Plentiful fossils of dinosaur contemporary allow population study

Dinosaurs ruled the earth for hundreds of millions of years, then disappeared so completely that to find even a partially complete skeleton of a single multi-ton animal is rare. Meanwhile, the Virginia Museum of Natural History has scores of fossils of Tanytrachelos ahynis, a 12 to 18-inch reptile that also lived millions of years ago, at the same time as the earliest dinosaurs.

Tanytrachelos is a long-necked reptile that was related to the perhaps better-known nine-foot (up to three meter) long Tanystropheus, also known for its very long neck. They belong to the Protosauria, a suborder of aquatic and terrestrial reptiles. As a contribution to efforts to create a family tree for all protosaurs, a Virginia Tech graduate student is taking advantage of the plentiful and nearby collection of Tanytrachelos fossils to study their preservation and population dynamics.

Michelle Casey of Elk River, Minn., a master’s degree student in geosciences at Virginia Tech, will present her findings at the Society of Vertebrate Paleontology meeting in Denver Nov. 3-6 and at the Geological Society of America 116th annual meeting Nov. 7-10, also in Denver.

All but one of the fossils of the small aquatic reptile are from the Solite Quarry, located on the Virginia-North Carolina border not too far from the museum in Martinsville, Va. and all specimens are currently accessible through the Martinsville museum.

The quarry reveals the late Triassic Cow Branch Formation. The location was a rift basin lake, one of a series of lakes from South Carolina to New Jersey that formed inland when the European continent pulled away from the American continent, forming the Atlantic Ocean. “Working with Tanys is cool because you find a lot of them still articulated – the pieces of bone are still put together,” said Casey. “They must have been buried really fast or left completely undisturbed after they died.”

The bones are still in the rock. “They are too fragile to remove from the sediment.” Nevertheless, with 90 specimens so far, Casey has measured everything she can about all the little bones – shape, size, relationships – more than 100 linear measurements. She plots each fossil using statistical methods to condense multiple-dimensional space into two-dimensional plots and then looks for variation between different groups of Tanys.

So far, Casey, museum vertebrate paleontology curator Nicholas Fraser, and Virginia Tech geosciences professor Michal Kowalewski have discovered there are no differences in morphology between populations of fossils from different times. “We have fossils from different lake cycles, about 350,000 years apart, but found no statistically significant differences in the size of specimens from the different periods,” Casey said.

The researchers also tested for size differences between two different types of Tany based on a bone in the pelvic region that about 40 percent of them have. One might be female and one male, Casey said. “We found no significant differences in shape or size between the two forms, suggesting that they definitely belong to the same species.”

In addition to the morphometric studies, the team is doing taphonomic analyses, trying to determine what effects the exceptional preservation might have on other Tany studies.

Casey’s bone measurements also note when bones were missing, “so all the missing values are data. For instance, we can look at how many are missing a tibia, how many are missing a femur, how many are missing both, and how many fossils have all or most of the variables.” “We are looking at which variables are most common and which are least common. Even in this collection, where there is exceptional preservation of articulated specimens, the outlook is bleak,” Casey said. “Only 47 percent of specimens have both of the two most common variables, femur length and femur width. Most have more missing variables than stuff you are looking for.”

Casey said that such preservation trends are often linked to environmental factors rather than any intrinsic differences between the fossils themselves. “These specimens were preserved at the bottom of a lake, where there was not much oxygen. In that environment, there were not many scavengers. Perhaps there were differences in these factors in the two horizons the fossils come from,” she said. “Another research group from the museum is working with Dr. Fraser on the geochemistry to address these issues.”

The paper, “Morphometric and Taphonomic Analyses of Tanytrachelos ahynis from the Late Triassic Cow Branch Formation, Solite Quarry, Virginia,” by Casey, Fraser, and Kowalewski, will be presented at the Society of Vertebrate Paleontology meeting at 2:15 p.m. on Saturday, Nov. 6, at the Adam’s Mark Hotel and at the Geological Society of American meeting at 9:30 a.m. on Tuesday, Nov. 9, in rooms 111/113 of the Colorado Convention Center. “It is unusual to have this kind of data to present at the SVP,” Casey said, “because the sample sizes are usually so small.”