Acristavus: North America’s New Hadrosaur

Hadrosaurs don’t get enough respect. Often called the “cows of the Cretaceous,” these big herbivores are often cast as relatively uninteresting animals that primarily served as fodder for the more charismatic tyrannosaurs and other predators. Even I fall into this trap—there is a relative scarcity of posts about hadrosaurs on this blog. A new paper by Terry Gates and colleagues in the Journal of Vertebrate Paleontology, however, gives me a chance to start evening up the score.

As a group, the hadrosaurid dinosaurs differed from their ancestors and earlier relatives—known as iguanodontians—in exhibiting an impressive array of ornaments on their skulls. The snouts of these dinosaurs, according to Gates and colleagues, “display a variety of protrubrances, paddles, and scoops,” and the only one to lack such modifications was one of the last hadrosaurs, Edmontosaurus. Rather than being a retention of the archaic, unornamented state, Gates and co-authors argue, the plain profile of Edmontosaurus probably represents a reversal from an ornamented ancestor. The discovery of a previously unknown species of hadrosaur from even older rock informs this hypothesis.

The new dinosaur, named Acristavus gagslarsoni, lived about 79 million years ago in western North America. Its remains have been found in both the Two Medicine Formation of Montana and the Wahweap Formation of Utah, and the dinosaur is represented by at least two nearly complete skulls and other skeletal elements. Contrary to what might be expected, though, what makes this dinosaur special is that, in the terminology of the paper’s authors, it was “unadorned.” Dinosaurs with weird structures such as sails, crests and arrays of horns often make the news, but in this case, the lack of specialized structures is more important.

Placed in an evolutionary context, Acristavus belonged to a peculiar subgroup of hadrosaurs known as the Brachylophosaurini—a group proposed in the paper that contains Maiasaura and (surprise) Brachylophosaurus. Whereas the other two dinosaurs expressed modified, ornamented snouts, Acristavus had a more archaic-looking skull which lacked such specializations. The significance of this is that the skull of Acristavus is consistent with the idea that the earliest hadrosaurid dinosaurs did not have ornamentation on their skulls. This means that the array of cranial ornaments seen among the two major subgroups of hadrosaurs—lambeosaurines, such as the long-crested Parasaurolophus, and hadrosaurines like Maiasaura—evolved independently in each lineage.

As the authors note, Acristavus is just one discovery. It is entirely possible that, like Edmontosaurus, this dinosaur secondarily lost ornamentation that was present in its ancestor, and this would indicate that crests were a common hadrosaurid feature which simply became modified differently in the two sides of the family tree. Nevertheless, the age and evolutionary position of Acristavus appears to favor the hypothesis that each of the two major hadrosaurid subgroups independently developed different modes of ornamentation. With luck, future discoveries will help paleontologists better understand how hadrosaurs wound up with such fancy skulls.

References:

Gates, T., Horner, J., Hanna, R., & Nelson, C. (2011). New unadorned hadrosaurine hadrosaurid (Dinosauria, Ornithopoda) from the Campanian of North America Journal of Vertebrate Paleontology, 31 (4), 798-811 DOI: 10.1080/02724634.2011.577854

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Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.