Compared to some of its later, gargantuan cousins, the 190-million-year-old sauropodomorph dinosaur Sarahsaurus aurifontanalis was a rather tiny herbivore. Only 14 feet long, this dinosaur lived in the early days of the Jurassic, and, according to a team of paleontologists led by Jackson School of Geosciences paleontologist Timothy Rowe, this newly-described dinosaur from Arizona does not fit with the popular image of dinosaurs as creatures that quickly evolved to be large and overran the planet.
The dinosaur story starts around 251 million years ago, in the wake of the catastrophic Permian mass extinction. More than 90 percent of all known marine species vanished and more than 70 percent of all species on land also fell into extinction, but the surviving lineages underwent a major evolutionary radiation. Among these groups were the dinosauromorphs, small creatures that we can recognize as being more closely related to dinosaurs than any other group of prehistoric reptiles, and around 230 million years ago one lineage of these dinosaurs gave rise to the first true dinosaurs. Like their ancestors, dinosaurs remained relatively small and were marginal parts of ecosystems in the southern hemisphere. Then, at the transition between the end of the Triassic and the beginning of the Jurassic, there was another major extinction event. Dinosaurs were lucky enough to survive, and Sarahsaurus was one of the forms that originated in the few million years after this second extinction pulse.
Until recently, Early Jurassic dinosaurs akin to Sarahsaurus were thought to have been part of a dinosaurian invasion of the Northern Hemisphere in which they quickly became the dominant land animals on the global stage. (Though it should be noted that the meat-eating theropod dinosaurs had made the jump north by the end of the Triassic and survived the extinction event.) Compared alongside other sauropodomorph dinosaurs from around this time period in North America, however, the new find suggests that there was instead a drawn-out pattern of dispersal in which dinosaurs moved north multiple times before finally gaining a foothold. This is evident the way Sarahsaurus relates to two of its Early Jurassic cousins: Anchisaurus from Connecticut and the recently-described Seitaad from Utah. Had sauropodomorphs moved to North America only once it would be expected that these dinosaurs would be one another's closest relatives, but instead they fell within different parts of the sauropodomorph family tree. Each is representative of a different dispersal event from the south to the north.
Given its degree of completeness—the majority of its skeleton was recovered—Sarahsaurus is also relevant for understanding the timing of evolutionary changes going on among sauropodomorph dinosaurs just prior to the evolution of truly large sauropod dinosaurs. As interpreted by Rowe and co-authors, Sarahsaurus had column-like hind legs and other skeletal peculiarities often seen among larger dinosaurs. This may mean that many of the classic sauropod traits evolved in small animals first and then were co-opted as lineages of sauropods grew larger (a trend similar to what Raptorex hinted at for tyrannosaurs). The dispersal and evolution of the Early Jurassic sauropods requires further study to test this hypothesis, but it may be that changes among small dinosaurs made the evolution of giants possible.
Timothy B. Rowe, Hans-Dieter Sues, and Robert R. Reisz (2010). Dispersal and diversity in the earliest North American sauropodomorph dinosaurs, with a description of a new taxon Proceedings of the Royal Society B : 10.1098/rspb.2010.1867