The evolution of the sauropod dinosaurs has to be one of the most fantastic transitions in the fossil record. Though some were the largest creatures to ever walk the land—long-necked behemoths such as Giraffatitan and Argentinosaurus—this impressive group of dinosaurs has its evolutionary roots in much smaller, bipedal dinosaurs that ran about during the Late Triassic. Rather than there being a single, straight evolutionary line from the small sauropod forerunners to famous Jurassic and Cretaceous giants, however, there were multiple flowerings of diversity among the early forms. Yet another new discovery from South America adds some resolution to the big picture.
Within the broad dinosaur family tree, the sauropodomorpha is one of the most prominent branches. This group contains the great sauropod dinosaurs as well as their closest relatives and evolutionary forerunners. Within this scheme, the sauropods were a particular and specialized lineage of a wider group of sauropodomorph dinosaurs that began to spread and diversify many millions of years before there was anything like Diplodocus stomping around. The roughly 231-million-year-old dinosaur Panphagia found in Argentina comes quite close to the beginnings of the sauropodomorph lineage. This dinosaur, named just two years ago, was a bipedal and relatively slender animal that nevertheless represents the approximate ancestral stage for the sauropodomorphs.
This month, another, more specialized sauropodomorph from Argentina was described in the journal PLoS One by paleontologists Cecilia Apaldetti, Ricardo Martinez, Oscar Alcober and Diego Pol. They have named the animal Leyesaurus marayensis. Relatively little of the approximately 199-million-year-old dinosaur was found: A skull, several neck and tail vertebrae, and a few elements of the limbs were all that were recovered, and the animal is estimated to be about eight and a half feet long. Nevertheless, these bones appear to be distinctive enough to separate the new dinosaur out as a previously unknown genus and species from the latest Triassic or earliest Jurassic of northwestern Argentina.
Leyesaurus falls in an intermediate place between the earliest, Panphagia-type forms and the earliest true sauropod dinosaurs. While this sauropodomorph dinosaur already had an elongated neck and spoon-shaped teeth suited to an herbivorous diet, Leyesaurus lacked the column-like limbs of the giant sauropods and could probably switch between walking on two legs or all fours. This can be inferred from the hypothesis of Apaldetti and co-authors that Leyesaurus was most closely related to Massospondylus, a better-known sauropodomorph from the Early Jurassic of South Africa that had shorter forelimbs than hindlimbs. But Leyesaurus was not ancestral to the giant sauropods of later Mesozoic time. Instead this dinosaur, like its close relatives, was part of an array of sauropodomorph dinosaurs which spread all over the world during the later part of the Triassic and Early Jurassic. This time period was one of great change for dinosaurs, and the more we understand about creatures such as sauropodomorphs from this time the better we will be able to comprehend how the giants of the Jurassic and Cretaceous came to be.
Apaldetti, C., Martinez, R., Alcober, O., & Pol, D. (2011). A New Basal Sauropodomorph (Dinosauria: Saurischia) from Quebrada del Barro Formation (Marayes-El Carrizal Basin), Northwestern Argentina PLoS ONE, 6 (11) DOI: 10.1371/journal.pone.0026964