SUBSEQUENT BENTHIC BIOTIC CRISES IN THE WAKE OF THE LATE PERMIAN MASS EXTINCTION EVENT

New paleontological data from a number of localities around the world shows that initial recovery of marine ecosystems following the late Permian mass extinction occurred locally within the first substage of the Early Triassic (i.e. Griesbachian; early Induan). Yet advanced ecosystem recovery is often not recorded until the late Spathian (latest Olenekian) and full ecosystem recovery not until the Pelsonian or Illyrian (Anisian). One hypothesis is that repeated environmental disturbances following the late Permian mass extinction event, recorded by carbon isotope fluctuations, directly delayed biotic recovery. Global analyses of ammonoid and conodont fossil records in the wake of the extinction support this hypothesis, and show that their recovery was interrupted by further crises that coincide with carbon isotope perturbations at the Early Triassic substage boundaries. For the benthos, however, there is little evidence that they were affected by these subsequent crises. Here, we investigate if the marine benthos were impacted by events at the sub-stage boundaries during the Early Triassic.

Quantitative relative-abundance data of fossil benthic macroinvertebrates were collected from rocks in northern Italy that were deposited in a mixed siliciclastic-carbonate ramp setting on the northwestern margin of the Paleotethys Ocean that provides a continuous fossil record through the entire Early Triassic. Taxonomic diversity of the Werfen Formation shows elevated extinction rates during the Dienerian and at the Smithian/Spathian boundary. Functional richness - a measure of ecological diversity - was, however, only demonstrably affected in the late Dienerian in association with a cluster of isotope perturbations. Benthic assemblages from the Griesbachian, Dienerian and Smithian have very similar compositions, mainly characterised by various cosmopolitan bivalve, gastropod and microconchid biofacies. A significant change in composition was recorded across the Smithian/Spathian boundary with a change to Natiria costata, Neoschizodus ovatus and Holocrinus biofacies. Our results show, therefore, that the recovery of benthic ecosystems was interrupted by additional crises in the late Induan (Dienerian) and mid-Olenekian (Smithian/Spathian) only.