Is being big clam on the block a factor in species success?

Body size is one of the most important biological characteristics in the study of organisms, telling a researcher a lot about how a particular animal lives and interacts with it’s environment and with other species. Despite this importance, there has been little study of body size trends of ancient life.

Now, using marine life forms as models, three Virginia Tech doctoral students in geological sciences have launched a long-term research project to see what can be learned about life across millions of years. At the Geological Society of America’s 114th annual meeting in Denver, Oct. 27-30, Richard Krause Jr. will present early findings from his, Jennifer Stempien’s, and Susan Barbour Wood’s work.

So far, findings suggest that body size may not be directly related to evolutionary or ecological success.

The trio focused initially on bivalves and brachiopods. Bivalves, which include clams, mussels, and scallops, and brachiopods, which appear similar to clams but have a fundamentally different anatomy, are easily compared because “there is a really good fossil record for both groups,” says Krause.

The scope of the project is huge. The researchers want to measure what has happened all over the world and over millions of years. “Obviously we can’t go out and collect fossils from each age and area,” says Krause. So they are using photographs that accompany published research. This way they can look at and measure shells from many different time periods all over the world.

The research is already yielding some promising results. The students report that early in the history of life, size of the organisms from these groups was increasing along with diversity, which has not been previously documented. “Most interesting, as diversity begins to drop at the end of the Ordovician period, during a major extinction interval (440 million years ago), the overall size of the organisms of both groups was unchanged. The extinction itself wasn’t size selective,” says Krause.

Another interesting point that Krause will focus on at the GSA meeting is the changing places of bivalves and brachiopods. “What we are finding is that from the early Ordovician to the Silurian, or between 500 million and 400 million years ago, bivalves were considerably bigger than brachiopods,” says Krause. This is very similar to the present-day situation for these groups. Bivalves living in the oceans today are, on average, significantly larger than modern brachiopods.

But, while their size differences haven’t changed much, these groups have done a major switch ecologically over the last 400 million years. Brachiopods were very diverse and successful in the Ordovician and Silurian, while bivalves were somewhat rare in many environments. The situation is exactly reversed in modern oceans, says Krause.

“This seems to say that diversity and evolutionary success may not have anything to do with how big an organism is. In this case, the culprit is likely the fact that bivalves’ metabolism is higher. They are more active. That may be what is controlling size, rather than environment,” says Krause. “The fact that this size difference seems to have not changed in the last 400 million years despite major ecological changes is really interesting, and a bit unexpected.”

Krause will present the paper, “Differences in size of early Paleozoic bivalves and brachiopods: The influence of intrinsic and extrinsic factors on body size evolution,” at 9:15 a.m. on Sunday, Oct. 27 in Room A108/110 at the Colorado Convention Center. Co-authors are Stempien, Virginia Tech geological sciences professor Michal Kowalewski, and Arnold I. Miller at the University of Cincinnati.

Contact information: Richard Krause. rkrause@vt.edu. 540-231-1840

Richard Krause’s major professor is Michal Kowalewski.

PR Contact: Susan Trulove, strulove@vt.edu, 540-231-5646.