Benthic ecosystem dynamics following the Late Triassic mass extinction event: Palaeoecology of the blue llas formation, Lyme Regis, UK

The Blue Lias Formation of southwest UK records the evolution of a marine, shelf ecosystem following the Late Triassic extinction event. In order to investigate responses of the local palaeocommunity to environmental changes during this critical interval in Earth history, a quantitative palaeoecological analysis of the well exposed, fossiliferous and historically important sedimentary succession between Pinhay Bay and Lyme Regis, southern England, was undertaken. Quantitative palaeoecological data were collected from 19 limestone beds of the Rhaetian-Sinemurian Blue Lias Formation. For each sampled bed, all macroinvertebrate fossil remains found within two 50 x 50 cm quadrats placed on the exposed bedding plane surface were counted and identified. Palaeoecological changes were assessed through analyses of diversity, richness, evenness, abundance and occupation of ecospace. There is a general trend from palaeocommunities with low diversity, richness, abundance and evenness around the Triassic/Jurassic boundary to those with higher diversity, richness, abundance and evenness in the late Hettangian, but there are intervals of relative stasis as well as significant shifts within biozones and even between adjacent beds. Most of the Rhaetian-Hettangian palaeocommunities are dominated by low-level, surficial, suspension feeders, but the appearance of higher- and deeper-tier organisms, and an increase in motility and feeding styles, characterise key stages of post-extinction recovery. The earliest Sinemurian assemblages show a return to low diversity, low evenness assemblages, but with high abundance. Where there is a stratigraphic overlap, the palaeoecological changes recorded in this study are comparable to those recorded from other southwest UK sites, despite differences in sampling methodology, and are mirrored by changes in the trace fossil assemblages. Local palaeocommunities were probably responding to local and global environmental and climatic changes, perhaps driven by changes in atmospheric CO2 through the aftermath of the Late Triassic extinction event.