The Jurassic Shield

Some dinosaurs are better at hiding than others. There’s no single reason for their edge. Some were rare. Others too small to gain much notice from the paleontologists who were set on finding large, imposing specimens for museum centerpieces. Still more became preserved in slices of rock that haven’t gained the attention of the most famed quarries. Whatever the reasons, though, a big, heavily-armored dinosaur was able to escape the notice of paleontologists despite over a century of Jurassic exploration.

North America’s Morrison Formation is a fossiliferous wonderland. Spectacular bonebeds dot its reach from Canada to Mexico, and it was one of the focal points for the great Bone Wars between O.C. Marsh and E.D. Cope in the late 19th century. Apatosaurus, Allosaurus, Camarasaurus, Diplodocus and more were the products of that feud.

Strangest of all was Stegosaurus – a 30-foot-long quadruped with a tiny head, flashy plates along its back, and a set of four spikes jutting from the tip of its tail. Marsh named the bizarre animal in 1877, and ever since Stegosaurus has held the banner for the plated, spiky armored dinosaurs that were eventually replaced by ankylosaurs.

Often thought of as Cretaceous creatures, ankylosaurs were low-slung dinosaurs bore dense coverings of osteoderms from snout to tail, giving them greater protection from the teeth and claws of the predators they evolved alongside. It seemed they were the improved successors of the stegosaurs, taking over the same niche of prickly browser in the same places, but the tale isn’t so clean cut. Ankylosaurs were already present when Stegosaurus was trundling around, 150 million years ago.

I was hoping to get an up-close look at one of these early ankylosaurs as I sat down to work the edge of eastern Utah’s Hanksville-Burpee Quarry in the late May heat. Last year, while scratching away in the spot I selected, a McLennan Community College student uncovered a set of rounded osteoderms. The Jurassic skin bones gave away the dinosaur almost immediately. The student had uncovered the first signs of an ankylosaur in the quarry. It seemed a good bet that there’d be still more waiting in the Morrison Formation’s coarse sandstone, and I got the ok from expedition lead Scott Williams to try for more ankylosaur.

Up until 1994, no one expected that an ankylosaur would be found in these rocks. The Morrison Formation had been frequently prospected and excavated by generations of paleontologists. All the same, while working in western Colorado’s Mygatt-Moore Quarry, paleontologists found bones that were unquestionably ankylosaur. Jim Kirkland and Ken Carpenter named it Mymoorapelta maysi, and, along with a later find dubbed Gargoyleosaurus, the dinosaur pushed the age of North American ankylosaurs back into the Late Jurassic.

Once paleontologists knew what to look for, Mymoorapelta started turning up at other sites. It’s a rare dinosaur, but so far it has been found in at least seven different localities within the Morrison Formation. The Hanksville-Burpee Quarry, excavated by the Burpee Museum of Natural History, is the latest spot Mymoorapelta has appeared.

I had done some prep work on a big ankylosaur osteoderm before, but I had never found the remains of one in the field. I would have been happy to find any bone – the quarry is brimming with the bones of Jurassic celebrities – but I was really hoping for some armored dinosaur. Given that the 11-foot-long ankylosaur had curved shoulder spikes and rounded scutes pebbling its body in addition to the limbs, vertebrae, and other bones that come standard in a dinosaur, I was hoping that skeletal statistics would be in my favor.

But excavation can’t be rushed. The dinosaurs of HBDQ have been waiting for about 150 million years. I figured that they could be patient as I carefully applied an air-powered microjack to the pebbly sandstone, watching every grain fly off the surface in the hope that it’d reveal pale bone. The quarry is so dense that it didn’t take long before the first piece of skeleton started to peek out of the sand, but what was it? Field identification is tricky, and often initial guesses turn out to be wrong. I decided to leave it as “bone” as I tried to excavate around the fossil to find its full extent.

The trouble was that I kept finding bones. It became impossible to uncover one without running into another unidentified piece. They were almost as pale as the surrounding sand, meaning that I had to hold my face so close to the sediment that I had to occasionally snort out the sand the airscribe sent flying up my nostrils.

By the second-to-last day, I had a set of at least six little mystery bones. They were not limb bones, ribs, or skull pieces. “Vertebrae” was my best guess, but of what? That’s when I found the cutest scute I’ve ever seen. It looked like nothing more than a little sliver of white bone. I figured it was just a weathered bone shard, a broken piece that had been rounded as it tumbled along with the pebbly sand that buried it. I popped it off its little Jurassic perch with my awl in the expectation that it was set for the “frag bag”, but when I turned it over I saw a little keel coming up to a point. This was a tiny piece of armor that was once carried in the skin of an armored dinosaur. What the other bones turn out to be will have to wait for preparation and identification in the lab, but I could leave the quarry content that I had found my Mymoorapelta.

Reference:

Foster, J. 2007. Jurassic West. Bloomington: Indiana University Press. pp. 119-120, 215-216

Kirkland, J., Carpenter, K. 1994. North America’s first pre-Cretaceous ankylosaur (Dinosauria) from the Upper Jurassic Morrison Formation of Western Colorado. BYU Geology Studies. 40: 25-42