Susan Barbour Wood, a Ph.D. student, studied an area in Cincinnati, Ohio, where abundant road cuts show outcrops well preserved along major highways and interstates. The outcrops studied are Ordovician in age. At the time the fossils in those outcrops were deposited approximately 450 million years ago, that land would have been located underwater in a tropical setting south of the equator in a prime storm area, she said. Potentially, any number of things could have disturbed the sea floor--organisms burrowing through it, storms, for example--but that doesn't appear to have happened.
"The fossils were preserved in a series of stacked or amalgamated horizons of rock totaling approximately one foot thick," Barbour Wood said. "The horizons are believed to have been produced by multiple storms over a long period of time. The oldest storm-derived deposits are preserved at the bottom of the horizon and the youngest at the top. During the Ordovician, storm waves and currents would have pounded against the ancient ocean shelf as they do today. The storms could have shifted or scrambled the ancient sea floor habitats across geographical space or potentially dug up buried deposits of mud and dead organisms and mixed them with live or recent ones (time averaging) or simply washed them away to another area."
Barbour Wood collected samples every 10 meters along a 130-meter-long outcrop, or transect, and at the same foot-thick horizon at four other locations in the area. "I analyzed spatial and microstratigraphic patterns of fossil distribution preserved in an amalgamated, multi-storm event Cincinnatian horizon that is traceable for a lateral distance of some 60 km," she said. She looked at organisms within the storm beds across the transect to see how the beds varied over space and time.
After analyzing 72 bulk samples, or 5,150 specimens, Barbour Wood found that "greater heterogeneity was preserved among the samples than can be expected by statistical chance." In other words, she found that the fossils were not randomly distributed, but seemed to be in a non-random distribution pattern such as that expected in the real world. "If they had been random, it would have meant that some process, most likely the storms, had mixed them up. The non-random placement means that fossils are being preserved in or very close to their original habitat and not being mixed around the sea floor randomly by the storm currents or other processes," she said.
Barbour Wood's demographic study will help paleontologists in their studies. "When we collect fossils in a rock bed, we like to think the bed preserves the original structures of the organism's habitats; and, after analysis, I believe I'm seeing close to the original structure of the sea floor."
In other words, scientists looking for a past analog for what they are seeing today, such as the diversity of organisms in a certain time interval or geographical area or movement of organism habitats through space and time, can rely on the fossil record to show them what actually did happen in the past. "If we're trying to compare ecological research today with past analogs, we can be fairly certain what we see in the fossil record is comparable to what happened in the past," she said.
Barbour Wood will present her paper, "Microstratigraphy of an Amalgamated Storm Bed: Spatio-Temporal Resolution in the Fossil Record," Topic 2715, at the meeting of the Geological Society of America at the Colorado Convention Center in Denver Oct. 30 at 9 a.m. Her presentation is part of the session "Seafood Through Time-The Ecologic Context of the History of Life I: In Honor of Richard K. Bambach." Bambach is a former professor of geology at Virginia Tech who is now at Harvard. He was the advisor for Barbour Wood's undergraduate thesis research at Virginia Tech on predation in the fossil record.
Susan Barbour Wood can be reached at susanwood@vt.edu or 540-231-1840 except when she is at the conference Oct. 27-30.
PR Contact: Sally Harris, 540-231-6759, slharris@vt.edu