PRICE, R.M.; Univ. of North Carolina, Chapel Hill: Why neogastropod shell ornamentation does not correlate with latitude in the North Eastern Pacific

In a pervious study, I found that the amount of external ornamentation in neogastropods from the North Eastern Pacific is independent of latitude, a result that rejects the hypothesis that morphological diversity parallels the well-established latitudinal species diversity trend. Here, I explore why ornamentation and latitude are not correlated. The analysis includes 659 species and their latitudinal ranges (compiled by K. Roy, D. Jablonski, and J. Valentine) and 25 presence/absence characters describing ornamentation. Four clades that lack ornamentation and are largely restricted to the tropics (the cones, marginellids, olives, and olivellids) decrease the median ornamentation score of tropical species. Analyses that replaced smooth species with simulated, ornamented species confirm that the null model does not predict the observed pattern; the effect that the smooth species decrease ornamentation scores in the tropics is statistically significant. Results from a Principal Coordinate Analysis concur that external ornamentation scores for all species are not correlated with latitude. The first two principal coordinates are not correlated with latitude (Spearman�s Rank test, P < 0.07). Furthermore, the number of species with a character correlates more closely with species richness than with latitude (for spiral ornamentation, for example, P << 0.001 for richness, but P = 0.38 for latitude; F-test in multiple regression). Thus, external ornamentation in neogastropods is not significantly higher in the tropics because (1) smooth families obscure differences in ornamentation across climatic zones, (2) the total amount of external ornamentation is not correlated with latitude, and (3) the probability that a character is present in a species is independent of latitude.