Allosaurus has one of the dullest names in all of paleontology. The famous dinosaur's moniker simply means "different reptile"—a bit of a letdown for one of the top predators of Jurassic North America. Early on, the name fit well— Allosaurus was a very unusual dinosaur compared to other large, predatory species—but since 1878 bone hunters have found a slew of closely related dinosaurs whose relationships are still being worked out by paleontologists.
Among the most puzzling of the allosauroids has been the high-spined Acrocanthrosaurus from the Early Cretaceous of North America. Looking like a larger, beefier version of Allosaurus, with a raised ridge of spines along its back, this dinosaur was considered to be the closet relative of Allosaurus until discoveries in South America and Africa started turning up remains of similar creatures. Called carcharodontosaurids, these giant predators were clearly related to Allosaurus and its closest kin. Where Acrocanthosaurus fit among these two lineages has been a matter of debate, but a paper just published by Drew Eddy and Julia Clarke in PLoS One seeks to resolve the issue.
In 2000, paleontologists Ken Carpenter and Phil Currie described the almost entirely intact skull of an Acrocanthrosaurus from Oklahoma. The skull was still partially encased in rock when initially studied, though. Now that the specimen has been completely prepared, Eddy and Clarke decided to give it another look. By comparing this exceptional Acrocanthrosaurus skull to those of other allosauroids and more distantly related theropod dinosaurs, they hoped to determine where the high-spined dinosaur fit among similar species.
The new paper by Eddy and Clarke serves as a minutely detailed guidebook to the Acrocanthosaurus skull, and the scientists were able to draw a few conclusions from their in-depth study. Eddy and Clarke confirmed that Acrocanthrosaurus was a carcharodontosaurid, being particularly close to Eocarcharia from Niger. This means that Allosaurus and Acrocanthrosaurus were only cousins that belonged to separate lineages within the larger group Allosauroidea.
Allosaurus and Acrocanthosaurus shared a common ancestor at a distant point in time, and this recognition can tell us something about how dinosaurs moved around the world. Around 143 to 134 million years ago, Eddy and Clarke state, dinosaurs could have moved from prehistoric Europe to North America by way of land connections including Greenland and island chains. Since the dinosaur Neovenator—identified as an early carcharodontosaurid in the new study—was present in Europe just after this time, the scientists propose that carcharodontosaurids could have radiated out of Europe into Africa, Asia and North America through the various pathways open around that time. Members of the dispersals would have been adapted in different ways on each continent, with Acrocanthosaurus being unique to North America.
But we are really only just beginning to understand the origin and evolution of this group of dinosaurs. For a long time Allosaurus and Acrocanthosaurus seemed like North American oddballs—predators unlike carnivorous dinosaurs elsewhere—but new discoveries are allowing scientists to slowly piece together their relationships and history. There are probably many allosauroid dinosaurs waiting to be discovered, and the recognition of these yet-unknown dinosaurs will further flesh out the story of some of the biggest predators to have ever lived.
Eddy, D., & Clarke, J. (2011). New Information on the Cranial Anatomy of Acrocanthosaurus atokensis and Its Implications for the Phylogeny of Allosauroidea (Dinosauria: Theropoda) PLoS ONE, 6 (3) DOI: 10.1371/journal.pone.0017932