Why Whorl?

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Science  03 Oct 2003:
Vol. 302, Issue 5642, pp. 19
DOI: 10.1126/science.302.5642.19a

Snails, or gastropods, come in a wide variety of shapes, some with many coils of their tube-shaped shells, others with just one or two large coils, and some with irregular or straightened tubes. Is this just some random evolutionary experiment or are there environmental forces that drive the snails to curl? Fossilized gastropod larval shells from the Paleozoic (450 to 250 million years ago) are largely open-coiled, but later larval shells are mostly tightly coiled. Nützel and Fryda have compiled a database of larval shells showing that the number of species with open whorls dropped from 61% in the Ordovician to 2% in the Permian, at the end of the Paleozoic. There are no open-whorled larval shells in the fossil record from the Triassic, the beginning of the Mesozoic. A snail starts life as a planktonic larva in an egg capsule, and a tight coil would be stronger and better protected from predators. Thus gastropods may have evolved from an open coil to a tight coil because of predation. Unfortunately there is no evidence in the fossil record of predator-prey relationships for gastropods, and although healed cracks and other evidence of possible predation are found in extant larval shells, their planktonic predator remains unknown. — LR

Geology 31, 829 (2003).

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