BIOENERGETICS OF EARLIEST JURASSIC MARINE ECOSYSTEMS: SLOW METABOLISMS, SLOW CARBON CYCLING

Marine fauna that flourished in the aftermath of the end-Triassic mass extinction differed profoundly from pre-extinction occupants, in terms of taxa, ecological structure, and biogeography. These new communities maintained prominence throughout the first two million years of the Jurassic period, but reasons for this relative ecological stability are unclear. I apply simple bioenergetics models to reconstruct plausible trophic structure and first-order carbon cycling rates for new ecosystems. The baseline for these models is recent research into the inverted trophic pyramids of many modern-day marine systems: compared to terrestrial settings, biomass is more heavily concentrated in predatory tiers, and carbon cycles more rapidly through the standing stock of autotrophs. This structure has been demonstrated across myriad different modern settings, from lagoons to the open ocean. In the Early Jurassic, benthic habitats were dominated in many places by siliceous demosponge meadows, representing large accumulations of biomass at a low trophic level, in animals with potentially very low metabolic rates. Many Early Jurassic pelagic settings were dominated by ammonites, particularly cosmopolitan species with simply-coiled shells that reached large sizes (>40 cm). The main food source for these ammonites was probably other ammonites, which introduces the challenge of defining trophic ranks to cannibals. The benthic analysis compares the sponge regimes to a Triassic-style coral reef system, and the pelagic model compares the ammonites to a modern-day market squid population-scale model. In both cases, the earliest Jurassic communities represent more bottom-heavy structures than their modern or pre-extinction counterparts. Perhaps more interesting, if not surprising: the potential for relatively low metabolic rates in the dominant animals of the earliest Jurassic allows considerably lower carbon cycling rates at the ecosystem scale. I compare these models to contemporaneous carbon isotopic data, and speculate that particular faunas can heavily influence the pace of whole-ecosystem recovery from mass extinction.