not fossilize and because the bellow-like air sacs in living birds only rarely penetrate (pneumatize) skeletal bone and thus leave a record of their presence. Methodology/Principal Findings We describe a new predatory dinosaur from Upper Cretaceous rocks in Argentina, Aerosteon riocoloradensis gen. et sp. nov., that exhibits extreme pneumatization of skeletal bone, including pneumatic hollowing of the furcula and ilium. In living birds, these two bones are pneumatized by diverticulae of air sacs (clavicular, abdominal) that are involved in pulmonary ventilation. We also describe several pneumatized gastralia (“stomach ribs”), which suggest that diverticulae of the air sac system were present in surface tissues of the thorax. Conclusions/Significance We present a four-phase model for the evolution of avian air sacs and costosternal-driven lung ventilation based on the known fossil record of theropod dinosaurs and osteological correlates in extant birds: (1) Phase I—Elaboration of paraxial cervical air sacs in basal theropods no later than the earliest Late Triassic. (2) Phase II—Differentiation of avian ventilatory air sacs, including both cranial (clavicular air sac) and caudal (abdominal air sac) divisions, in basal tetanurans during the Jurassic. A heterogeneous respiratory tract with compliant air sacs, in turn, suggests the presence of rigid, dorsally attached lungs with flow-through ventilation. (3) Phase III—Evolution of a primitive costosternal pump in maniraptoriform theropods before the close of the Jurassic. (4) Phase IV—Evolution of an advanced costosternal pump in maniraptoran theropods before the close of the Jurassic. In addition, we conclude: (5) The advent of avian unidirectional lung ventilation is not possible to pinpoint, as osteological correlates have yet to be identified for uni- or bidirectional lung ventilation. (6) The origin and evolution of avian air sacs may have been driven by one or more of the following three factors: flow-through lung ventilation, locomotory balance, and/or thermal regulation.

Taxon distributions and the tetrapod track record

Abstract Vertebrate tracks are a unique, abundant source of fossil data that supplements the skeletal record in many ways. However, the utility of ichnofossil data depends on how specifically the authors of tracks can be identified. Despite this fact, there is little consensus about how to identify potential trackmakers, and existing methods differ in their bases, assumptions, and corresponding implications. In this paper we support the proposal that trackmakers should be identified primarily by skeletal structures that are both preserved in the ichnofossils and synapomorphies of some body-fossil clade. This synapomorphy-based technique enables certain taxa to be positively identified as candidate trackmakers and others to be excluded from consideration. In addition, the diagnostic level of the synapomorphy (i.e., to a higher or lower level) corresponds to that of the trackmaker. Additional features, such as body size and provenance, can be used in association with synapomorphies as additional differentiae of trackmaker identity. Trackway analyses are dependent on the level of trackmaker diagnosis, but not all analyses require the same diagnostic specificity. Palichnostratigraphic correlations to the stage level are shown to require at least a genus-level identification of a trackmaker, whereas studies of vertebrate distributions (i.e., origins, extinctions, ranges) accommodate much coarser designations. Anachronistic occurrences of trace and body fossils result in range extensions for either the skeletal taxon or the feature in question. For example, the temporal distribution of theropods can be extended on the basis of the footprint record, resulting in an earlier estimated divergence time for Dinosauria.