Another day, another dinosaur. The last to trundle out into public view was Rhinorex, a shovel-beaked herbivore that roamed southeastern Utah around 75 million years ago and given a scientific name just last week. Now paleontologists have unveiled another southwestern dinosaur that lived very close in time to the “nose king”, but was a vastly different animal. The newest dinosaur to step on to the scene was covered in armor.
Found in northwestern New Mexico’s Bisti/De-na-zin Wilderness, the dinosaur was excavated in 2011 by a crew led by New Mexico Museum of Natural History and Science paleontologist Robert Sullivan. The recovered parts included a nearly-complete skull, two spiky half-circles of neck armor, and other fragments. That might not sound like very much, but the bones were distinctive enough to identify this animal – one of the heavily-armored ankylosaurs – as a new dinosaur. In a PLoS One study out this week, University of Alberta ankylosaur expert Victoria Arbour joined Sullivan and other researchers in naming the dinosaur – Ziapelta sanjuanensis, San Juan County’s “sun shield.”
Ziapelta was a prickly-looking dinosaur. The back of the herbivore’s skull sported large, outwardly-curved horns, and the cervical half rings that covered the dinosaur’s neck were decorated with high triangular spikes of bone. The anatomical details of these osteological remnants distinguish Ziapelta from the other ankylosaur found in the same rock layers – named Nodocephalosaurus – but also belie a curious relationship that might have implications for the big picture of dinosaur evolution in North America.
During the past decade, paleontologists have picked up an odd pattern among the Late Cretaceous dinosaurs of western North America. The animals seem to be split into separate evolutionary provinces, with those found in places like Alberta differing significantly from those found in New Mexico and Utah. This implies some kind of barrier that caused dinosaur evolution to diverge, eventually spinning off the different species of horned dinosaurs, tyrannosaurs, ankylosaurs, hadrosaurs, and other fauna that have been recovered in the roughly 75 million year old rock of the west.
How Ziapelta fits into this pattern isn’t yet clear. The dinosaur lived about 73 million years ago, meaning that it didn’t live at the same time as the ankylosaurs of Utah’s Kaiparowits Formation or Alberta’s Dinosaur Park Formation. The closest northern equivalent, Arbour and coauthors point out, is in a lower part of Canada’s Horseshoe Canyon Formation, but no ankylosaurs have been found in those rocks yet.
But even though it was a bit too late to be a neighbor of better-known ankylosaurs, Ziapelta is still relevant to the ongoing investigation of dinosaur provincialism during the Late Cretaceous. That’s because Arbour and colleagues found that Ziapelta was closely related to northern ankylosaurs such as Ankylosaurus, Euoplocephalus, and Dyoplosaurus. The contemporary Nodocephalosaurus, on the other hand, appears to be a close cousin of ankylosaurs that lived in Cretaceous Asia.
Why were the southern ankylosaurs closely related to dinosaurs to the north and from Asia? There are a few possibilities. Perhaps their ancestors traveled to southern North America millions of years earlier and became trapped there by an as-yet-undiscovered geographic barrier. Then again, maybe the biogeographic blockade was broken by 73 million years ago and dinosaurs from the north could again travel south. Or the story of dinosaur evolution in western North America could be much more complex than the current provincialism model accounts for.
Whatever the case, Ziapelta is another actor in life’s great Cretaceous drama. There are more questions about the dinosaur than definitive answers – the most pressing being whether researchers will ever find more of the dinosaur – but some of these mysteries will begin to clear as paleontologists consider Ziapelta in the framework of how life changed. Dinosaurs did not exist in isolation. They were components of ever-changing ecologies. Discovering and naming dinosaurs is just an initial step in exploring the evolving nature of lost worlds.
Arbour, V., Burns, M., Sullivan, R., Lucas, S., Cantrell, A., Fry, J., Suazo, T. 2014. A new ankylosaurid dinosaur from the Upper Cretaceous (Kirtlandian) of New Mexico with implications for ankylosaurid diversity in the Upper Cretaceous of Western North America. PLoS ONE. 9, 9: e108804. doi:10.1371/journal.pone.0108804