Ashley Dineen

ABSTRACT The end-Permian mass extinction was the most devastating loss of life in Earth’s history. Evidence suggests that ecological devastation following this event was protracted and may have lasted 5 million years until the Middle Triassic (Anisian). Despite this, the timing and nature of full biotic recovery is not completely understood. Previous work has based the onset of biotic recovery on generic and species diversity and the reappearance of metazoan reefs, largely from broad global datasets. However, emerging research shows that community recovery should be based on more than these parameters, and that it can vary in time and space. Further fieldwork is needed to understand patterns of post-extinction community ecology. Previous understanding of the timing and spatial pattern of recovery has been confined mostly to shallow water environments in low latitude settings. However it has been proposed that shallow-marine environments may have acted as refuges from toxic deep ocean conditions, and thus it is hypothesized that shallow and deep marine communities recovered very differently. Recent data has also shown that Triassic recovery has paleogeographic and clade-specific dynamics (Brayard, 2009; Song et al., 2011). But before this can be tested, a working definition of recovery needs to be established. A previous definition of recovery (Krassilov, 1996) states that a community can be considered fully recovered when normal ecosystem functioning has resumed and previous dominance and diversity are regained. The study herein has defined community recovery not only by high diversity and abundance, but also by larger organism size and high evenness and tiering. This study aims to establish the spatial and temporal nature of ecosystem recovery following the end-Permian mass extinction. Fieldwork was conducted on the Lower Triassic (Olenekian) Anshun and the Middle Triassic (Anisian) Qingyan Formations in Guizhou Province, south China. Preliminary results indicate that while Middle Triassic benthic marine communities were characterized by high diversity, tiering, evenness, and organism size were not comparable to those of pre-extinction (Permian) communities. This demonstrates that full biotic recovery may have taken longer than previously recognized in south China.

ABSTRACT 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.

ABSTRACT Published data has been interpreted as indicating that marine ecological devastation following the end-Permian mass extinction was protracted and may have lasted 5 million years into the Middle Triassic (Anisian). However, a review of previous literature shows that understanding of biotic recovery is typically based on only a few components of the ecosystem, such as on taxonomic diversity, a single genus/phylum, or shallow water facies. Typically, paleocommunities are considered fully recovered when 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 and ecologic structure changed, with the extinction marking the faunal shift from brachiopod-rich Paleozoic Evolutionary Fauna (EF) to the mollusc-rich Modern EF. This suggests that the extreme reorganizational nature of the Triassic does not adhere to the standard definition of recovery, which is a return to previous conditions. Thus, we propose the term “restructuring” to describe this interval, as Early and Middle Triassic communities might not exhibit the typical characteristics of a “normal” Permian one. To more fully characterize Triassic ecologic restructuring, paleoecologists should take into account functional diversity and redundancy. We quantified functional richness and regularity in four different paleocommunities from classic Permian and Triassic sections. Functional richness was low in paleocommunities after the end-Permian mass extinction, but increased to high levels by the Middle Triassic. In contrast, functional regularity was low in the Middle Permian, but high in all the Triassic paleocommunities. The change from low to high functional regularity/redundancy at the P/T boundary may be a factor of the highly stressful Triassic environmental conditions (i.e. anoxia, hypercapnia), as high regularity in a community can boost survival in harsh environments. Parameters such as these will more accurately establish if the biotic patterns represent either failed biotic restructuring or a fully restructured marine community adapted to harsh Triassic environments.