Public Release: 

Impact of geography on species challenges paleontologists

Virginia Tech

Blacksburg, Va., Oct. 25, 2002 -- It can be difficult for paleontologists to determine if differences among some ancient life forms, such as clams, represent different species. After all, the scientists are working with fossils of organisms dead for millions of years, so who knows if they were able to breed or had evolved to a point where that was not possible.

A study by Virginia Tech geological sciences doctoral student Jennifer Stempien may provide some evidence of whether bivalves that lived along the eastern U.S. coast 3.5 million years ago were close distant cousins of each other. She will present her research at the Geological Society of America's 114th annual meeting in Denver Oct. 27-30.

Stempien is studying the small bivalve, Mulinia, which started out in South America and took over the entire east coast 3.5 million years ago. She began her study of the 1-centimeter bivalve that lived off the Virginia coast based on the specimens in the Virginia Natural History Museum. At first, Stempien was studying Mulinia's response to an environmental change that took place between 3.5 and 3 million years ago. During that period, an active coast line, with a lot of waves and strong currents, became muddy and quieter. "The critter didn't change with the environment," Stempien reports.

Out of curiosity, she decided to compare the Virginia Mulinia to those at other locations. That is when she began to see differences. "The samples from different states had wide variability. There was more of a geographic influence in terms of body type, than there was as a result of a half-million years of environmental change at a single location," Stempien says.

There were differences in shell thickness, size, placement of muscle scars, hinge attributes, and so on. "The populations were obviously not mixing."

Did these different physical characteristics mean there were different species? Or would these various forms of Mulinia still be able to breed, if they had met?

Stempien is now looking at living species, the descendents of Mulinia -- specifically, Mulinia coloradoensis, a larger clam recently from the Gulf of California, and modern Mulinia lateralis from Florida, which is more like its ancestor in terms of size. "The variability between these organisms and their ancestor is about the same as is seen in ancient Mulinia from two different localities. It might mean that M. congesta and M. lateralis are one species, but I'm still processing data."

Stempien will deliver the paper, "Spatial and environmental dimensions of shelly morphospaces: Geometric morphometrics of the matrid bivalve Mulinia," at 4:15 p.m. (Tuesday, Oct. 29) in A108/110 of the Colorado Convention Center. Co-authors are Virginia Tech geological sciences professor Michal Kowalewski and G.M. Daley of the University of Wisconsin.

Stempien, who is from Cicero, near Syracuse, N.Y., received her undergraduate degree from the State University of New York, Binghamton.


Contact information: Jennifer Stempien, or 540-231-1840

Jennifer Stempien's major professor is Michal Kowalewski

PR Contact Susan Trulove, 540-231-5646,

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