Acristavus: A New Hadrosaur from North America
Discovery of Acristavus
In the realm of dinosaurs, bizarre structures like sails and horns often steal the spotlight. However, the discovery of a new hadrosaur species, Acristavus gagslarsoni, has shed light on a different aspect of these ancient herbivores: their lack of ornamentation.
Acristavus lived approximately 79 million years ago in western North America. Its remains have been found in the Two Medicine Formation of Montana and the Wahweap Formation of Utah, represented by nearly complete skulls and other skeletal elements.
A Plain Profile
Unlike many other hadrosaurs, Acristavus lacked the elaborate skull ornaments that characterized its relatives. This “unadorned” appearance sets it apart and provides valuable insights into the evolutionary history of hadrosaurs.
Evolutionary Context
Hadrosaurs are a diverse group of dinosaurs that evolved from earlier relatives known as iguanodontians. One of the defining features of hadrosaurs is their array of cranial ornaments, including snouts with protrusions, paddles, and scoops. However, Acristavus’s plain skull suggests that ornamentation was not a universal trait among hadrosaurs.
Independent Evolution of Ornaments
The discovery of Acristavus supports the hypothesis that the two major subgroups of hadrosaurs, lambeosaurines and hadrosaurines, independently developed different modes of ornamentation. Lambeosaurines, such as Parasaurolophus, had long, crested skulls, while hadrosaurines, like Maiasaura, had more subtle cranial modifications.
A Switch from Ornamented Ancestors?
Acristavus’s evolutionary position suggests that the earliest hadrosaurid dinosaurs did not have skull ornamentation. This implies that ornamentation evolved independently in each of the two major subgroups. However, it remains possible that Acristavus secondarily lost ornamentation that was present in its ancestor, as seen in the later hadrosaur Edmontosaurus.
The Significance of Acristavus
Acristavus is just one discovery, and further research is needed to fully understand the evolutionary history of hadrosaur ornamentation. However, its unique features and evolutionary position provide a valuable piece of the puzzle.
A Peculiar Subgroup: Brachylophosaurini
Acristavus belonged to a peculiar subgroup of hadrosaurs known as the Brachylophosaurini, which also includes Maiasaura and Brachylophosaurus. While Maiasaura and Brachylophosaurus expressed modified, ornamented snouts, Acristavus retained a more archaic-looking skull.
Future Discoveries
The discovery of Acristavus has opened up new avenues of research into hadrosaur evolution. With luck, future discoveries will help paleontologists better understand how these ancient giants acquired their distinctive skull ornaments.
