This is the third post in a series called “Cretaceous Cold Cases” in which the science of taphonomy, or prehistoric forensics, is explained 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.
One brisk fall day in 2001, I was prospecting for fossils in the steep Cretaceous-era badlands of southern Utah. The crumbling rock under my feet was about 79 million years old and difficult to navigate safely. Luckily, I was following the cows. That is to say, I was following the well-worn path made by grazing cattle, which always find the best route in and out of steep terrain.
After rounding a rather beefy switchback I saw what looked like several bones on a small plateau below me. Excited by the first fossils I had seen all day, I tossed caution by the wayside and jumped down the remaining 10-foot cliff. Sure enough, three duck-billed dinosaur vertebrae lay on the surface. After careful searching up the cliff, I found their original location, along with more vertebrae from the tail of this dinosaur.
Seven years and one huge hole later we had the remains of two duck-billed dinosaurs – one large and one small. What species were they? How did they get here? Could we gain important insights into the lives of these dinosaurs? I had to know…
My name is Bucky Gates, and I’m a taphonomist.
Before I performed the postmortem examination of the dinosaur skeletons- in fact, before they were even out of the ground- my mind was busy trying to piece together what the site originally looked like. There was fine-grained sediment (mud), dark colored rock, leaf bits everywhere, amber pieces and amazing fossilized logs. Not one or two, but nearly a dozen ancient trees that had now turned into coal crisscrossed the bones.
All of this evidence pointed toward a watery swamp located a long distance from any river. Pine-like trees stood within the swamp, and fallen trees scattered across the ground.
After the bones were carefully cleaned, each was examined for features that could tell me what kind of duck-billed dinosaurs these were and for evidence of a carnivore’s attack. There were no bite marks, but there were broken bones. So we know that something (or some things) stepped on the bones before they were buried. And burial in this swamp would have taken a long time because it was not near a river to provide the necessary dirt. None of the bones were buried as if stuck in mud, so we know the dinos likely did not die from miring.
As for the species, unfortunately, there was not enough of each skeleton to say for sure (although I suspect a new species). So for now they remain “John Dinos.”
Incomplete skeletons tell us that either the bones washed away or were carried away by carnivores. To solve this part of the mystery we analyzed the direction in which each bone was buried, and it turns out no preferred direction was detected, which means the missing bones were probably not removed by water. To understand this concept, picture yourself stepping into a river, only to fall into the rushing water. As you drift downstream you grab a tree branch and the water turns you and the branch parallel to the current because that is the most efficient way for the water to flow. The same thing happens to bones – those that are not picked up and carried away will often position themselves parallel to a water’s current.
So despite no chew marks, there likely were carnivores after all. But how did these dinos die? Were they attacked? Did they break a bone on fallen trees?
It is notoriously difficult to determine the cause of death for fossil animals. This case was no exception. As we dug the specimens from the rock hypotheses flew: ambushing tyrannosaurs, a mired baby duck-billed dinosaur and its protecting mother, or even vice versa, a dead mother and helpless baby.
Unfortunately, all real evidence of how these dinosaurs died has been lost to time.
What we know is that the smaller individual is one to two years old (based on the microscopic structure of the interior bone) and probably the same species as the larger individual. The skeletons were close to one another and not mingled very much. The bones were mostly all scattered. Therefore, they either died several meters away from each other, or the smaller duck-bill was pulled away from the mother by scavengers before being ripped apart.
I very much wanted evidence for parental behavior to be found at this site, but nothing remains that can tell us about the final moments of this tragedy. Sometimes, a cold case stays cold.
Sites such as this one are sometimes used as evidence for dinosaur sociality – you know, protective parents, dinosaurs living in groups, etc. However, it takes very special circumstances to preserve fossils in such a way that we can infer social behavior. Next time, I will tell you how a colleague of mine found just the right evidence in just the right place to piece together a gruesome tale of unsociable behavior.