PATTERNS AND PROCESSES IN THE HISTORY OF BODY SIZE IN TURRITELLINE GASTROPODS, JURASSIC-TO-RECENT

Previous studies have shown that Phanerozoic marine organisms increased in size mostly by differential diversification of larger clades. This is not the same as support for Cope’s Rule, which is an increase in size within a lineage, presumably driven by natural selection. Turritelline gastropods have been cited as an example in which maximum size increased during the Cenozoic. We investigated the size evolution of modern and fossil turritelline gastropods to explore the dynamics of within-group size evolution.

In Recent turritelline gastropods in New Zealand, mixed effects models revealed no significant relationship between shell length and sea surface temperature or chlorophyll concentration, but did show an increase in size with decreasing latitude in two of four species. These results are similar to previous studies of latitudinal size trends in molluscs that fail to reveal the specific environmental parameters co-varying with latitude that would provide selection pressure.

We conducted time series analysis of over 380 turritelline species type specimen shell lengths spanning their entire evolutionary history and first differences of 22 time bins of geologic environmental proxy data tested against min, max, and mean shell length. Preliminary results do not support a driven trend, but rather an unbiased random walk (URW) composed of a combination of URW in the Mesozoic and stasis in the Cenozoic. This result could be interpreted as a response to selection pressures that lack a unidirectional trend through time and the role of regional selective pressures such as those observed in the Recent of New Zealand.

Turritelline gastropods originated at a moderate size in the Jurassic and experienced a substantial reduction of minimum shell length in the Mesozoic. Selective benefits of smaller size might include lower metabolic demands, shorter development time, and less risk exposure before reproduction. The dispersion of minimum and maximum shell length slowed near the Mesozoic/Cenozoic boundary, suggesting that while rapid initial length expansion may represent an increase in shell length variance through diffusive diversification from a moderate sized ancestor, it was also tempered by physiological lower limits and selective upper limits on size.