Escargot through Time: Comparative Energetics of Neogene and Early Mesozoic Gastropod Assemblages

The global genus diversity of marine gastropods has increased by more than an order of magnitude over the past 250 million years. Much of this diversity increase occurred within the Neogastropoda, a primarily carnivorous clade. Gastropods also exhibit an increased prevalence of anti-predatory shell morphologies over the same interval. These and other observations have led several researchers to suggest that the taxonomic radiation of the Gastropoda was associated with an increase in the overall delivery of nutrients and primary productivity to marine ecosystems and a consequent escalation of energy use. Here we use body size and abundance data from fossil assemblages, metabolic data from living counterparts, and macroecological scaling principles to compare Middle and Late Triassic (pre-diversification) gastropod assemblages with Neogene (post-diversification) assemblages. We find that, due to an increase in mean body size as well as a shift in mean trophic level, the per-individual energy use of gastropod assemblages was likely significantly higher in the Neogene than in the Triassic. Estimates of energy use for tropical Neogene assemblages are similar to those of modern tropical shallow subtidal gastropod communities, suggesting that fossil data adequately capture energetic trends. Both Neogene and Triassic shallow subtidal energy use estimates are higher than estimates for modern gastropod assemblages in extremely resource-poor abyssal environments. These findings provide quantitative support for suggestions that the standing biomass and energetic demand of the benthos has increased through time, and that energy demand has tracked energy availability at least to a first order. However, additional data on absolute (vs. relative) abundance of gastropods and other major benthic groups are necessary to validate this hypothesis.