Cretaceous Cold Cases No. 1: A Case With Legs

This is the first post in a series called “Cretaceous Cold Cases” in which the science of taphonomy, or prehistoric forensics, is explained and exemplified by fascinating cases from the files of Terry “Bucky” Gates, a research scientist with a joint appointment at NC State and the North Carolina Museum of Natural Sciences.

Summer, 2001. It was a blistering day as I made my way across the badlands of Utah to the Grand Staircase-Escalante National Monument. I’d been called to the scene of some suspected Cretaceous-era foul play – the remains of a 10-meter long duck-billed dinosaur that had died at the edge of a river over 75 million years ago. The fossil was largely intact, present, and accounted for, except for two meaty details:  its legs were missing.

My name is Bucky Gates, and I’m a taphonomist.

I closely examine the clues surrounding a fossil – such as the types of rock surrounding it, and how the bones are arranged – to complete a prehistoric forensic analysis of fossil sites.  My goal is to reconstruct, as closely as possible, everything that happened to a fossil animal from the time that it died to the time that we dig it up.

This duck-billed dinosaur, dubbed “Jodi’s Hadrosaur,” had died on the edge of a river, half of its body in the middle of the channel, the other half out.  Eventually, the skeleton was buried in sand, not to be seen again until 2001.

I accompanied a team of fellow researchers from the University of Utah three miles into the badlands and up a 100 foot tall hill to view the partially exposed fossil. On the surface, we could see back vertebrae and fragments of other fossil bone.  But it wasn’t until the ground was swept clean of all loose dirt that we understood that a mostly complete skeleton and partial skull of a huge duck-billed dinosaur lay before us.

After thousands of hours of excavation, we finally saw “JH’s” skeleton as it had lain 75.5 million years ago.  The tail, hip, and most of the vertebrae were articulated, that is, arranged together as one would find them in life, but the rest of the bones were spread apart, or disarticulated.  The rock surrounding the skeleton at the tail was the kind of sandstone found in the center of a riverbed. As you moved toward the skeleton’s head, the rock changed to the sand found at the edge of a river.  But there was one very peculiar detail:  the legs were missing when almost everything else was present.

So what did this tell me about Jodi’s Hadrosaur?  First, that this dinosaur was dead when it hit the ancient riverbed.  I don’t know how it died, and we likely never will (this will be discussed more fully in a future post), but I know that once it hit the river the animal did not move again of its own volition.

Second, I know that the long tail, which was located near the middle of the river, became buried relatively quickly. I know this because it is fully articulated and there are impressions of the skin that surrounded the tail preserved in 3-D. The remainder of the skeleton took longer to bury. The bones of the skull fell apart and drifted downstream in the current for a few meters, whereas the arm and shoulder bones did not go very far from where they disarticulated. This portion of the body was covered by sand characteristic of the river’s edge and therefore, likely was exposed on the ground surface for weeks until sufficient flood waters covered it with sand.

One mystery remained surrounding JH. Where were the legs?

In taphonomy we use a principle called size equivalence.  This simple but powerful rule states that if bones smaller than missing bones in a skeleton are present at a fossil site, the missing elements were not taken away by water.  And since every other bone in the hadrosaur skeleton was smaller than the femur (thigh bone) and tibia (shin bone), and most of those smaller bones were still on the skeleton, that ruled out water as our leg thief.

There was only one reasonable alternative…theft by the large tyrannosaur roaming southern Utah 75 million years ago, Teratophoneus. The large carnivore likely pulled off the meatiest portion of the hadrosaur and left the rest. Unfortunately, we will never know how much time there was between dinner and the leftovers being buried. In all likelihood, there was a lot more scavenging of the hadrosaur by smaller raptor dinosaurs such as Talos, yet we have no evidence of tooth marks on the bones.

To this day I envision Jodi’s Hadrosaur tumbling off a steep bank into a Cretaceous river, never to move again.  And I just want to know: what made it fall that fateful day?

Additional Case Notes:

Jodi’s Hadrosaur is now on display at the Natural History Museum of Utah.  Check it out.