TURRITELLIDS TAKE OVER: LATITUDINAL RANGE EXPANSION AND BODY SIZE INCREASE IN TURRITELLID GASTROPODS SINCE THE LATE CRETACEOUS

Cope’s Law of the Unspecialized predicts that the start of a clade lies in an unspecialized organism whose upper evolutionary limit for body size far exceeds the lower limit. We might predict an overall increase in body size and number of species when using the gastropod genus Turritella as a model. To test this hypothesis, log-transformed shell length of type specimens, first appearance date (FAD), and the biogeography of 341 fossil and recent species of Turritella were analyzed.

Spearman rank analysis found a significant inverse relationship between the age of turritellid species origination (analyzed as FAD values) and the maximum shell lengths of type species. Kruskal-Wallace analysis by period (Cretaceous, Paleogene, Neogene) displayed no significant difference in the variance of shell length however additional analysis at finer temporal resolution may reveal an increase in variance through time. Turritellids imperfectly embody Cope’s rule. They originate at a medium-small size in the Early Cretaceous and as they diversified through the Recent they expanded in maximum size as predicted, but in contrast with Cope’s hypothesis turritellids also exhibit a decrease in minimum size.

There was a significant difference in the latitudinal occupation of turritellid gastropods through time. Turritellids underwent a significant increase in latitudinal occupation from the Cretaceous to the Paleogene. Turritellids were more diverse in non-tropical latitudes during the Cretaceous and significantly increased their occupation of lower latitudes in the Cenozoic, via tropical migration, while still retaining coverage in the upper latitudes. There was no significant difference between shell size in tropical vs. non-tropical species defined as above or below 30° latitude. Additional turritellid diversity and body size data will contribute to refining these findings.