Patterns and processes in the history of body size in turritelline gastropods, Jurassic-to-Recent

Body size is an important trait with implications for energy use and
ecology as well as generation time and evolutionary rates. Turritelline
gastropods are widely distributed through geologic time and space, making
them an excellent group for evaluating macroevolutionary patterns. To
evaluate the pattern of body size change in turritelline gastropods, we
compiled a dataset of shell lengths of 316 species of turritelline
gastropods spanning the Jurassic to Recent. Type specimens were almost
always significantly larger than specimen distributions from the same
species. We found that 1) turritelline gastropod size was inversely
correlated with latitude, a trend likely driven by the Neogene-Recent
diversification of small-bodied Southern Hemisphere taxa. A time series
model was applied to distinguish among three possible macroevolutionary
patterns: unbiased random walk (no directional trend), biased random walk
(directional trend), and stasis (no net change). We found that 2)
turritelline gastropods have experienced stasis in body size throughout
their evolutionary history, adding to the growing literature documenting
directionless body size trends in marine invertebrate clades. Stasis of
geographically widespread clades may be the result of ecological
variability across the environmental range occupied by the group or
differential diversification into opposing environments. Turritelline
life-history patterns, especially their reproductive strategy that
combines a short lifespan and decline in growth rate around 1 year of age
to reallocate energy to reproduction, might circumvent selection for
longevity and larger size while further decrease in minimum size is likely
limited by feeding efficiency and anti-predatory defense. The expectation
that species or clades should continue to evolve to occupy larger size
classes conflicts with the evolutionary advantages of small size, which in
turritelline gastropods include high generational turnover and larger
population sizes that yield opportunities for genetic variance.