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auropodomorph dinosaurs were incredibly successful animals, by any standard. They attained high levels of alpha-taxonomic diversity, with more than 100 valid genera known currently (Galton 1990; McIntosh 1990; Galton and Upchurch 2004; Upchurch et al. 2004) and were the dominant animals, in terms of numerical abundance and biomass, in many Middle and Late Mesozoic terrestrial biomes (e.g., Young 1951; Dodson et al. 1980; Russell et al. 1980; Galton 1986; Dong 1992). Phylogenetic evidence, and the known stratigraphic distributions of these animals, suggests that the Sauropodomorpha originated in the early Late Triassic, though there are also recent reports of Middle Triassic prosauropod material (Flynn et al. 1999). Sauropodomorph monophyly is well established (Charig et al. 1965; Gauthier 1986; Benton 1990; Sereno 1997, 1999; Galton and Upchurch 2004), and early in its history the clade split into two major lineages: the Prosauropoda and Sauropoda. Although some authors regard the prosauropods as paraphyletic to the Sauropoda (Gauthier 1986; Benton 1990; Yates 2003), the current consensus is that both groups are monophyletic (Galton 1990; Gauffre 1993; Upchurch 1995, 1998; Sereno 1997, 1999; Wilson and Sereno 1998; Benton et al. 2000; Pisani et al. 2002; Galton and Upchurch 2004; Upchurch et al. 2004). Prosauropods attained a global distribution during the Late Triassic and Early Jurassic, before disappearing close to the Early–Middle Jurassic boundary (Galton 1990). Sauropods first appeared in the Late Triassic (Buffetaut et al. 2000) and achieved a global distribution by the Middle Jurassic, persisting thereafter on almost all of the major continental landmasses until the latest Cretaceous (McIntosh 1990; Weishampel 1990). One of the reasons for this success may be the early adoption of facultative or obligate herbivory in the Sauropodomorpha. In analyses on living amniotes, Moore and Brooks (1995, 1996) demonstrated an evolutionary association between the origin of herbivory and amniote diversification by showing that clades of extant herbivorous amniotes (principally mammals) are, on average, 16.8 times more speciose than their respective carnivorous sister-groups. Comparisons between taxic diversity estimates 125 FOUR Sauropodomorph Diversity through Time PALEOECOLOGICAL AND MACROEVOLUTIONARY IMPLICATIONS Paul M. Barrett and Paul Upchurch S for herbivorous and carnivorous dinosaurs throughout the Mesozoic have yielded similar results, with herbivores being between 3 and 10 times more diverse than carnivores at any one time (Barrett 1998). Such analyses, and the apparent rapid diversification of amniotes after the origin of herbivory in this clade during the Late Carboniferous, provide support for the suggestion that herbivory can be regarded as a “major adaptive zone” or a “key innovation” (e.g., Moore and Brooks 1995, 1996; Hotton et al. 1997; Reisz and Sues 2000). Adaptive zones represent particular discrete ways of life that, although they may be difficult to enter evolutionarily , allow the diversification of a group once the zone has been entered, whereas key innovations represent the acquisition of a particular character state or states that promote the radiation of a clade (e.g., Brooks and McLennan 1991). Prosauropod feeding mechanisms appear to have been relatively uniform, with little variation in either the craniodental or the postcranial characters involved in the collection and processing of plant food (Galton 1985, 1986; Barrett 1998). Jaw actions were simple orthal movements ; there was little or no tooth–tooth contact; oral processing of food was probably negligible; and fermentative digestion in elongate digestive tracts and/or the mechanical action of gastric mills seem to have been the dominant methods for breaking down plant food (Galton 1986; Farlow 1987; Norman and Weishampel 1991; Barrett 2000). Some prosauropods may have been omnivorous, while others were facultative or obligate herbivores (Barrett 2000). The long necks, large body size, and bipedal gaits of these animals further suggest that some were capable of browsing up to 5 m above ground level (Barrett 1998). Until recently, sauropod feeding was also viewed as simple and stereotyped, with little variation in the feeding apparatus between taxa (e.g., Bakker 1971; Coombs 1975; Dodson 1990). However, detailed reconsideration of sauropod anatomy, phylogeny, and paleoecology has revealed a surprising diversity of form and function in the feeding mechanisms of these animals (Fiorillo 1991, 1998; Barrett and Upchurch 1994, 1995; Calvo 1994; Christiansen 1999, 2000; Upchurch and Barrett 2000). All sauropods appear to have been obligate herbivores, and they experimented with a wide range of tooth types, occlusal patterns, and jaw actions; in addition , many taxa had specialised craniodental structures for food gathering, such as the presence of tooth combs. Sauropod postcranial adaptations were likewise variable, including differences in body size, absolute and relative...

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