Titanosauriformes was a globally distributed, long‐lived clade of dinosaurs that contains both the largest and smallest known sauropods. These common and diverse megaherbivores evolved a suite of cranial and locomotory specializations perhaps related to their near‐ubiquity in Mesozoic ecosystems. In an effort to understand the phylogenetic relationships of their early (Late Jurassic–Early Cretaceous) members, this paper presents a lower‐level cladistic analysis of basal titanosauriforms in which 25 ingroup and three outgroup taxa were scored for 119 characters. Analysis of these characters resulted in the recovery of three main clades: Brachiosauridae, a cosmopolitan mix of Late Jurassic and Early Cretaceous sauropods, Euhelopodidae, a clade of mid‐Cretaceous East Asian sauropods, and Titanosauria, a large Cretaceous clade made up of mostly Gondwanan genera. Several putative brachiosaurids were instead found to represent non‐titanosauriforms or more derived taxa, and no support for a Laurasia‐wide clade of titanosauriforms was found. This analysis establishes robust synapomorphies for many titanosauriform subclades. A re‐evaluation of the phylogenetic affinities of fragmentary taxa based on these synapomorphies found no body fossil evidence for titanosaurs before the middle Cretaceous (Aptian), in contrast to previous reports of Middle and Late Jurassic forms. Purported titanosaur track‐ways from the Middle Jurassic either indicate a substantial ghost lineage for the group or – more likely – represent non‐titanosaurs. Titanosauriform palaeobiogeographical history is the result of several factors including differential extinction and dispersal. This study provides a foundation for future study of basal titanosauriform phylogeny and the origins of Titanosauria. © 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2012, 166, 624–671.

BackgroundThe 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 FindingsWe 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/SignificanceThe 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.

BackgroundTooth replacement rate can be calculated in extinct animals by counting incremental lines of deposition in tooth dentin. Calculating this rate in several taxa allows for the study of the evolution of tooth replacement rate. Sauropod dinosaurs, the largest terrestrial animals that ever evolved, exhibited a diversity of tooth sizes and shapes, but little is known about their tooth replacement rates.Methodology/Principal FindingsWe present tooth replacement rate, formation time, crown volume, total dentition volume, and enamel thickness for two coexisting but distantly related and morphologically disparate sauropod dinosaurs Camarasaurus and Diplodocus. Individual tooth formation time was determined by counting daily incremental lines in dentin. Tooth replacement rate is calculated as the difference between the number of days recorded in successive replacement teeth. Each tooth family in Camarasaurus has a maximum of three replacement teeth, whereas each Diplodocus tooth family has up to five. Tooth formation times are about 1.7 times longer in Camarasaurus than in Diplodocus (315 vs. 185 days). Average tooth replacement rate in Camarasaurus is about one tooth every 62 days versus about one tooth every 35 days in Diplodocus. Despite slower tooth replacement rates in Camarasaurus, the volumetric rate of Camarasaurus tooth replacement is 10 times faster than in Diplodocus because of its substantially greater tooth volumes. A novel method to estimate replacement rate was developed and applied to several other sauropodomorphs that we were not able to thin section.Conclusions/SignificanceDifferences in tooth replacement rate among sauropodomorphs likely reflect disparate feeding strategies and/or food choices, which would have facilitated the coexistence of these gigantic herbivores in one ecosystem. Early neosauropods are characterized by high tooth replacement rates (despite their large tooth size), and derived titanosaurs and diplodocoids independently evolved the highest known tooth replacement rates among archosaurs.

