Tracking the origin of the dinosaurs has been one of the most difficult tasks paleontologists have faced, but since the 1990s, multiple discoveries in South America have provided scientists with a look at what some of the earliest dinosaurs were like. Eoraptor, Herrerasaurus and the recently-described Panphagia are among the oldest representatives of the famous vertebrate group, and all come from the 231-million-year-old rock of Argentina's Ischigualasto Formation. A new species from the same slice of time, described yesterday in the journal Science, has added to the diversity of early dinosaurs.
Named Eodromaeus murphi by Ricardo Martinez, Paul Sereno and colleagues, this early dinosaur is currently represented by a partial skeleton which is still missing a few parts of the skull, tail, ribs and other parts of the skeleton. Despite this missing fragments, however, it is clear what sort of dinosaur it was. The long, low skull of Eodromaeus was filled with sharp, recurved teeth, and anatomically it resembles both its contemporary Herrerasaurus and the 215-million-year-old predatory dinosaur Tawa. Even though our knowledge of early dinosaurs remains scrappy, comparison with its relatives shows Eodromaeus to be a theropod dinosaur, which was one of the earliest-known carnivorous groups.
But one of the most significant aspects of the new paper does not directly relate to Eodromaeus. Paleontologists are constantly reexamining ideas about early dinosaur evolution as new species are found, and thanks to the discovery of both Eodromaeus and Panphagia, one of the more famous Ischigualasto dinosaurs has been given a new identity. Eoraptor was thought to have been one of the earliest theropod dinosaurs and representative of the humble beginnings of this group, but the new study by Martinez and co-authors repositions this dinosaur as a sauropodomorph closely related to Panphagia.
If the new study is correct, Eoraptor was not a precursor to Allosaurus, Tyrannosaurus, and other predators giants, but instead was on the evolutionary stem which eventually gave rise to the immense sauropod dinosaurs. This seems especially apparent in the teeth of Eoraptor. Compared to the teeth of Eodromaeus, the teeth of Eoraptor are more leaf-shaped and seem better suited to a varied diet, indicating that it was probably an omnivore that regularly consumed plants. Nevertheless, it should be kept in mind that this new interpretation of Eoraptor is a hypothesis; it will require further discovery, investigation, and analysis to determine just what sort of dinosaur it was.
The researchers behind the Eodromaeus description also use the opportunity to appraise the pattern of early dinosaur evolution. By 231 million years ago there were already multiple genera of different dinosaur carnivores and omnivores (if not dedicated herbivores), and they appear to have made up a significant part of the local fauna. This might indicate that the oft-discussed "rise of the dinosaurs" may have occurred later than thought, but as has been recently stressed in reference to other dinosaur sites, we must be careful in our counts of dinosaur diversity at any one place and time. An exceptional richness in dinosaurs or a particular type of dinosaur may mean that those species actually accumulated over a longer span of time and did not live side-by-side after all. This well-known concept is called time-averaging, and parsing the fine details of what dinosaurs lived alongside one another will be critical to studies of their early evolution.
For more, see Bill Parker's post on Eodromaeus at Chinleana.
Martinez, R., Sereno, P., Alcober, O., Colombi, C., Renne, P., Montanez, I., & Currie, B. (2011). A Basal Dinosaur from the Dawn of the Dinosaur Era in Southwestern Pangaea Science, 331 (6014), 206-210 DOI: 10.1126/science.1198467