Cretaceous diplodocids in Asia? Re‐evaluating the phylogenetic affinities of a fragmentary specimen

Abstract

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 ...

Osteology of Huabeisaurus allocotus (Sauropoda: Titanosauriformes) from the Upper Cretaceous of China

“…13). In most titanosauriforms, the POCDF is deeper and better developed than the POSDF (e.g., Mendozasaurus [50]), but in H. allocotus the reverse is true, representing an autapomorphy. The postzygapophyseal articular surfaces are large, flat, lack hyposphenes, and face ventrolaterally.…”

A Diplodocid Sauropod Survivor from the Early Cretaceous of South America

“…Tentative reports from Asia [34] were later suggested to be non-diplodocid [12]. For Gondwanan landmasses, only the Upper Jurassic of Tendaguru beds of Tanzania produced remains of this family, known today as Tornieria africana [35].…”

The early evolution of titanosauriform sauropod dinosaurs

“…However, the addition of new taxa to analyses has outpaced the addition of characters and thus outpaced discovery of stable (i.e. repeatedly recovered) synapomorphies (Whitlock et al ., 2011). Many of the analyses aimed at resolving basal titanosauriform relationships have included a substantial number of characters that were parsimony‐uninformative (Table 2), or informative only to the relationships of non‐titanosauriforms such as diplodocoids.…”

“…For example, East Asian endemism has been suggested for sauropods during various time periods, from the Middle Jurassic to the Cretaceous (Russell, 1993; Upchurch, Hunn & Norman, 2002; Wilson & Upchurch, 2009). Recently, all Cretaceous East Asian sauropods have been recognized as titanosauriforms, in contrast to an array of non‐neosauropods that characterized the Jurassic of Asia (Wilson, 2005; Whitlock, D'Emic & Wilson, 2011). Resolving the role of endemism and the details of this faunal turnover for the sauropods of East Asia also depends on their lower‐level phylogenetic relationships.…”