Nov. 7, 2001 (Blacksburg, Va., Nov. 7, 2001) -- Shelled sea creatures were relatively free of predators 250 million years ago. But geological scientists have discovered a tiny ancient brachiopod that was drilled by predators at rates similar to that seen in modern mollusks.
The research will be presented during the 113th national Geological Society of America meeting in Boston, Nov. 1-10, 2001.
In 1956, paleontologist G. Arthur Cooper documented a small brachiopod, Cardiarina cordata, in a collection from Pennsylvanian strata in New Mexico. No one had studied the specimens in detail until they received close scrutiny by Virginia Tech geological sciences Ph.D. student Alan Hoffmeister this year.
Hoffmeister is researching predation of brachiopods from the late Paleozoic (290 to 250 million years ago) as part of a National Science Foundation-funded, long-range study that will help to understand how marine ecosystems have changed over time.
"While the brachiopods left their shells behind, the predators only left marks on those that they attacked," explains Hoffmeister. Drilling predators left characteristic holes in a number of species of brachiopods in the Late Paleozoic. Brachiopods in general display low rates of drilling predation but drilling in bivalve mollusks today is common. "This is the highest drilling rate I've found in the Paleozoic -- it is similar to today's levels," says Hoffmeister.
The family of brachiopods that Cardiarina cordata belongs to also shows high rates of predation. Of nine known species in the family, six are reported as having been drilled. Cardiarina cordata appears to be the most heavily attacked, with 32 percent of the 400 specimens studied from the Smithsonian collection displaying evidence of drilling.
This high incidence of drilling in Cardiarina cordata is interesting for a couple of reasons, says Hoffmeister. "First, they are very small -- an average of 1.5 millimeters (mm) in length, compared to many brachiopods, which can reach three centimeters in length. The holes in the smaller brachiopods are also very small. Second, the predators are highly selective in terms of size of prey and where on the valve they attack. "
Why these small brachiopods are being targeted is unclear, but the researchers have a couple of ideas, Hoffmeister says. "The predator may be a juvenile picking on small brachiopods. Or it may be a predator that, throughout its life cycle, drills the same prey. Because Cardiarina cordata was common, the predator was able to drill many of them and get enough food.
Hoffmeister says the fact that larger brachiopods suffer lower drilling intensities than small brachiopods suggests that two systems may have been at work with different resource requirements and dynamics.
Now that he has begun looking at size as a factor, Hoffmeister has found a report of a second brachiopod in the same family as Cardiarina cordata from the same New Mexico unit that was also drilled, but much less frequently. "This brachiopod, Minysphenia conopia, is even smaller than Cardiarina cordata, indicating just how selective the predator was."
How sure are the researchers that the culprit is a predator rather than a parasite? "What we know about predators is that they only drill a single hole, this hole is only in one valve, the holes are perpendicular to the shells, and the holes are frequently slightly beveled and counter sunk. The drill holes found in Cardiarina fit these characteristics. This ancient predator is producing drill holes similar to those made by naticid gastropods today. All indications are that this is a case of predation, not a parasitism," says Hoffmeister.
Incidents of drilling among brachiopod fossils are rare after the Paleozoic as brachiopods become less abundant in the oceans. However, in the late Mesozoic and Cenozoic (75 million years ago to the present) drilling predation in bivalve mollusks often exceeds 15 percent of the specimens.
Such observations are the result of studies by many researchers. "We are trying to build data sets that will allow us to determine ecosystem dynamics through time," Hoffmeister says. An example of modern ecosytems dynamics is the predation by many animals that helps to control populations of sea urchins. If this were not the case, grazing by the urchins would reduce the extent of kelp beds and the marine ecosystem would change.
Hoffmeister grew up in Cincinnati, Ohio, where Paleozoic rocks abound. "I grew up looking at rocks full of brachiopods and developed an interest in Paleozoic creatures," he says. "When I came to Virginia Tech, Dr. (Michal) Kowalewski had just received the NSF grant to investigate predation in this era."
The paper, "Intense drilling predating of brachiopod Cardiarina cordata (Cooper 1956) from the Pennsylvanian of New Mexico (Dis 22515)" is in session 104, Marine Invertebrate Paleontology I. It will be presented in room 106 of the Hynes Convention Center at 10:15 a.m., Wednesday, Nov. 7. Authors are Hoffmeister, Kowalewski, and geology professor Richard Bambach, all of Virginia Tech, and Tomasz K. Baumiller of the University of Michigan Museum of Paleontology.
An article, "Intense Drilling Predation on the Brachiopod Cardiarina cordata (Cooper 1956) from Pennsylvanian of New Mexico," by Hoffmeister, Kowalewski, Bambach, and Baumiller will be submitted to an international paleontological journal.
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