Seitaad ruessi, the “Sand Monster” of the Navajo Sandstone

Even though the first dinosaurs had evolved by 228 million years ago, it was not until the early Jurassic (about 201 million to 176 million years ago) that they were established as the dominant large vertebrates on land. It was during this time that various groups of dinosaurs diversified and began to be adapted in ways which made them quite different from their ancestors, and among these groups were the sauropodomorphs. These were the early relatives of the immense sauropod dinosaurs of the Jurassic and Cretaceous, and an unexpected discovery from southern Utah has shown that these dinosaurs were more widespread that was previously understood.

During the heyday of the early sauropodomorphs, many of the world's landmasses were still connected, which allowed the dinosaurs to travel between continents that are separated by oceans today. Their remains have been found Africa, Antarctica, Asia, North America and South America, but in some places they are more rare than in others. In North America, especially, the bones of sauropodomorphs are difficult to come by, but as reported in the journal PLoS One by paleontologists Joseph Sertich and Mark Loewen, a new partial skeleton greatly adds to our understanding of these dinosaurs from this part of the world.

The remains, found in the approximately 190-million-year-old rock of southern Utah's Navajo Sandstone, primarily consist of portions of the hips, partial hindlimbs, most of the forelimbs and shoulders, and several vertebrae and ribs. From its preservation and the geological details surrounding the bones, it appears that the animal died and was subsequently buried by a collapsed sand dune, hence its name Seitaad ruessi, for a sand monster in Navajo lore (Seitaad) and naturalist Everett Ruess (ruessi) who disappeared in southern Utah in 1934. It was preserved articulated within the rock, a body without head or tail.

What is most significant about this fossil, however, is that it is the best-preserved sauropodomorph yet found from the western United States. Paleontologists have been finding fragments of them for years, but this is the first time that enough has been found to compare the dinosaur to its relatives from elsewhere in the world. When Sertich and Loewen did so they found that Seitaad was  most closely related to either Plateosaurus from Europe and its close relatives or Adeopapposaurus from South America and its kin. The trouble was that most of the comparisons made for these dinosaurs so far have relied upon characteristics of bones not preserved in this particular specimen (such as the skull). But it is most certainly a variety of sauropodomorph that probably spent much of its time walking on two legs (like its distant cousin Aardonyx).

Joseph J. W. Sertich, Mark A. Loewen (2010). A New Basal Sauropodomorph Dinosaur from the Lower Jurassic Navajo Sandstone of Southern Utah PLoS One, 5 (3) : 10.1371/journal.pone.0009789

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Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.