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The origin and early radiation of dinosaurs

>From Earth-Science Reviews:


The origin and early radiation of dinosaurs

Stephen L. Brusatte, Sterling J. Nesbitta, Randall B. Irmis, Richard
J. Butler, Michael J. Benton and Mark A. Norell

Available online 4 May 2010


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 pre-eminent 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.


Michael D. Barton
Butte, MT
Graduate Student, History (of Science)
Participant, John Tyndall Correspondence Project
Email: darwinsbulldog@gmail.com