Thanks to their prominent appearances in museum displays and the Jurassic Park film franchise, many people are very familiar with what dromaeosaurid dinosaurs looked like. Relatively small and lightly-built, these predators had long, grasping hands and a hyperextendable second toe on each foot tipped in a large sickle-shaped claw. But a newly-discovered "raptor" from the Late Cretaceous rock of Transylvania has an altered body plan that sets it apart from its evolutionary cousins. It is one of the strangest raptors ever discovered.
Announced by a team of paleontologists led by Zoltán Csikia in this week's issue of PNAS, the dinosaur Balaur bondoc comes from Romanian geological deposits representing an ancient island which was home to a variety of dwarfed herbivorous dinosaurs. The herbivores are relatively easy to find at these sites, but bones from predatory dinosaurs are much rarer, and the partial skeleton used to identify Balaur is the most complete set of dromaeosaurid remains yet found from the area. This alone makes it an important discovery—a first, detailed look at one of the predators that lived on the prehistoric island—but what really makes Balaur unique are its hands and feet.
Thanks to the discovery of numerous other dromaeosaurids, paleontologists have recognized that the standard set of predatory equipment for raptors includes three-fingered hands and a foot with two toes touching the ground and one (the second toe) held off the ground to support a large, recurved claw. What makes Balaur different is that, while it only had two fingers on each hand, its first toe was rotated forward and supported a second sickle claw. This was not just an aberrant, polydactyl individual; the additional hyperextendable toe was a highly modified version of the standard first toe seen in other raptors.
As strange as it is, though, Balaur does not appear to occupy some weird new branch of raptor dinosaurs. Instead, Csikia and colleagues found that it was most closely related to the well-known Velociraptor, meaning that the lineage Balaur belonged to became highly modified after the time of its split with the common ancestor of Velociraptor. Why it is so different from its relatives is still unknown. The authors of the new paper state that the "island effect," which caused the herbivorous dinosaurs to become dwarfed, may account for the strange anatomy of Balaur, but exactly what this means is left unclear. Nevertheless, Transylvania was clearly a strange place during the Late Cretaceous: an island where strange theropods such as Balaur stalked the dwarfed sauropods and hadrosaurus of the day.
Csiki, Z., Vremir, M., Brusatte, S., & Norell, M. (2010). From the Cover: An aberrant island-dwelling theropod dinosaur from the Late Cretaceous of Romania Proceedings of the National Academy of Sciences, 107 (35), 15357-15361 DOI: 10.1073/pnas.1006970107