New Dinosaur Rhinorex Raises Cretaceous Quandary

Rhinorex is an evocative name for a dinosaur. It sounds like a snarling theropod with a huge nasal horn, like Ceratosaurus amped up to eleven. But dinosaur names can be deceiving. As fierce as the name Rhinorex might seem, the actual animal was a much gentler creature – one of the shovel-beaked, herbivorous hadrosaurs. And if you know your Greek and Latin, you can already pick out the distinguishing feature of this new dinosaur. Rhinorex is the “nose king.”

On a purely superficial level, Rhinorex isn’t as wildly unusual as some other dinosaurs that have been making headlines lately. Described by North Carolina State University’s Terry Gates and Rodney Scheetz of the Brigham Young University Museum of Paleontology, the 75 million year old hadrosaur is quite similar to another dinosaur named Gryposaurus. The features that distinguish Rhinorex as a new dinosaur take an anatomist’s eye to see – a hook-shaped flange on the nasal bone, and the orientation of part of the premaxilla, or front of the jaw.

But appearances aren’t everything. What makes Rhinorex remarkable is the dinosaur’s place in space and time.

Rhinorex skin impressions. From Gates and Scheetz, 2014.
Rhinorex skin impressions. From Gates and Scheetz, 2014.

The body of Rhinorex – including a skull, skin impressions, and a partial skeleton still encased in sandstone – was discovered in 1992 by a pair of University of California, Riverside geology students who were mapping sections of eastern Utah’s Book Cliffs. Paleontologists haven’t found many dinosaurs here, the only other published example being part of a tyrannosaur’s lower leg described just last year. To have a new dinosaur from this area is a significant milestone, and one that raises questions about the evolutionary explosion that was going on in southern Utah at the time.

Rhinorex lived somewhere around 75 million years ago. The date is a little shaky, as Gates and Scheetz point out, but places the hadrosaur in the middle of a major dinosaur radiation.

Based on other recent finds – many of them in southern Utah – paleontologists have been starting to piece together a picture of distinctive dinosaur groups evolving in isolation from each other. On the subcontinent of Laramidia, the landmass to the west of the great interior seaway, the dinosaurs that lived in prehistoric Alberta were different genera and species than those living further to the south in Cretaceous Utah and Mexico. The dinosaur communities were similar – with horned dinosaurs, tyrannosaurs, hadrosaurs, ankylosaurs, and more in each – but the species were different. This implies some kind of physical barrier that split dinosaur populations and caused their evolution to diverge along different paths.

What’s strange about Rhinorex is that this different dinosaur lived near its close relatives. If Rhinorex really did live 75 million years ago, then it overlapped in time with Gryposaurus species that inhabited swamps just 155 miles away. That’s not terribly far for 30 foot long animals that could have ranged widely across Utah’s lost coast.

The skull of Rhinorex. From Gates and Scheetz, 2014.
The skull of Rhinorex. From Gates and Scheetz, 2014.

So why is Rhinorex distinct? Gates and Scheetz suspect that habitat may have made the difference. Rhinorex was found near an estuary that was closer to the seaway, while Gryposaurus was a bit more upland in a swamp associated with a more floodplain-like environment. Then again, the researchers note that Gryposaurus is thought to have been a generalist that could have grazed in a variety of habitats, so figuring out why Rhinorex evolved may not be as simple as tracking habitats.

Another possibility is that Rhinorex lived a little bit after 75 million years ago. If this is the case, Gates and Scheetz write, then there are no hadrosaurs known from this part of the world. Perhaps, then, Rhinorex was a descendant of Gryposaurus, connecting that dinosaur to the slightly later Kritosaurus. Rather than being an example of extreme habitat partitioning, Rhinorex could help make sense of an evolutionary gap between North America’s hadrosaurs.

The Rhinorex mystery will take a lot more work to solve. Paleontologists have learned a great deal about the flora and fauna of the Kaiparowits Formation – the land of Gryposaurus – but the Book Cliffs largely remain an undiscovered country. And Rhinorex is beckoning scientists to return, not just because dinosaurs are weird and wonderful, but for what they can tell us about our world’s grand evolutionary tale.

Reference:

Gates, T., Scheetz, R. 2014. A new saurolophine hadrosaurid (Dinosauria: Ornithopoda) from the Campanian of Utah, North America. Journal of Systematic Paleontology. doi: 10.1080/14772019.2014.950614

3 thoughts on “New Dinosaur Rhinorex Raises Cretaceous Quandary

  1. I skimmed the paper last night. Maybe I’m becoming a lumper in my old age, but I don’t see a genus-level distinction here. Gryposaurus cordupus seems perfectly reasonable, especially given the potential overlap.

    I’m curious if you’ve read Nick Longrich’s recent paper in Cretaceous Research about the (potential) presence of Pentaceratops and Kosmoceratops in Alberta. If he’s right, that would throw an interesting wrench into the north-south endemism theory.

    http://www.sciencedirect.com/science/article/pii/S0195667114001293

  2. I agree with Zach Miller. The analyses find it either the sister group to three species of Gryposaurus or in a polytomy with them. No reason this couldn’t have just been called Gryposaurus condrupus.

    Of course that’s just the name and has nothing to do with the niche partitioning question.

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