The Sauropods: Evolution and Paleobiology

3 • Phylogenetic and Taxic Perspectives on Sauropod Diversity

104 he sauropoda represent one of the most diverse and geographically widespread dinosaurian radiations. After their origin during the Late Triassic, sauropods increased rapidly in diversity, and acquired a nearly global distribution by the Middle Jurassic. Diversity appears to peak in the Late Jurassic, at which point sauropods were very abundant and dominated many terrestrial environments . Although sauropod diversity apparently declined very rapidly in the Early Cretaceous , several lineages remained as a significant component in many faunas, and the titanosaurs underwent a major radiation during the mid- or Late Cretaceous. These changes in diversity took place against a backdrop of profound geological and biotic events. The gradual fragmentation of Pangaea affected physical environments by altering sea levels and climatic regimes, which in turn could have had an impact on sauropod diversity. In addition, evolutionary interactions with predators, competing large-bodied herbivores , and plants may also have affected sauropod diversity. Before we can attempt to tease apart the effects of these physical and biological factors, the basic pattern of sauropod diversity must be established accurately. No summary of sauropod evolutionary history and palaeobiology is complete without a consideration of diversity (e.g., how the number of sauropod taxa fluctuated through time). Yet this aspect of sauropod history has been relatively neglected. Most studies have been based on direct observation of numbers of sauropod species and/or genera in the fossil record. Although this approach has the potential to provide an accurate measure of diversity, it can be seriously affected by non-random sampling biases. In principle, such problems can be partially corrected when the phylogenetic relationships of taxa are taken into account (Smith 1994), but to date only one study (Weishampel and Jianu 2000) has applied this method to sauropod diversity. This neglect is not surprising given the confused nature of much of sauropod species- and genus-level systematics and the lack (until recently) of detailed cladistic analyses for this group. Fortunately, several recent studies (Upchurch 1995, 1998, 1999; Wilson and Sereno 1998; Curry Rogers and Forster 2001; Wilson 2002; Upchurch et al., three Phylogenetic and Taxic Perspectives on Sauropod Diversity Paul Upchurch and Paul M. Barrett T P H Y L O G E N E T I C A N D TA X I C P E R S P E C T I V E S O N S A U R O P O D D I V E R S I T Y 105 2004) have gone a long way toward removing the obstacles to detailed analysis of sauropod diversification patterns. Thus, the main aims of the current chapter are (1) to present and compare alternative estimates of sauropod lineage diversity and (2) to evaluate the implications of these diversity patterns for our understanding of sauropod evolutionary history and sampling biases. The reader is also referred to the following companion chapter by Barrett and Upchurch, which examines the relative diversity of different sauropodomorph clades and considers the possible relevance of morphological characters related to feeding strategy. PREVIOUS WORK Few studies have examined sauropod diversity in detail. This topic has, however, been treated in a superficial fashion by several studies that have attempted either an “overview” of dinosaur evolutionary history (Bakker 1977, 1978; Sereno 1997, 1999) or an assessment of the factors controlling dinosaur diversity (Horner 1983; Weishampel and Horner 1987; Haubold 1990; Barrett and Willis 2001). All of these studies, as well as the detailed treatment of sauropod diversity presented by Hunt et al. (1994), are based on the taxic approach to diversity estimation, in which diversity is measured by counting the number of taxa present in the fossil record at each point in time. The resulting “consensus ” is that sauropods gradually increased in diversity throughout the Jurassic, reaching a peak in the Kimmeridgian–Tithonian. At the Jurassic– Cretaceous boundary, however, sauropods seem to undergo a major extinction event: although many of the familial lineages survive into the Early Cretaceous, the overall level of genus or species diversity is drastically reduced (Bakker 1978). Sauropods maintained a lowered level of diversity until the end of the Cretaceous, at which point the number of genera increased as a result of the titanosaur radiation. In many of these studies , fluctuations in the numbers of sauropod genera or species are accepted as genuine changes in diversity, even though it is possible that such variations actually reflect changes in the quality of the fossil record. The larger data sets employed by Hunt et al. (1994) and Barrett and Willis (2001) enabled...