PRICE, RM; University of Chicago: Constant function or random evolution: changes in the apertural ornamentation of fusiform neogastropods since the Cretaceous

Columellar folds, ridges visible on the inner lip of a gastropod’s aperture, are useful in distinguishing species, but the evolution of folds has hardly been considered even though they evolved in at least 6 neogastropod clades. I test the hypotheses that most fold shapes in fusiform neogastropods evolve in a short span of the Cretaceous, rather than slowly accumulating through time, and that once a particular fold shape appears, it persists into the Recent. The latter hypothesis implies either that specific functions require certain fold shapes and functional demands have remained unchanged, or that fold evolution was rapid and random, but limited by architectural requirements. In preliminary analyses, I characterized 289 specimens with 14 binary and multi-state characters describing the number, orientation, position, spacing, uniformity, and definition of columellar folds. 158 morphotypes are present, and maximum dissimilarity is 0.5, meaning that half of the character states are shared between the most disparate taxa. An eigenvalue decomposition reduced an unordered similarity matrix to two axes that summarize 85% of the data and represent the morphotypes as a single cloud of points. I grouped specimens into 10-million year bins to study whether fold shapes have changed through time. The boundaries of the fold-space appear early in history and remain virtually unchanged, supporting both hypotheses. Since fold shapes persist, no morphotype seems to offer any adaptive advantage over others from a species-selection point of view; no single fold type can explain the neogastropod radiation. Knowledge of fold function and the phylogeny of neogastropods are required to determine whether constant function or random change forms the pattern of fold evolution.