An Evolutionary Shell Game

An academy scientist finds evidence for one of evolution's driving forces.

For hundreds of millions of years, Earth's organisms have been engaged in a longstanding battle. Known as the biological arms race, or the Theory of Escalation, predator and prey, parasite and host, over time evolve better methods and tools to "outdo" one another and gain an edge in survival. Though the presence of biological enemies is considered a driving force for the evolution of new species, evidence of the phenomenon has remained largely elusive. Until now.

Academy paleontologist Peter Roopnarine and colleague Amy Beussink were sifting through the fossil record looking for clues when they uncovered proof of an ancient war. A group of predatory snails attack their clam prey, Chione erosa, by drilling holes through the clam's protective shells. But about 1.5 million years ago, C. erosa went extinct and was replaced by a close relative, C. elevata, an event that offered the scientists a chance to see how hunters and the hunted react to one another.

Initially, the snails chose small individuals of C. elevata, but in time moved on to larger meals, suggesting that the snails developed better ways to drill through the shells. But the battle didn't end there, as the new clam species in time evolved thicker shells.

The team is now looking to present-day relationships in the Gulf of California for more clues. By comparing the morphological variations of prey throughout the Gulf to geographical location, substrate type, and the depth and salinity of water, the team hopes to better understand what other factors may affect predator-prey interactions, and thus, contribute to evolution.

Gulf of California as seen from low orbit (North is toward the bottom of the figure). Map: Colleen Sudekum. Satellite photo NASA.

Chione erosa (left) and C. elevata (right) from the Pliocene and Pleistocene of Florida respectively. Note the large drill hole on the specimen of C. erosa. Photos Peter Roopnarine.

Close-up of naticid drill hole. Photo Peter Roopnarine.
Left: Typical drill hole from the Gulf of California, Mexico. Right: Edge-drilled Chione from the Gulf of California, Mexico. Photo Peter Roopnarine.

Web Links

Peter Roopnarine - Academy Curator
http://www.calacademy.org/research/izg/roopnarine/peter.php

Ancient Snail Wars
http://www.abc.net.au/science/news/stories/s20485.php

Extinction and Predation of the Bivalve Chione in Neogene of Florida.
http://palaeo-electronica.org/1999_1/bivalve/issue1_99.php