By Hans-Dieter Sues
A lively debate continues regarding the cause(s) of the extinction of dinosaurs (other than their descendants, birds), along with many other organisms, at the end of the Cretaceous Period some 65 million years ago.
While this subject has tremendous appeal, the biologically interesting issue of the origin and early evolution of these reptiles has received much less attention. This lack of interest is surprising because a number of remarkable discoveries in the last two decades have shed much light on this subject.
Now a new paper by a group of Argentine and Brazilian paleontologists in Biological Reviews and a forthcoming survey by a team of American and British researchers in Earth-Science Reviews have summarized recent work in this exciting area of research on dinosaurs.
Ultimately, the origin of dinosaurs can be traced all the way back to the mass extinction at the end of the Permian Period, about 251 million years ago. This event is generally considered the most severe crisis in the history of life and led to the disappearance of many groups of animals and plants, especially in the sea.
On land, several lineages of therapsids (the group including the precursors of mammals) and other vertebrates vanished. Among the survivors of the end-Permian extinction were the immediate precursors of the Archosauria (“ruling reptiles”), which are made up of crocodylians, dinosaurs, flying reptiles (pterosaurs), and a few other forms.
The Triassic Period became the heyday of archosaurs. New discoveries have documented that already early in this period, they split into two major lineages–one including dinosaurs (and birds) and pterosaurs, the other comprising crocodylians and their relatives. The former are generally referred to as Ornithodira (“bird necks,” based on their bird-like necks) and the latter as Crurotarsi (“cross ankles,” based on the distinctive structure of their ankle joints).
Skull of the early predatory dinosaur Coelophysis bauri from the Late Triassic of Ghost Ranch, New Mexico.
Courtesy and copyright of Robert Reisz (University of Toronto at Mississauga).
The first undisputed dinosaurs appeared in the fossil record in the early Late Triassic, about 230 million years ago. Another mass extinction occurred at the end of the Triassic, about 200 million years ago, and may have been triggered by intense volcanic activity related to the initial breakup of the supercontinent Pangaea. This end-Triassic event led to the disappearance of most lineages of crurotarsan archosaurs, including phytosaurs, a group of superficially crocodile-like predators, and rauisuchians, which included the top predators on land during most of the Triassic.
By contrast, dinosaurs apparently did not suffer any significant casualties during this extinction, and the available evidence suggests that they rapidly increased in both diversity and size at the dawn of the Jurassic Period. They became the dominant large land animals for the remainder of the Mesozoic Era.
Surprisingly, the oldest known dinosaurs, from the Ischigualasto Formation (early Late Triassic) of northwestern Argentina, already include representatives of the three major groups–saurischians (“lizard-hipped” dinosaurs), with the predominantly predatory theropods and the primarily plant-eating sauropodomorphs, and ornithischians (the plant-eating “bird-hipped” dinosaurs). Obviously, these are not the earliest dinosaurs, so the origin of the group must be sought in even older formations.
For many years, paleontologists argued that dinosaurs must have had anatomical and physiological adaptations that gave them a competitive edge over other groups of land animals during the Triassic Period.
The fully upright limb posture of dinosaurs, resulting in greater speed and maneuverability compared to other reptiles whose limbs sprawl out sideways from the body, has been most frequent reason given for their success.
However, the authors of the Earth-Science Reviews paper argue that the emergence of dinosaurs may have been simply due to chance and opportunism.
Dinosaurs survived the end-Triassic extinction, which claimed most of their potential rivals. The disappearance at the end of the Triassic of other top predators as well as a variety of other meat- and plant-eaters could have created new ecological opportunities–a “lucky break” for dinosaurs.
However, the situation is not that clear-cut.
For much of the Late Triassic, the supposed competitors existed side-by-side yet dinosaurs already underwent considerable diversification. Some crurotarsans even evolved body forms similar to those of dinosaurs.
Furthermore, recent studies on the structure of the bony tissues of long bones indicate that most dinosaurs grew at much faster rates than crurotarsans, possibly reflecting higher rates of basal metabolism. Additional, as yet unidentified biological differences could also account for the evolutionary success of dinosaurs.
Testing hypotheses concerning competition is often problematic even for different kinds of present-day organisms. Thus, it should not come as a surprise that it is difficult, perhaps impossible, to ever say for certain why dinosaurs came to dominate Mesozoic land ecosystems.
Hans-Dieter (Hans) Sues is a vertebrate paleontologist based at the National Museum of Natural History in Washington, D.C. He is interested in the evolutionary history and paleobiology of vertebrates, especially dinosaurs and their relatives, and the history of ecosystems through time.
A former member of the National Geographic Committee for Research and Exploration, Hans has traveled widely in his quest for fossils and loves to share his passion for ancient life through lectures, writings, and blogging.