Early Triassic recovery from the end-Permian extinction of benthic ecosystems in the palaeotropics

摘要

The end-Permian mass extinction is widely accepted to have been the greatest biotic crises in the history of meatzoan life. The understanding of the subsequent recovery during the Early Triassic is of utmost importance to address fundamental question in earth system science: (i) how do ecosystems respond to large-scale environmental stress, (ii) how quickly do ecosystems recover, and (iii) how do evolutionary processes operate under the unusual conditions of vacated ecospace? The first studies on Early Triassic communities suggested that no significant recovery took place during the entire stage with an approximate duration of 5 million years. It has been proposed that this pattern was caused by persistent environmental stress which delayed ecological and taxonomic recovery. However, this notion rests on data with limited taxonomic, stratigraphic, and environmental resolution. Furthemore, with the exception of some early studies, proposed models to explain the recovery pattern were exclusively based on extrinsic (abiotic) controlling factors whereas macro-ecological consequences of the extinction itself were largely ignored. The aim of the presented thesis is to reconstruct a detailed picture of the benthic recovery at the eastern and western tropical shelf of Pangaea. We use quantitative palaeoecological analysis of selected successions in the Western U.S. and the Dolomites to distinguish between local and global signals. The analysis of different depositional settings (e.g. deep vs. shallow shelf) should elucidate whether recovery was restricted to certain environments. Another central aspect of this thesis is to revisit the role of intrinsic controlling factors on the recovery. A first significant finding is that benthic ecosystem show a unexpectedly early global recovery signal during the upper Griesbachian, only some 0.5 Million years after the extinction. This questions the influence or the spatial and temporal extent of stress factors such as shallow marine anoxia. The data from the succeeding Dienerian time interval show that benthic ecosystem declined at least on an interregional scale approximately 1 million years after the extinction. The next and most significant recovery signal is observed in lower Spathian strata. During this time interval, 2 Million years after the extinction, many benthic organisms that dominated Mesozoic ecosystems such as bivalves, gastropods, porifers, echinoderms and articulated brachiopods became established in relatively diverse communities and represent the rootstock of the subsequent radiation. The comparison of both regions also shows that local stress factors may influence the overall recovery signal. The overall signal however, suggests that instead of persisting, repeated but short environmental perturbations contributed to the delayed recovery. The second aspect advanced by this thesis is the reconsideration of intrinsic factors on the recovery patterns. The analysis of the Virgin Formation of southwestern Utah has shown that many subhabitats were inhabitat by many generalistic species. Such traits were also traditionally interpreted to reflect environmental stress. This is, however, at variance with the finding that these communities were comparatively complex and diverse. Alternatively, we propose that the low number of specialised benthic organisms reflects a generally low rate of competition within these habitats. The aftermaths of large mass extinctions are typically characterised by a low habitat (alpha) diversity, which would imply a low rate of niche overlap and thus low rate of competition within ecosystems. Such conditions would allow species to thrive outside their ecological optima. This should be reflected in a low between habitat (beta) diversity. Accordingly, after a sufficiently severe mass extinction event, beta diversity should rise when alpha diversity reaches a level which drives ecosystems into a mode allowing for competitive exclusion. This evolutionary principle should than cause species’ to find “their” ecological optima along environmental gradients. A conclusive test of this model for Lower Triassic strata of the western U.S. confirms at least one prediction of this model: Whereas alpha diversity rises successively throughout the Early Triassic, beta diversity remained constantly low. If this model is correct, it could explain why the actual radiation of many benthic clades lags behind the definite onset of recovery. Future studies which will consider Middle Triassic and Permian ecosystems will help to test the validity of this model. The general low biodiversity especially during the earliest Triassic has usually been interpreted to reflect environmental stress. In this thesis, it is argued that the general usability of the relationship between low diversity and hostile conditions was at least diminished in the aftermath of the mass extinction which per definition reduces diversity in the first place. This thesis represents a significant advance in the understanding and reconstruction of ecosystem in the aftermath of the greatest mass extinction recorded in the Phanerozoic. The observed data a best explained by (i) short phased of environmental stress and (ii) the nonactualistic ecology which was a direct consequence of the dramatic loss in biodiversity and collapse of marine ecosystems.