The body size of ancient creatures, bivalves and brachiopods, could tell geoscientists a lot about the creatures’ life history and about the ecology of the times in which they lived. However, traveling the world to measure these creatures' fossils would take several life-times and more travel funds than scientists usually have.
Since the same creatures also have become abundant in scientific literature since the mid 1800s, a team of Virginia Tech researchers is determining whether measuring photos of fossils collected worldwide would be a reliable way to compile body size data. Richard A. Krause Jr., of Greenfield, Wis., a Ph.D. student in the Department of Geosciences at Virginia Tech, will report on his assessment of this nontraditional approach at the joint meeting of the Northeastern and Southeastern Sections of the Geological Society of America, to be held March 25-27 in Tysons Corner, Va.
Bivalves (i.e. clams) are common today while brachiopods, which look like clams but have a different biology, were most common in ancient oceans. Both types of organism have been around for more than 500 million years and still exist today, which makes them good subjects for an assessment of long-term body size trends. Changes in body size could reveal whether an organism evolved and became more efficient or whether environmental changes had an impact. "But no one has compiled a list of how body size changed for a group of animals over long time periods," Krause says.
He and colleagues want to look at two time periods – from 500 million to 300 million years ago and from 200 million to 25 million years ago.
"Since paleontology became a science, people have been publishing papers on new species from all over the world. With the advent of photography, authors began to provide volumes of photographic plates that document everything they thought was important about a species. If we could find a way to get size data from the photos, we could travel around the world simply by going to the library," says Krause.
But there were two big questions. Are the photos an accurate representation of size, and, were the specimens selected to be photographed actually representative of those collected?
Virginia Tech professor of geosciences Michal Kowalewski addressed the first question by measuring brachiopods in photos and then measuring the actually specimens that were retained at museums. He reported, along with several colleagues, that the difference was negligible and was within the error range of the instruments used to take the measurements (Kowalewski et al., 2000).
Krause and colleagues have been researching the second question by examining the specimens field geologists originally shoveled into their collection boxes. They compared the range of sizes of the specimens in each box with those finally selected for the limelight in a scientific journal. "The author can't photograph every specimen due to space," says Krause. He went to museums to examine the boxed fossils originally collected for 41 species of brachiopods and 45 species of bivalves from a half-dozen countries.
"The distribution of those that were photographed were generally within the range we observed by measuring the collections," Krause says. However, authors did tend to pick more of the larger specimens to photograph than would have actually been representative, the Virginia Tech researchers discovered. "In both the bivalve and brachiopod samples, about one-third of the specimens photographed were in the top 10 percent of those collected in terms of size," says Krause.
"The taxonomic literature is biased,” Krause says, “but it is biased in the same direction and roughly the same magnitude for both brachiopods and bivalves. So, we feel that the methodology of measuring photographs in the literature can be useful," says Krause, "although, it is still controversial. There are geoscientists who are not comfortable with this approach."
He and his colleagues will continue to double check the methodology as they collect large amounts of size data over the next several years. They expect that these data will help answer many important questions about the history of life, and it will probably spark some new ones as well.
Krause will present the paper, "Assessing the usefulness of literature-derived estimates of body size" (60-2), at 8:20 a.m. Saturday, March 27, in the Lord Thomas Fairfax Room of the Hilton McLean Tysons Corner. Co authors are Krause, Virginia Tech graduate student Jennifer Stempien, Kowalewski, and Arnold Miller of the Department of Geology at the University of Cincinnati.
Krause says he has always been interested in geology and biology. As an undergraduate at the University of Wisconsin, he discovered he could combine the interests. His bachelor's degree is in geology from the University of Wisconsin and his master's degree in geology is from the University of Cincinnati.
Kowalewski, M. Simões, M.G., Torello, F.F., Mello, L.H.C., and Ghilardi, R.P. (2000) Drill holes in shells of Permian benthic invertebrates. Journal of Paleontology , 74:532-543.
Trends in body size may be related to a range of evolutionary, paleoecologic and taphonomic questions. Efficient assessment of these trends is therefore important for our understanding of the history of life. Here, we evaluate the bias inherent in the use of size data provided by photographs in monographs.
The observed monographic bias can be assessed with respect to four different end-member outcomes. In the best case, monographs yield unbiased estimates of the central tendency of the sampled population. Alternatively, the monographs may yield biased but predictable (“inaccurate but precise”) estimates of size. This is acceptable as long as the bias does not vary across time, space, or taxa. Finally, monograph data would be deemed unacceptable for the reconstruction of size trends if they follow either an “accurate but imprecise” or “inaccurate and imprecise” model (i.e., high imprecision implies very low informative value of monograph estimates).
We targeted several species of Neogene bivalves (n=20) and Ordovician brachiopods (n=10) for which we could acquire, from the same locality, data on both monographic and bulk sample estimates for a given species. For each species, we then compared the central tendencies of the monographic measurements against the central tendencies of the bulk sample. The size bias of monographic specimens was also estimated by its percentile value, relative to the corresponding bulk sample estimates. These two approaches allow us to quantify the magnitude and consistency of the monographic bias and test our data with respect to the four end-member scenarios.
Results suggest that monographic specimens are consistently larger than the average sizes of those from bulk samples for the corresponding species. For brachiopods and bivalves, the majority of monographic specimens are located above the 60th percentile of the corresponding bulk samples. However, the central tendencies of monograph estimates for each species show significant correlations with estimates derived from the corresponding bulk samples. Thus, the data follow the “inaccurate but precise” scenario very well. Just as importantly, the bias appears to be consistent across higher taxa and through time. Thus, monographic data should yield meaningful estimates of body size trends through time.