Giant 'cow of the Cretaceous' discovered almost 100 years ago identified as new duck-billed dinosaur

Scientists have discovered an enormous species of duck-billed dinosaur that lived in what is now New Mexico about 75 million years ago.

The dinosaur, Ahshislesaurus wimani, likely had a flat head and a bony crest low on its snout, researchers revealed in a study. The findings, which is due to be published in the Bulletin of the New Mexico Museum of Natural History and Science, suggest that duck-billed dinosaurs, or hadrosaurids, were more diverse and overlapping during the last 20 million years of the Cretaceous period (145 million to 66 million years ago) than previously thought.

Hadrosaurids were large, plant-eating dinosaurs that lived during the last 24 million years of the Cretaceous. They have "sometimes been colorfully called 'the cows of the Cretaceous,'" study co-author Steven Jasinski, a paleontologist at the Harrisburg University of Science and Technology in Pennsylvania, said in a statement. "While this may not be a perfect metaphor, they likely were living in herds and would have been conspicuously present in the environments of northern New Mexico near the end of the Cretaceous."

According to the statement, A. wimani could potentially have grown up to 40 feet (12 meters) long.

One set of A. wimani fossils discovered in 1916 were previously identified as a member of the hadrosaurid genus Kritosaurus. But existing fossil specimens are frequently being reevaluated as more data and fossils become available.

In the new study, the researchers revisited that set of fossils — including an incomplete skull, lower jawbone, and several vertebrae — from the Kirtland Formation in New Mexico. The fossils were housed at the Smithsonian National Museum of Natural History.

"As a general rule … skulls are really the basis for identifying differences in animals," study co-author Anthony Fiorillo, the executive director of the New Mexico Museum of Natural History and Science, said in a separate statement. "When you have a skull and you're noticing differences, that carries more weight than, say, you found a toe bone that looks different from that toe bone."

By comparing the skull to those of other hadrosaurids, the team found that its shape and features were distinct enough from other hadrosaurid skulls to suggest it was likely a different species. A. wimani is closely related to Kritosaurus, suggesting that their evolutionary lines had split not long before.

"Kritosaurus is still a valid genus with species of its own," study co-author Edward Malinzak, a paleontologist at Penn State University Lehigh Valley, said in the second statement. "We took a specimen that was lumped in as an individual of Kritosaurus and determined it had significantly distinct anatomical features to warrant being its own genus and species."

It's not clear yet how the related species co-existed in the same environment, the researchers wrote in the study. But tracing the history and extent of different species could help scientists understand the environment they lived in, as well as the evolutionary history of duck-billed dinosaurs.

"The lineages appear to have co-existed in the region for a time," Malinzak said. "It showed that this group not only exploded with diversity across the continent at one point, but also contributed to the world-wide spread of this group in the Late Cretaceous."