A newly discovered 4.5-tonne dinosaur has the largest horns ever found on a dinosaur, with a set that were 1.2 metres long each, according to palaeontologists.
A team led by Mark Loewen at the Utah Museum of Natural History announce the new species unearthed in Mexico, in the book New Perspectives on Horned Dinosaurs to be released this week by Indiana University Press.
The name of the new species, Coahuilaceratops magnacuerna, translates in part to "great horned horny face," and the dinosaur lives up to its description.
In addition to the two enormous horns above each eye, the hefty herbivore also had an unusual, rounded nose horn not seen before on any other dinosaur.
"The large horns certainly would have been heavy to haul around, but we know from related animals that horned dinosaurs had very large neck muscles to take care of this problem," says Loewen.
He and colleagues recovered the new ceratopsid (horned) dinosaur in arid, desert terrain within the Mexican state of Coahuila.
When the dinosaur was alive 72 million years ago the region was a humid estuary with lush vegetation.
Based on the dinosaur's remains, the researchers believe that it was rhinoceros-sized - about 6.7 metres long as an adult, 1.8 to 2.1 metres tall at the shoulder and hips, and with a 1.8-metre-long skull.
Both males and females of the four-legged, plant-eating dinosaur had the massive horns, which were probably used to attract mates and to fight with rivals of the same species.
Different from rhino horns
Although the dinosaur looked somewhat like a rhino, the horns were very different.
Rhino horns are made of soft tissue, while Coahuilaceratops horns had a bony core surrounded by soft tissue, similar to the horns of modern sheep, goats and cattle.
Loewen says that "based on the position and orientation of the horns, it might have engaged in 'horn locking' as seen in some modern three-horned chameleons".
In addition to this dinosaur, the researchers found the remains of possibly five or more other new dinosaur species. Two were duck-bills, and one of those has already been named Velafrons coahuilensis.
They also found another horned dinosaur and the remains of carnivores, including large tyrannosaurs that were smaller, older relatives of Tyrannosaurus rex, and more diminutive Velociraptor-like predators armed with sickle-claws on their feet.
While exploring the region, the palaeontologists noticed large fossil deposits with jumbled dinosaur skeletons, suggesting that mass death events occurred at the site, possibly due to hurricanes and other storms that occur there today.
"We know very little about the dinosaurs of Mexico, and this find increases immeasurably our knowledge of the dinosaurs living in Mexico during the Late Cretaceous," says Loewen.
"Mexico's dinosaurs have been elusive, in part, because many areas there are so remote, and partly because there are so few palaeontologists in Mexico who study dinosaurs."
High sea levels
From about 97 to 65 million years ago, high global sea levels resulted in flooding of the central, low-lying portion of North America.
As a result, a warm, shallow sea extended from the Gulf of Mexico to the Arctic Ocean, splitting the continent into eastern and western landmasses.
Central America had not formed at the time, so Mexico was the southern tip of an island continent.
Western North America was connected to the region, so the dinosaurs of Mexico were most closely related to species from there.
Mexico's dinosaurs were less similar to those in South America, since the sea served as a large barrier between the two areas for much of the Cretaceous.
Dr Don Brinkman, a researcher at the Royal Tyrrell Museum who studies early non-dinosaur vertebrates from Mexico, says, "Dinosaurs from this particular period are important because this is a time that is relatively poorly understood."
"The locality in Mexico goes a long way to filling in a gap in our knowledge of the record of changes in dinosaur assemblages throughout the Late Cretaceous," says Brinkman
Coahuilaceratops specimens are currently on exhibit at the Museum of the Desert in Saltillo, Mexico. The horned dinosaur's skull will be unveiled at the museum later this year.