EVOLUTION WITHIN A GENERAL SHELL-SPACE AMONG EARLY GASTROPODS

The general forms of gastropod shells commonly reflect compound features produced by combinations of specific shell features. Thus very different combinations of specific features produce the same general gross trait whereas fairly similar combinations of specific features sometimes produce different gross traits. Although iterative evolution of general shell forms is thought to be common among gastropods, such features provide the basis for much of higher taxonomy (especially among Paleozoic clades). To ideas about general shell forms, we explored evolutionary patterns within a general shell form space. We scored over 600 latest Cambrian - Devonian gastropod species for 1) Raup’s translation; 2) umbilicus size; 3) gross aperture shape; 4) long-axis of apertural orientation; 5) apertural inclination; 6) left-right symmetry; and, 7) base shape. Species clusters based on these features are similar to traditional classifications, but fairly different from estimated phylogenetic clusters. For example, macluritiforms (nearly-planispiral with flat bases and long axis parallel to the coiling axis) appear at least 7 times. Trochiforms (moderate spire height, inclined apertures, round aperture shapes) appear at least 8 times. Subulitiforms (high spired with highly asymmetrical apertures and siphonate bases) appear at least 3 times, and raphistomiforms (moderate spire with asymmetrical apertures and weakly siphonate bases) appear at least 4 times. Monte Carlo tests reject the hypothesis that phylogeny and independent character evolution autocorrelation explain species distributions in the general character space. However, these tests also reject the hypothesis that gross shell forms are randomly distributed among clades. These results suggest that: 1) general forms generally reflect functional biology; 2) similar functional morphologies evolved repeatedly during the Paleozoic; and, 3) deriving some functional types was more probable in some clades than in others.