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989 Early Mesozoic Continental Tetrapods and Faunal Changes Hans-Dieter Sues Most of the principal groups of present-day continental vertebrates, or their closest relatives, first appeared in the fossil record during the Triassic Period–mammals, turtles, archosaurian reptiles (crocodylians and the precursors of birds, dinosaurs), lepidosaurian reptiles (rhynchocephalians and squamates), frogs, and salamanders (because they are the sister-group of frogs). This chapter briefly reviews the distribution of the principal assemblages of Triassic and Early Jurassic continental tetrapods in time and space and discusses the profound changes that occurred among tetrapod communities during the early Mesozoic. For a comprehensive recent survey of Triassic life on land and the biotic changes during this period the reader is referred to Sues and Fraser (2010). During the Late Permian, the most diverse and abundant tetrapods on land were therapsids, but many members of this group vanished during the Late Permian or at the end of the Permian Period in what is generally considered the greatest episode of extinction in the history of life (Erwin 2006). At the beginning of the Triassic Period, a few remaining therapsid lineages (especially dicynodonts) still constituted the vast majority of terrestrial tetrapods although their taxonomic diversity was low. By the beginning of the Jurassic Period, dinosaurs had become the dominant group of large continental tetrapods (Olsen et al. 2002; Brusatte et al. 2010; Langer et al. 2010). Archosaurian reptiles rapidly diversified during the Triassic, while only a few groups of therapsids (including the precursors of mammals) persisted in moderate diversities. The possible causes for this major change in continental vertebrate communities are not yet fully understood, but the basic structure of land ecosystems established during the early Mesozoic has persisted to the present day. Subsequently, the principal change in continental vertebrate communities was the replacement of dinosaurs other than birds by mammals in many ecological roles at the beginning of the Cenozoic Era. However, one lineage of dinosaurs–birds–vastly outnumbers mammals in terms of species numbers even today. For the entire length of the Triassic, the continents formed a single vast landmass–the supercontinent Pangaea (Fig. 41.1). Apparently few, if any, significant barriers to dispersal of land-living animals across Pangaea existed. The breakup of the supercontinent began during the Triassic, but significant separation of landmasses did not occur until the Jurassic. Late Triassic and Early Jurassic assemblages of continental vertebrates share some taxa across Pangaea, but there are also differences in faunal composition. During the Triassic, there was a worldwide climatic trend toward warmer and increasingly drier conditions (Sellwood and Valdes 2006), which was, at least in part, driven by a northward drift of Pangaea through 41 Hans-Dieter Sues 990 some 10 degrees of latitude (Golonka 2007; Fig. 41.1). The sole exception to this trend appears to have been an interval of increased humidity during the Carnian (Simms and Ruffell 1989; Prochnow et al. 2006). The slow northward drift of Pangaea continued into the Jurassic Period so that, by the Early Jurassic, paleogeographic reconstructions place more than 50% of the exposed land surface of the supercontinent north of the paleoequator. Paleontologists have long attempted to correlate the various early Mesozoic tetrapod assemblages with the stages of the Triassic Period on the Standard Global Chronostratigraphic Scale (SGCS)–Induan and Olenekian (Early Triassic); Anisian and Ladinian (Middle Triassic); and Carnian, Norian, and Rhaetian (Late Triassic). Such efforts are fraught with difficulties because most of the SGCS stages are formally based on sequences of marine strata in the European Alps. Direct correlation between continental and marine sedimentary rocks has proven challenging because only in rare instances have fossils of continental tetrapods and plants been preserved in Triassic marine strata. Radiometric dates for volcanic rocks interbedded with sedimentary deposits and paleomagnetic data increasingly provide ways for reliable correlation of marine and continental strata. Lucas (1998) proposed a comprehensive scheme for the global correlation of Triassic continental strata based on their vertebrate assemblages. He defined a succession of eight “Land Vertebrate Faunachrons” (LVFs) during the Triassic Period. Each LVF is characterized by the first appearance in the fossil record of a particular tetrapod taxon. For example, Lucas defines the first (oldest) LVF, the Lootsbergian, based on the first appearance of the dicynodont therapsid Lystrosaurus. He augmented the definition of each LVF by adding other characteristic tetrapod taxa. Thus, the Lootsbergian LVF is also characterized by the presence of the procolophonid parareptile Procolophon, the cynodont therapsid Thrinaxodon, and the archosauriform reptile Proterosuchus. Lucas’s LVFs provide a temporal...

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