GAINESVILLE --- Understanding why an ancient oyster became as large as a dinner plate may help answer the evolutionary riddle of how generations of dinosaurs and other animals grew.
Throughout the animal world, descendents are often bigger than their ancestors, but scientists have been stumped by whether this phenomenon is due to creatures actually growing faster over time or simply living longer, said Douglas Jones, a UF paleontologist studying growth rings in shells of fossilized oysters.
In the rings, which act like tree rings in revealing age, Jones found that oysters living during the Jurassic Period about 200 million years ago actually grew faster over succeeding generations.
"It answers a basic question about how evolution occurs and raises the issue of how pervasive a pattern it is," Jones said. "Paleontologists at least a century ago noticed this trend with fossil horses and other organisms, that things often started out small and got bigger through geologic time. The same methodology used in this study could be applied to creatures like dinosaurs, which have annual increments in their bones, or mammals, which have them in their teeth."
Jones, who works at the Florida Museum of Natural History on the UF campus, is writing a paper on his findings with Stephen Jay Gould, an eminent paleontologist, Harvard professor and bestselling author. The research is scheduled to be presented in a talk titled "Cutting the Devil's Toenails" at the annual meeting of the Geological Society of America in Denver today (10/31) on Oct. 31.
Determining why the fossil oyster Gryphaea evolved the way it did is a classic riddle that has befuddled scientists since the publication of a provocative paper by paleontologist Edward Trueman in 1922. One of the best documented cases of evolution in the fossil record, the paper showed how the oyster changed from being as small as a penny and flat to larger and coiled, Jones said.
Even though Gryphaea is a fairly simple organism, it has been one of history's most studied fossils because its subtle changes over time show a clear picture of how evolution occurs, Jones said. In that sense, it is like the small fruit fly Drosophila that is so commonly used in genetics experiments, he said.
"Gould once said that every field has its Drosophila," Jones said. "For paleontology the Drosophila is Gryphaea," Jones said.
Jones traveled to the British Isles to do his research, collecting several thousand specimens of fossilized oysters from the great cliffs and rugged sea coast that have made it a classic area of the world for rocks containing fossils from the Jurassic Period.
To verify that the oyster's growth rings indeed represented annual age increments, Jones tested them by sawing the oysters in half and analyzing seasonal changes in their oxygen isotopes. The ratios of these isotopes change as the temperature of the ocean gets colder during the winter, and by drilling holes in the growth rings and analyzing the samples Jones was able to confirm that each ring marked the passing of a year.
Using that information, he charted growth curves for the oysters at different stages of their evolutionary development.
"I found that the growth rate was slower for the earliest specimens because at certain ages they were not as big as the later specimens," he said. "So the later ones got their larger sizes by growing faster. Some of them grew to be as big as dinner plates."
The other major unanswered question about the ancient oyster's development was whether it truly became more coiled through time or just looked that way because of its growing size, Jones said.
The oysters were so curved that the rocks where they were found appeared to be popping out of the ground, resulting in the local people of the British Isles calling them `devil's toenails,' he said.
Jones, was able to separate the size and coiling trends. He found that coiling did not actually increase as the oysters grew in size.
Color or black & white photo available with this story. For information, please call News & Public Affairs photography at (352) 392-9092.