A new paleoecological look at the Dinwoody Formation (Lower Triassic, western USA): intrinsic versus extrinsic controls on ecosystem recovery after the end-Permian mass extinction

Abstract

The Dinwoody Formation of the western United States represents an important archive of Early Triassic ecosystems in the immediate aftermath of the end-Permian mass extinction. We present a systematic description and a quantitative paleoecological analysis of its benthic faunas in order to reconstruct benthic associations and to explore the temporal and spatial variations of diversity, ecological structure and taxonomic composition throughout the earliest Triassic of the western United States. A total of 15 bivalve species, two gastropod species, and two brachiopod species are recognized in the study area. The paleoecological analysis shows that the oldest Dinwoody communities are characterized by low diversity, low ecological complexity and high dominance of few species. We suggest that this low diversity most likely reflects the consequences of the mass extinction in the first place and not necessarily the persistence of environmental stress. Whereas this diversity pattern persists into younger strata of the Dinwoody Formation in outer shelf environments, an increase in richness, evenness and guild diversity occurred around the Griesbachian–Dienerian boundary in more shallow marine habitats. This incipient recovery towards the end of the Griesbachian is in accordance with observations from other regions and thus probably represents an interregional signal. In contrast to increasing richness within communities (alpha-diversity), beta-diversity remained low during the Griesbachian and Dienerian in the study area. This low beta-diversity reflects a wide environmental and geographical range of taxa during the earliest Triassic, indicating that the increase of within-habitat diversity has not yet led to significant competitive exclusion. We hypothesize that the well-known prevalence of generalized taxa in post-extinction faunas is primarily an effect of reduced competition that allows species to exist through the full range of their fundamental niches, rather than being caused by unusual and uniform environmental stress.

Recovery of benthic marine communities from the end‐Permian mass extinction at the low latitudes of eastern Panthalassa

“…This procedure has been applied successfully in Hofmann et al . (, b ) to detect recurrent assemblages of taxa in comparable data sets. All subclusters and respective samples with a Morisita similarity approximately higher than 0.4 were pooled into associations (Aberhan ).…”

“…Our basic measure of alpha‐diversity is species richness, whereas beta‐diversity (between associations/habitat diversity) is measured by the mean minimum beta‐diversity approach proposed by Hofmann et al . (). Data for this paper, including an Excel file with data sheets showing the full lists of species occurrences, associations and computed alpha‐ and beta‐diversities, are available in the Dryad digital repository (Hofmann et al .…”

“…, ; Hofmann et al . , ) provided evidence that the recovery from the end‐Permian mass extinction was more volatile than traditionally portrayed (Hallam ; Schubert and Bottjer ; Twitchett and Wignall ), suggesting that ecosystems suffered from several short‐term environmental perturbations during the Early Triassic rather than from long‐lasting environmental stress. Furthermore, it has been suggested that intrinsic aspects (Schubert and Bottjer ), most notably reduced interspecific competition, had an important impact on the pace of recovery (Hofmann et al .…”

“…Furthermore, it has been suggested that intrinsic aspects (Schubert and Bottjer ), most notably reduced interspecific competition, had an important impact on the pace of recovery (Hofmann et al . , b ). The Lower Triassic of the western USA provides one of the best records of Early Triassic benthic ecosystems worldwide, which facilitates tracing benthic recovery from the end‐Permian mass extinction through different time slices, represented by comparable sets of habitats from the same palaeogeographical region.…”

“…; Hofmann et al . , b ), we re‐evaluated the taxonomy and palaeoecology of the Griesbachian–Dienerian and the Spathian faunas from the western USA. Here, we add data on Smithian – early Spathian benthic palaeocommunities of the same region.…”