The ascendancy of dinosaurs on land near the close of the Triassic now appears to have been as accidental and opportunistic as their demise and replacement by therian mammals at the end of the Cretaceous. The dinosaurian radiation, launched by 1-meter-long bipeds, was slower in tempo and more restricted in adaptive scope than that of therian mammals. A notable exception was the evolution of birds from small-bodied predatory dinosaurs, which involved a dramatic decrease in body size. Recurring phylogenetic trends among dinosaurs include, to the contrary, increase in body size. There is no evidence for co-evolution between predators and prey or between herbivores and flowering plants. As the major land masses drifted apart, dinosaurian biogeography was molded more by regional extinction and intercontinental dispersal than by the breakup sequence of Pangaea.

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Late Cretaceous (Cenomanian) fossils discovered in the Kem Kem region of Morocco include large predatory dinosaurs that inhabited Africa as it drifted into geographic isolation. One, represented by a skull approximately 1.6 meters in length, is an advanced allosauroid referable to the African genus Carcharodontosaurus. Another, represented by a partial skeleton with slender proportions, is a new basal coelurosaur closely resembling the Egyptian genus Bahariasaurus. Comparisons with Cretaceous theropods from other continents reveal a previously unrecognized global radiation of carcharodontosaurid predators. Substantial geographic differentiation of dinosaurian faunas in response to continental drift appears to have arisen abruptly at the beginning of the Late Cretaceous.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. ABSTRACT-Archosaurs first appeared in the Middle Triassic and within a short interval of time came to dominate all faunas of large vertebrates for the remainder of the Mesozoic. It is widely held that shortly after archosaurs first appeared the group split into two clades, now termed "Pseudosuchia" and "Ornithosuchia." Each clade independently evolved a rotary-style ankle design ("crocodile-normal" and "crocodile-reversed," respectively) and each independently evolved from semi-erect to erect limb posture. Erect posture, in turn, has been identified as the key adaptation (possibly associated with locomotor stamina) that resulted in the archosaurian radiation.These hypotheses are examined in light of a numerical cladistic analysis of basal archosaurs. Contrary to previous schemes, Archosauria is divided into Crurotarsi and Ornithodira. Crurotarsi (Suchia + Ornithosuchidae + Parasuchia) is characterized by the rotary crurotarsal ankle joint and other postcranial synapomorphies; this style of ankle joint thus appears to have evolved once rather than twice. Ornithodira is divided into Pterosauria and Dinosauromorpha. Pterosaurs, therefore, occupy a basal position within Ornithodira and do not constitute the sister-group to Dinosauria as has been suggested. The supposed pterosaur precursor, Scleromochlus, is reexamined and its proximity to Pterosauria is questioned.The analysis underscores the perils of employing functional constructs, such as ankle "types," rather than character data in phylogeny reconstruction. Current scenarios for the evolution of upright posture in archosaurs-either as an "improvement" in design or as a correlate of locomotor stamina-are not supported by the cladistic pattern. Erect archosaurs coexisted for millions of years alongside more abundant sprawling or semi-erect rhynchosaurs and synapsids and radiated only after their demise. Osteological features associated with locomotor stamina in living tetrapods do not appear to be strongly correlated with erect posture among archosaurs. Once erect posture had been achieved in archosaurs, however, it may have promoted the evolution of bipedal locomotion, which appears at about the same time. Bipedal locomotion, in turn, may have permitted more extensive modification of the forelimbs, which in a short interval of time were fashioned as wing supports for powered flight in pterosaurs.Archosaurs include familiar forms, such as crocodiles, pterosaurs, and dinosaurs, as well as a variety of less familiar forms that went extinct before the end of the Triassic, including the gavial-like phytosaurs, armored aetosaurs, and a variety of quadrupedal and bipedal rauisuchians and dinosaur precursors. Although several monophyleti...