EVOLUTION OF ALTERNATIVE ECOLOGICAL STATES IN THE EARLY TRIASSIC

The end-Permian mass extinction represents the largest loss of taxonomic diversity in the history of life. The Early and Middle Triassic represent the longest adjustment interval observed for benthic environments in a post-mass extinction system. During this time almost all functional groups present in the Permian were represented somewhere in the Triassic. However, taxonomic diversity, community composition, and ecological stability varied widely. The predominance of short-lived “disaster taxa” may represent the establishment of post-perturbation alternative state communities which thrived in the aftermath of the extinction and dwindled as environmental disturbance intensity and frequency diminished by the Middle Triassic.

‘Flat-clams’ and microbialites have been labeled as “disaster taxa” because of their dominance directly after the end-Permian mass extinction and their decreased abundance by the Middle Triassic. Recently, microconchids and unique foraminifera have been deemed additional “disaster taxa”. The co-occurrence of multiple disaster species in the Griesbachian Stage in the Italian Werfen Formation as well as sections in South China, Hungary, and Western Panthalassa might be better described as complex, wide-spread alternative state communities. For example, microconchids relied on microbialites and flat-clams to provide a substrate for colonization. Microbialites may have been responsible for oxygen production and seafloor stabilization. Griesbachian alternative state communities thrived under extreme temperature and low oxygen conditions but were easily displaced by Dienerian mollusc-dominated benthic communities. The re-establishment of complex ichnofauna and a more ‘Mesozoic-like’ benthos marked the end of alternative ecological states.

Frequent perturbations in the Early Triassic may have created additional ‘reactive communities’ among benthic gastropods and pelagic ammonoid cephalopods. Multiple extreme temperature perturbations likely advanced microgastropod dominated communities while low oxygen and low nutrient conditions may have maintained selection for small body size. Ammonoids exhibited high resilience to environmental change, through the re-occupation of diverse shell morphologies despite repeated taxonomic diversity loss.