See Psittacosaurus Run

Brandon Hedrick always wanted to be a paleontologist. “That’s kind of the norm in my field—you figure out when you’re three or four that you’re interested in dinosaurs,” says the doctoral candidate in Earth and Environmental Sciences. At Penn, he’s been able to link that first love with his interests in math and biology to give a better picture of how dinosaurs looked and walked.

He and his advisor Peter Dodson, a Penn Vet anatomy professor with an appointment in paleontology in Earth and Environmental Sciences, were working on computer modeling of dinosaurs when they discovered a problem. “A lot of people don’t realize most dinosaurs are based on just one or two specimens, and usually incomplete specimens,” says Hedrick. And the bones they do have aren’t in great shape: “They’ve been in the ground for more than 65 million years, so they get messed up.” The modeling being done hadn’t accounted for this.

Fortunately, there are about 100 different specimens of the Psittacosaurus (si-TAC-o-saurus), an early horned dinosaur. Hedrick and Dodson used a new method, three-dimensional geometric morphometrics, to analyze the shape of Psittacosaurus skulls statistically. The resulting article, published in PLoS-ONE, was the first 3D geometric morphometric analysis ever done on dinosaurs. Hedrick is also comparing the right and left sides of skeletons, along with looking at the amount of asymmetry in modern birds, to quantify the amount of expected irregularity. This yields insight into the amount of deformation that has affected the shape of the bone since the time it was buried.

In addition, Hedrick is using Psittacosaurus bones to visualize how the animal moved. Using traces left on the bones by muscles and the knowledge he’s gained by dissecting birds and alligators, he can ascribe different properties to the muscles. He then uses a computer program to see how the animal would move by iteratively ‘firing’ different muscle activation patterns. 

He’s examining the bone itself by cutting a section and grinding it so thin he can look at it on a slide. Using hundreds of microscopic photos, Hedrick can create a digital image of the cross-section that shows the original microstructure of the bone, which, he says, “is just completely amazing, even after you’ve been doing this forever.” Microscopic holes indicate where blood vessels and nerves went through the bone, and dark lines mark periods of arrested growth, showing how the animal grew while it was alive.

Hedrick is also tracking growth patterns through time, especially for very large long-necked dinosaurs. Until recently it was thought that it took dinosaurs about 100 years to become very large, but it has become apparent that they did most of that growing in only about 20 years. He’s examining a theory that a kind of gigantism affected their growth. 

“We’ve been studying dinosaurs a long time, but there’s still a lot to learn,” says Hedrick. “We’re finding new things all the time that could completely overturn some concepts.”