Almost everyone is familiar with the ongoing debate surrounding the extinction of the non-avian dinosaurs 65 million years ago, but the discussion over where dinosaurs came from in the first place is often overlooked. Hypotheses of dinosaur origins have been just as controversial as those of triggers for the end-Cretaceous mass extinction, but during the past few decades numerous new discoveries have helped scientists better understand the early evolution of dinosaurs. What we presently understand about the evolution of these famous vertebrates is presented in a new review by paleontologists Stephen Brusatte, Sterling Nesbitt, Randall Irmis, Richard Butler, Michael Benton, and Mark Norell in Earth Science Reviews.
As pointed out by the team of scientists, a good place to start the story of the dinosaurs is with the mass extinction that occurred about 251 million years ago at the end of the Permian. This event, the worst mass extinction in the history of life on Earth, drastically cut down the diversity of the dominant terrestrial vertebrates (such as synapsids, a group containing our ancestors and creatures more closely related to us than to reptiles) and allowed the surviving groups to radiate in a changed landscape. Among the groups to evolve in the wake of the disaster were the first archosaurs, and these were the earliest representatives of the group to which crocodiles, pterosaurs, dinosaurs and numerous other lineages of reptiles belonged.
The Triassic became the heyday of the archosaurus—numerous groups evolved and radiated into a variety of forms such as the gavial-like phytosaurs and the terrifying rauisuchians—but dinosaurs were relative latecomers. Recent discoveries have identified creatures such as Asilisaurus as among their closest relatives, hence providing paleontologists with clues as to what the ancestors of the first dinosaurs might have been like, but it was not until about 230 million years ago in the Late Triassic that the first true dinosaurs appeared. They were relatively small animals that lived in a warm, seasonal world inhabited by many other archosaurs, but another mass extinction at the end of the Triassic about 200 million years ago (possibly triggered by intense volcanic activity) cleared away many of the archosaur lineages. Dinosaurs, however, persisted through the event, and the beginning of the Jurassic marked the beginning of their rise to ecological dominance.
Dinosaurs had split into two main groups relatively early in their evolution—the saurischians (containing theropods and the long-necked sauropods) and the ornithischians (the diverse group of dinosaurs that contains hadrosaurs, ceratopsians, ankylosaurs etc.)—but during the Jurassic the different lineages within these groups evolved into a dazzling array of creatures. New types of large predatory theropods like Dilophosaurus set out after sauropodomorphs such as Seitaad and some of the earliest true sauropods, and while the early history of ornithischian dinosaurs is less well-known, the transition from the Triassic into the Jurassic unambiguously shows that this group, too, was undergoing its own radiation. Indeed, not only were there more dinosaurs around during the Early Jurassic, but they were becoming increasing disparate from each other, or, in other words, becoming adapted into a variety of shapes and sizes not seen before.
The paper itself presents more important details of early dinosaur evolution that can be mentioned here, but suffice it to say that the information paleontologists have discovered since the 1980s has greatly revised old notions of the pattern of dinosaur origins. Contrary to what was believed during the mid-2oth century, the seeming evolutionary ascent of the dinosaurs was not the result of their inherent superiority over other groups of organisms (a notion that stemmed from the notion that evolution is progressive, pushing creatures towards greater levels of absolute perfection). Instead the emergence of dinosaurs was contingent on chance events that could not have been been predicted—chance and evolutionary history played important roles in the early evolution of dinosaurs. Had the mass extinction at the end of the Permian never occurred, for example, there may never have been the dramatic radiation of archosaurs seen during the Triassic, and this would have drastically changed the history of life on Earth.
Viewed as a whole, the story of dinosaurs is a tale of how life on Earth reacts to mass extinctions. Dinosaurs rose to prominence because of mass extinctions, yet most of them (the exception being their bird descendants) succumbed to another ecological catastrophe 65 million years ago. As the authors themselves conclude:
... there was nothing predestined or superior about dinosaurs when they first arose, and without the contingency of various earth-history events during the early Mesozoic, the Age of Dinosaurs might have never happened.
Brusatte, S., Nesbitt, S., Irmis, R., Butler, R., Benton, M., & Norell, M. (2010). The origin and early radiation of dinosaurs Earth-Science Reviews DOI: 10.1016/j.earscirev.2010.04.001