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BackgroundThe Late Cretaceous titanosauriform sauropod Huabeisaurus allocotus Pang and Cheng is known from teeth and much of the postcranial skeleton. Its completeness makes it an important taxon for integrating and interpreting anatomical observations from more fragmentary Cretaceous East Asian sauropods and for understanding titanosauriform evolution in general.Methodology/Principal FindingsWe present a detailed redescription of Huabeisaurus allocotus and a suite of anatomical comparisons with other titanosauriforms that demonstrate its validity via autapomorphies (e.g., division of some presacral vertebral laminae, reduced development of caudal ribs, the development of fossae relative to one another in caudal vertebral neural arches, high tibia-to-femur ratio). Huabeisaurus shares many features with other Cretaceous East Asian sauropods (e.g., pendant cervical ribs, anterior-middle caudal vertebrae with a nearly flat anterior centrum face and a concave posterior centrum face) that are absent in sauropods from other landmasses and strata, suggesting a close relationship among many of these forms within the clade Somphospondyli.Conclusions/SignificanceRestudy of Huabeisaurus provides further evidence for the existence of a clade of somphospondylans – Euhelopodidae – mainly found in the Cretaceous of East Asia. Euhelopodidae represents a fourth example of the evolution of narrow crowns within Sauropoda, along with diplodocoids, brachiosaurids, and advanced titanosaurs (lithostrotians). Despite being known from fewer species than Diplodocoidea, Brachiosauridae, or Lithostrotia, euhelopodids possessed a broader range of tooth shapes than any of these clades, suggesting that euhelopodids exemplified a comparably broad range of feeding strategies and perhaps diets.

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The recent description of an anterior caudal vertebra purportedly belonging to a diplodocid sauropod from the Early Cretaceous of China has the potential to drastically alter our interpretation of the evolution and timing of geographical dispersal of a major dinosaur lineage. However, comparison with a wider taxonomic sample points more strongly towards titanosauriform affinities for this specimen, which is in keeping with the affinities of all other sauropods known from the Cretaceous of Asia. We explain the disparity in phylogenetic interpretation of this isolated vertebra as a by-product of scoring differences and analysis of fragmentary material using repurposed data matrices. Rescoring the isolated vertebra based on our interpretation of the anatomy and rerunning the original analyses removes the specimen from Diplodocoidea but does not place it within Titanosauriformes, because of inadequacy in taxon and character sampling inherited from the repurposed data matrices. We suggest that phylogenetic analysis must begin with an initial hypothesis of affinity, based on comparative anatomy and spatiotemporal distributions, that must be adequately tested by the data matrix employed -i.e. data matrices should be tailored to sample anatomically, geographically and temporally relevant clades, and new characters should be added in tandem with new taxa so that the potential synapomorphy pool is not diluted. This is especially important for analyses of fragmentary specimens, which are likely to return coarse phylogenetic results with general evolutionary and palaeobiogeographical implications.Key words: Dinosauria, Diplodocoidea, Titanosauriformes, Early Cretaceous, phylogenetic systematics, homoplasy, taxon sampling.R ecently, an isolated anterior caudal vertebra from the Early Cretaceous of China was described as the first diplodocid sauropod from Asia (Upchurch and Mannion 2009). This claim has dramatic consequences for dinosaur palaeobiogeography, because it would represent the first diplodocid recovered outside the Late Jurassic and the only Asian diplodocoid. Upchurch and Mannion (2009) defended this conclusion with congruent phylogenetic results derived from two independent, large-scale data matrices (Wilson 2002; Upchurch et al. 2004a).The dinosaur and mammal faunas of Asia have received a great deal of attention because of their somewhat peculiar character, which is thought to have resulted from an interval of geographical isolation (Russell 1993; Upchurch 1995; Buffetaut and Suteethorn 1999; Luo 1999; Barrett et al. 2002; Upchurch et al. 2002; Zhou et al. 2003). Although there is disagreement about the mechanism and duration of the physical isolation of East Asia from the rest of Pangaea, the pattern in sauropod dinosaurs is striking. All known Triassic and Jurassic Asian sauropods (18 genera) most likely fall outside the main radiation of Neosauropoda (but see Upchurch et al. 2004a), but all Cretaceous Asian sauropods (28 genera) are members of the derived neosauropod subgroup Titanosauriformes ...

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