In the 1980s, Texas Tech University professor Tom Lehman was conducting research on rock layers at Rattle Snake Mountain and discovered badly deteriorated bones. With the help of other members of the university they collected them, but some were glued together, making it impossible to study them.
Subsequent research in the 1990s revealed an arched nasal crest that was considered a hallmark of the hadrosaurid Gryposaurus. At the same time, a peculiar morphology of the lower jaw was observed.
However, the specimen spent several years waiting for a complete description and it was not until a recent analysis that researchers realized that the specimen was more primitive than Gryposaurus and that the two main groups of 'duck-billed' dinosaurs.
"This new animal is one of the most primitive hadrosaurids known and thus can help us understand how and why the ornamentation on their heads evolved, as well as where the group initially evolved and migrated from," says lead author, Albert Prieto-Márquez of the Institut Català de Paleontologia Miquel Crusafont (ICP).
"Their existence supports the increasingly popular hypothesis, not yet confirmed, that the group appeared in the southeastern United States."
The 'duck-billed' dinosaurs, scientifically known as hadrosaurids, were the most common herbivorous dinosaurs to end of the Mesozoic. They all had a similar-looking snout, with a “U” -shaped front part of the jaws that held a hollowed-out beak that they used to cut the plants on which they fed.
Although the peak of some species of hadrosaurids It is wider than in others, no significant differences were observed, so the researchers believed that the way of eating was similar in all of them. Until Aquilarhinus was found. Their jaws have a peculiar “W” shape, creating a wide, flattened spoon at its base.
The origin of the ridge
This particular morphology allows us to imagine this dinosaur a few 80 million years shoveling wet sediments to collect aquatic plants from the marshes of an ancient delta, where today the Chihuahuan Desert is located. When the dinosaur died, some of its bones were carried downstream by the tide and were hidden in the vegetation. The daily flow of the tide was covering them and allowed their fossilization.
By analyzing the jaw and other characteristics of the specimen, the researchers found that they did not match that of saurophids, the main group of 'duck-billed' dinosaurs. Aquilarhinus is more primitive than this group, suggesting that there may have been a greater number of lineages than are currently recognized and that they developed before the great radiation that gave rise to the incredible variety of shapes of their ridges (without ornaments, solid, hollow ...) observed in this group.
While most saurophids had bony cranial ridges of many different shapes and sizes, that of Aquilarhinus it was simple, hump-shaped. The presence of this solid crest beyond those that appeared during the great radiation of the hadrosaurids supports the hypothesis that all the ridges derive from a common ancestor that had a very simple crest.
Albert Prieto-Márquez, Jonathan R. Wagner & Thomas Lehman (2019): «An unusual 'shovel-billed' dinosaur with trophic specializations from the early Campanian of Trans-Pecos Texas, and the ancestral hadrosaurian crest» Journal of Systematic Palaeontology, DOI : 10.1080 / 14772019.2019.1625078.