Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs

Abstract

The rise and diversification of the dinosaurs in the Late Triassic, from 230 to 200 million years ago, is a classic example of an evolutionary radiation with supposed competitive replacement. A comparison of evolutionary rates and morphological disparity of basal dinosaurs and their chief "competitors," the crurotarsan archosaurs, shows that dinosaurs exhibited lower disparity and an indistinguishable rate of character evolution. The radiation of Triassic archosaurs as a whole is characterized by declining evolutionary rates and increasing disparity, suggesting a decoupling of character evolution from body plan variety. The results strongly suggest that historical contingency, rather than prolonged competition or general "superiority," was the primary factor in the rise of dinosaurs.

Dinosaur morphological diversity and the end-Cretaceous extinction

“…Dinosaur disparity and diversity fluctuated throughout the Mesozoic 11,12,15,36,37 , and small increases or decreases between two or three time intervals (such as the late Campanian and Maastrichtian) may not be noteworthy within the context of the entire . The late Campanian-maastrichtian disparity decrease for global ceratopsids, global hadrosauroids, and north American hadrosauroids is found whenever sample sizes are equalized, down to a common size of two taxa in both time bins.…”

“…Unfortunately, records of dinosaur disparity across the entire Mesozoic are not yet available for comparison, and are difficult to compile because of low sample sizes in many stage-level intervals, but the establishment of these patterns should be a major goal of research on dinosaur biodiversity. Furthermore, there are known instances in the fossil record in which major vertebrate clades endured catastrophic diversity and disparity losses, both during mass extinctions and normal background times, but later rebounded [36][37][38][39][40] . A satisfactory understanding of the causes and tempo of the non-avian dinosaur extinction remains difficult, but the compilation of additional biodiversity metrics (such as disparity) and increased study of non-North American taxa is helping to bring clarity.…”

A Middle Jurassic abelisaurid from Patagonia and the early diversification of theropod dinosaurs

“…Together with the wide array of basal tetanuran taxa known from the Middle Jurassic [19] and the growing recognition that most, if not all coelurosaurian lineages also reach back to this time [20 -23], this underlines the rapid and apparently explosive radiation of theropod dinosaurs in the late Early to early Middle Jurassic. Whereas current research on the origin and success of dinosaurs mainly focuses on the Triassic -Jurassic boundary [24,25], these findings lend support to the hypothesis that the rise of dinosaurs was a two-step process, marked by extinctions of basal archosaurs at the Triassic -Jurassic boundary and of several basal dinosaur lineages in the Pliensbachian -Toarcian, followed by an explosive radiation of 'modern' clades [26,27]. New discoveries and more research into this topic are needed to establish the timing and patterns of early diversification of dinosaurs in the Jurassic.…”

The origin and early radiation of dinosaurs

“…During the Late Triassic, archosaurs of both major subgroups were exceptionally abundant in ecosystems across the globe. This period of time, from approximately 235-201 million years ago, witnessed the evolution of several morphologically distinctive archosaur clades that filled a variety of ecological roles (Nesbitt, 2007;Brusatte et al, 2008a;Nesbitt et al, 2010). Most of these groups, such as the long-snouted and semi-aquatic phytosaurs, the heavily armored aetosaurs, the sleek and predatory ornithosuchids, and the predatory and omnivorous "rauisuchians," became extinct by the end of the Triassic.…”