The origin and early radiation of dinosaurs

Abstract

Dinosaurs were remarkably successful during the Mesozoic and one subgroup, birds, remain an important component of modern ecosystems. Although the extinction of non-avian dinosaurs at the end of the Cretaceous has been the subject of intense debate, comparatively little attention has been given to the origin and early evolution of dinosaurs during the Late Triassic and Early Jurassic, one of the most important evolutionary radiations in earth history. Our understanding of this keystone event has dramatically changed over the past 25 years, thanks to an influx of new fossil discoveries, reinterpretations of long-ignored specimens, and quantitative macroevolutionary analyses that synthesize anatomical and geological data. Here we provide an overview of the first 50 million years of dinosaur history, with a focus on the large-scale patterns that characterize the ascent of dinosaurs from a small, almost marginal group of reptiles in the Late Triassic to the preeminent terrestrial vertebrates of the Jurassic and Cretaceous. We provide both a biological and geological background for early dinosaur history. Dinosaurs are deeply nested among the archosaurian reptiles, diagnosed by only a small number of characters, and are subdivided into a number of major lineages. The first unequivocal dinosaurs are known from the late Carnian of South America, but the presence of their sister group in the Middle Triassic implies that dinosaurs possibly originated much earlier. The three major dinosaur lineages, theropods, sauropodomorphs, and ornithischians, are all known from the Triassic, when continents were joined into the supercontinent Pangaea and global climates were hot and arid. Although many researchers have long suggested that dinosaurs outcompeted other reptile groups during the Triassic, we argue that the ascent of dinosaurs was more of a matter of contingency and opportunism. Dinosaurs were overshadowed in most Late Triassic ecosystems by crocodile-line archosaurs and showed no signs of outcompeting their rivals. Instead, the rise of dinosaurs was a two-stage process, as dinosaurs expanded in taxonomic diversity, morphological disparity, and absolute faunal abundance only after the extinction of most crocodile-line reptiles and other groups.

Anomalously high variation in postnatal development is ancestral for dinosaurs but lost in birds

“…These silesaurids and early-diverging theropods phylogenetically bracket Sauropodomorpha and Ornithischia; therefore, similar variation in early-diverging members of these groups should also be present, although the early ornithischian record is exceptionally poor (29). The Triassic sauropodomorphs Thecodontosaurus antiquus and Melanorosaurus readi have been interpreted as possessing robust/gracile variation in similarly sized limb elements (18,45), but given our interpretation of a similar ostensible dichotomy in early theropods, these taxa probably possess similar levels of variation in growth patterns.…”

“…In contrast to most of the early dinosaurian record, which rarely preserves greater than a single individual per species (29,30), the theropod Coelophysis bauri is known from the most wellsampled growth series of any nonavian dinosaur preserved as a single population in the same horizon (31). We used this taxon as a lens to interpret development among early dinosaurs, with additional support from another early dinosaur with a well-sampled growth series, Megapnosaurus rhodesiensis (24).…”

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 origins of modern biodiversity on land

“…The apparently coupled rises in diversity and disparity of crurotarsans and dinosaurs in the Late Triassic, and the continuing broad morphological range of crurotarsans (figure 5d ) suggest that there was no insistent competition driving other groups to extinction but rather that the dinosaurs occupied new ecospace opportunistically, after it had been vacated. New evidence (Nesbitt et al 2010) suggests that dinosaurian sister groups originated much earlier, perhaps by 240 Ma, and that the Dinosauromorpha (¼ dinosaurs and their closest stem-group relatives) existed at low diversity and low abundance for some 10 -15 Myr before diversifying and rising in abundance at community level worldwide (Brusatte et al 2010b).…”