September has been a good month for troodontid dinosaurs. Earlier this month paleontologist Xing Xu and colleagues described a new genus of the slender, sickle-clawed predators—Linhevenator—from Inner Mongolia in PLoS One. Now, in the same journal in which that dinosaur made its debut, paleontologists Lindsay Zanno, David Varricchio, Patrick O’Connor, Alan Titus and Michael Knell describe a similar creature from western North America during a time when a massive seaway divided the continent in two.
The new dinosaur has been named Talos sampsoni and is known from the hips, the nearly-complete remains of the hindlimbs and a few other elements. The bones were found in the roughly 75 million year old Kaiparowits Formation of southern Utah. This particular window into the past—much of it located within Grand Staircase-Escalante National Monument—has allowed paleontologists to perceive a unique pocket of dinosaur diversity quite different from what has been found before.
At the time of Talos, the area that is now southern Utah was a costal environment located near the Western Interior Seaway, a shallow sea which divided North America into eastern and western subcontinents. This division affected dinosaur evolution—species found in the east significantly differ from those found in the west at the same time—but there was probably another barrier that divided the north half of the western subcontinent from the southern half. Horned dinosaurs such as Utahceratops and Kosmoceratops, tyrannosaurs such as Teratophoneus, and other dinosaurs found in the Kaiparowits Formation were quite different from members of the same groups found in the north. Over time, the isolation of different dinosaur populations led to the origin over strikingly different species.
As described by Zanno and co-authors, the discovery of Talos supports the idea that the dinosaurs found in the Late Cretaceous of southern Utah were part of an isolated pocket of evolution. The dinosaur was not just part of a southern extension of a genus already better known from skeletons found to the north. Instead, Talos was part of an aggregation of unique dinosaur species that appear to have evolved in the south. Additional discoveries, as well as the description of already-discovered specimens, will help fill out the history of why the southern dinosaurs were so different.
The dinosaur may also help sort out the history of troodontid dinosaurs in North America. Although many species from this group have been found in Asia, their record in North America is poorly understood. Other than new genus Geminiraptor named last year, most of the troodontid remains have been attributed to the genus Troodon. Even the remains of Talos were initially thought to be Troodon bones. As the authors of the new study point out, this state of affairs means that Troodon would appear to have a 20 million year history that extended over almost the entire northern half of North America, an unlikely scenario that has been created by our incomplete understanding of North American troodontids. Many of these partial skeletons and teeth ascribed to Troodon probably belong to other, as-yet-undescribed species. Troodon has become something of a wastebasket for hard-to-identify remains, and the fact that some of those enigmatic remains turned out to be a new species makes it likely that other so-called “Troodon” specimens will also turn out to be distinct species of dinosaur.
Zanno, L., Varricchio, D., O’Connor, P., Titus, A., & Knell, M. (2011). A New Troodontid Theropod, Talos sampsoni gen. et sp. nov., from the Upper Cretaceous Western Interior Basin of North America PLoS ONE, 6 (9) DOI: 10.1371/journal.pone.0024487