Monday, November 29, 2010
Kraig Derstler on the Comparative Taphonomy of Tyrannosaurs
Illustration by James Seward, from the Vintage Dinosaur Art Flickr pool.
A couple weekends ago, I had the chance to attend a lecture by University of New Orleans paleontologist Kraig Derstler at the Children's Museum. One of Derstler's primary interests over the years has been the study of what happens to bones between the death of an animal and fossilization, a field of study called taphonomy. He got his start with invertebrates, and has done plenty of work dealing with the Western Interior Seaway. But on this November Saturday, Dr. Derstler's talk dealt specifically with his work comparing tyrannosaur fossils.
His inquiry in this area is an example of how sometimes a research path a scientist starts down veers in unpredictable directions. When he began, he wanted to know why paleontologists have turned up so many fine tyrannosaur fossils, and expected to come to some new insights. A basic tenet of ecology is that big predators are rare, sitting atop a pyramid made of stacked "trophic levels," each one representing a group of organisms that survives by eating the level below it. You can safely assume that in a given environment, you'll have lot more green leafy plants than deer, and a lot more deer than cougars.
Dr. Derstler's conclusion was that tyrannosaurs simply weren't rare, at least not as rare as common knowledge maintains. He used the Nemegt formation of Mongolia as an example. It's produced many bones of the big tyrannosaur Tarbosaurus. This bounty can't be explained by a predator trap, as the bones occur over a wide area and over a long period of time. For some reason, tyrannosaurs were abundant predators who hung out in places that were good for creating fossils. While the abundance of different species of dinosaurs in Montana did decrease dramatically towards the end of the Cretaceous, the overall number of animals was still pretty high, but they represented fewer species. This doesn't exactly overturn the trophic pyramid, but rather suggests that if there were a lot of big tyrannosaurs, there was an awful lot of prey for them. Herds of Triceratops, Edmontosaurus, and Parasaurolophus would certainly make for good snacking.
Dr. Derstler spoke about some of the tyrannosaurs he studied, and his description of a famous specimen called Peck's Rex was particularly vivid. One of the most disputed fossils of the modern era of paleontology, it had been through six separate dig teams and had even been partially mounted by the time Dr. Derstler came in to complete excavation. The animal had died in an oxbow lake that was so putrid, it couldn't support any life. The big tyrannosaur's carcass rotted away, fouling the lake further, apparently protected from scavengers. Its bones were scattered about the lake, chipped, and degraded. By the time it was fossilized, the only bones still articulated as they had been in life were a few vertebrae of the tail.
This was the case for the other tyrannosaurs he talked about, partial remains with pitted, damaged, nasty bones. Dr. Derstler believes that the reason so many tyrannosaur fossils have made it to us over millions of years is twofold. First, they're robust, tough bones. They do take a beating from the environment, but their size makes them more likely to stick around long enough to be fossilized. Second, they're popular and profitable dinosaurs to have on display, so a lot more money goes into their excavation and preparation, giving the impression that they're better preserved than they are.
Derstler said that this is the way research often goes. When he began, he had hoped to reach some new insight about how tyrannosaurs were fossilized. What he ended up with was a lot of data that bore no trends, offered no opportunity to generalize about the fossils. These were just big animals with tough bones.
Besides discussing his work in tyrannosaur taphonomy, Dr. Derstler also touched on the subject of hadrosaur "mummies," of which he's been able to study 73 specimens. Because of the way dried out hunks of "dino-jerky" hold bones together, his opinion is that many of the most complete skeletons of other dinosaurs began "life" as mummies as well. He also lamented missed opportunities, as with the T. rex nicknamed "Pete," an unremarkable specimen he excavated in 1992 that he believed may have been very special if it had been found a hundred years earlier. Dr. Derstler's research concluded that around that time, three different fossil hunting expeditions teams led by the Sternbergs, by Barnum Brown, all came tantalizingly close to Pete, stopping just short of him. By the time the specimen was found, the elements had their way with it, reducing what may have been a special fossil to an unexceptional one.
I'm grateful to Josh Estes of the Children's Museum for giving me the chance to attend the lecture. I don't often get to listen to working scientists discuss their research, and even though Dr. Derstler didn't blow apart every preconceived notion I held, he yielded plenty of insight into the ongoing story of life as revealed by paleontology. Seriously, it's nice just to sit and listen to an expert discuss dinosaurs for an hour. Even moments that may seem mundane to the speaker, asides added only for levity, can add nuance to our understanding of paleontology. Heck, even chatting with fellow attendees and Dr. Derstler after the talk was a rare and stimulating experience. Even though they had nothing to do with the talk itself, I was especially captivated by a couple of fossils Dr. Derstler brought along. I think I'll save those for my next post.