ESTABLISHING A “NEW NORMAL” IN THE EARLY AND MIDDLE TRIASSIC: NOVEL VIEWS ON MARINE COMMUNITY RECOVERY AFTER THE END-PERMIAN MASS EXTINCTION

Evidence suggests that marine ecological devastation following the end-Permian mass extinction was protracted and may have lasted 5 million years into the Middle Triassic (Anisian). Despite this, the timing and nature of full biotic recovery in the Triassic is not completely understood. Previous work has based biotic recovery on only a few components of ecosystem recovery, such as generic and species diversity, the reappearance of reefs, and broad global datasets. However, emerging Triassic research shows that recovery has spatial and temporal dynamics indicating a need to develop new parameters that more accurately describe the complexity of the recovery. For example, variations in the rate of faunal recovery have been identified between nektonic and benthic fauna (Brayard et al., 2009) as well as shallow and deep water environments (Beatty et al., 2008).

Biotic communities are typically considered fully recovered when previous dominance and diversity are regained and normal ecosystem functioning has resumed. However, in addition to the biodiversity crash at the end of the Permian, taxonomic structure changed as well, with the extinction marking the faunal shift from brachiopod-rich Paleozoic Evolutionary Fauna (EF) to the mollusc-rich Modern EF. Thus, the biota was evolutionarily and ecologically altered, resulting in Triassic marine communities that are biologically unlike their Permian counterparts. Therefore caution should be taken in comparing the two communities, as recovered Triassic communities might not exhibit the typical characteristics of a “normal” Permian one (e.g. large organism size, brachiopod dominance, high levels of tiering).

To more fully characterize the recovery, a new definition of “normal” should be established for Triassic communities. We propose that ecologic recovery in the Triassic should not only take into account diversity and abundance, but also evenness, organism size, and guild structure. These parameters will more accurately establish if the biotic pattern represents either a “recovering” community or a new “normal” for marine communities in the aftermath of the end-Permian mass extinction. Further discussion and innovative approaches to quantifying the Triassic recovery will help to advance all fields of research on this significant interval.