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331 Origin and Early Evolution of Dinosaurs Michael J. Benton 18 The dinosaurs arose in the Triassic, and probably during the Early to Middle Triassic. They entered a world far different from the typical Age of Dinosaurs scenes, a world in which the dominant herbivores were synapsids (dicynodonts and chiniquodontids) and rhynchosaurs, and carnivores were cynodonts and basal archosaurs of various kinds, previously called thecodontians (see Chapter 17, this volume). Into this world came the dinosaurs, initially small bipedal carnivores. They rose to dominance at some point during the second half of the Triassic. Certainly, by the end of the Triassic Period, dinosaurs were abundant and reasonably diverse, and all the major lineages had emerged and diversified. Since 1980, paleontologists’ views have changed dramatically, and new specimens and new methods have revolutionized our understanding of the origin of the dinosaurs. Most important has been the widespread use of cladistics as the key tool in disentangling the tree of life (see Chapter 11, this volume). Second, new, high-precision methods of dating the rocks give a much firmer timescale of events. There are still debates, however, about the relative importance of different groups of land vertebrates through the Triassic and the kinds of ecological processes that might have been involved in the rise and initial expansion of the clade Dinosauria. Three key topics will be explored here: phylogeny (defining what is a dinosaur, spurious early records, the first dinosaurs), geology (dating the rocks), and models (how evolutionary radiations happen). Richard Owen assumed in 1842 that his new group, the Dinosauria, was a real group, a monophyletic group, or a clade, in modern parlance (see Chapter 2, this volume). In other words, he assumed that the Dinosauria had a single ancestor, and that the group included all the descendants of that ancestor. This view was commonly held during most of the nineteenth century, but it was shaken by Harry Seeley’s demonstration in 1887 that there were two major dinosaur groups, the Saurischia and Ornithischia, distinguished by the nature of their pelvic arrangements (see Chapter 3, this volume). Perhaps, thought Seeley, the Saurischia and Ornithischia were distinct evolutionary branches that had arisen from separate ancestors. Seeley was merely espousing a commonly held view at the time called the persistence of types, a view promoted especially by Thomas Henry Huxley. The idea was that large changes in form could not happen readily in evolution, and the fossil record showed how major groups retained their main characteristics Definition of the Dinosauria Michael J. Benton 332 for long periods of time. Hence, paleontologists had to look for very long periods of initial evolution that led up to each major group, and often these initial spans of evolution were missing from the fossil record. Seeley’s view dominated during most of the twentieth century, and many dinosaur paleontologists made it even more complex. Not only had the Saurischia and Ornithischia evolved from separate ancestors, but so too had some of the subdivisions within those two groups, probably the two saurischian groups, the Theropoda and Sauropodomorpha, and possibly even some of the main ornithischian groups known in 1900, the Ornithopoda, Ceratopsia, Stegosauria, and Ankylosauria. In the end, the dinosaurs became merely an assemblage of large extinct reptiles of the Mesozoic that shared little in common. Hence, dinosaurs were seen as a polyphyletic group, deriving from two, three, or more sources among the basal archosaurs (reviewed in Benton 2004). The collapse of the polyphyletic view came quickly and dramatically in about 1984 (Fig. 18.1). This had been presaged in short papers by Bakker and Galton (1974) and Bonaparte (1976), who saw many unique characters shared by both saurischian and ornithischian dinosaurs. Some brief papers published in 1984 were followed by more substantial accounts (Gauthier 1986; Sereno 1986; Novas 1989, 1994; Benton 1990); all applied strictly cladistic approaches to the data, and they independently agreed strongly that the Dinosauria of Owen (1842) is a monophyletic group, defined by a number of synapomorphies, including the following:·  Possible absence of a postfrontal bone (Fig. 18.2B).·  Jugal branching backward into two distinct processes to contact the quadratojugal (Fig. 18.2B).·  Temporal musculature extending anteriorly onto the skull roof, marked by a distinct fossa on the frontal bone.·  Epipophyses (additional facets on the postzygapophyses) on the cervical vertebrae.·  Elongate deltopectoral crest on the humerus (Fig. 18.2A).·  Possible three or fewer phalanges in the fourth finger of the hand (Fig. 18.2C).·  Fully open acetabulum (Figs. 18.2A, 18.3A).·  Possible...

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