2010 has been a bumper crop year for Utah's dinosaurs. No fewer than eight new species have been named, including the iguanodonts Hippodraco and Iguanacolossus; the ceratopsids Utahceratops, Kosmoceratops and Diabloceratops; the sauropodomorph Seitaad and the sauropod Abydosaurus. (A few other Utah dinosaurs were previewed at the 70th annual Society of Vertebrate Paleontology meeting, too, but have not been published yet.) Now, just two and a half weeks before the end of the year, another Utah dinosaur has been described, and it is quite different from all the other new species.
Even though our understanding of dinosaurs is increasing at an astonishing rate, there are still significant gaps in our knowledge. Many of these gaps can be attributed to literal vacancies in the fossil record where creatures we would expect to find have not yet been uncovered. Such has been the case with troodontid dinosaurs in North America. These were small, lightly-built cousins of the famous "raptor" dinosaurs like Deinonychus. Although the evidence is disputable, they may have been present in North America as early as the Late Jurassic, and they were most definitely there by the Late Cretaceous. If this is so, however, it creates an Early Cretaceous gap in which no troodontids have been found.
A new, approximately 127-million-year-old troodontid from the Cedar Mountain Formation of eastern Utah now fills in this gap. Named Geminiraptor suarezarum and described by paleontologists Phil Senter, James Kirkland, John Bird and Jeff Bartlett in PLoS One, this dinosaur lived during a time when some of the dinosaurs we consider to be indicative of the Late Cretaceous, such as the troodontids, mixed with sauropods and other dinosaurs with a more Jurassic style. Looking at dinosaurs described this year alone, the large sauropod Abydosaurus and the iguanodonts Hippodraco and Iguanacolossus also were found within the Early Cretaceous rock of the Cedar Mountain Formation. This does not mean that all these dinosaurs were contemporaries—the Cedar Mountain Formation spans material from about 127 to 98 million years ago—but this collection of dinosaurs serves to illustrate the change from the Jurassic, sauropod-dominated world to a different mix of dinosaurs.
Frustratingly, all scientists found of Geminiraptor was part of the upper jaw (the maxilla). This makes it difficult to ascertain its closest relatives among the troodontids, and more complete material will be required to resolve its relationship to its kin. What is significant about the new dinosaur, however, is its size. It is big for an Early Cretaceous troodontid—closer in size to the Late Cretaceous, 6-foot-long Troodon than to Early Cretaceous types found in Asia like Sinusonasus—indicating that the larger forms of troodontid evolved earlier than previously thought. With any luck, paleontologists will find more of this unique dinosaur and its Cedar Mountain Formation contemporaries to better flesh out the strange world of the Early Cretaceous.
Senter, P., Kirkland, J., Bird, J., & Bartlett, J. (2010). A New Troodontid Theropod Dinosaur from the Lower Cretaceous of Utah PLoS ONE, 5 (12) DOI: 10.1371/journal.pone.0014329