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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SurE-TESTS (Surviving extinction: traits encouraging survival at times of stress)

Teaser

Summary

Understanding the causes of mass extinctions is one of the key issues being addressed in Earth Sciences. These ancient catastrophes record severe environmental changes and help us understand the extreme spectrum of conditions that life on earth has faced and inform us about the effects of extreme climate change that has relevance to our understanding of the present day planet. However, being able to recognise and disentangle the multiple proposed causes of mass extinctions is not easy. Causation has been linked with effects of giant meteorite impact for one mass extinction whereas the majority of the others are linked to large-scale volcanism. The effects of this latter cause include rapid emission of carbon dioxide to the atmosphere and its consequential effects: global warming, ocean acidification and changes in ocean circulation leading to widespread anoxia. The problem is working out the relative importance of these changes and why they lead to extinction. The objectives of this research are to compile a major database on marine invertebrate generic ranges in the interval from 270-190 million years ago at a high temporal resolution and use this to calculate extinction rates through an interval characterised by two mass extinctions and several minor crises. This will allow selectivity of extinction to be evaluated and thus the role of key parameters at times of environmental crisis. Extinction risk is primarily controlled by group and geographic range during times of background extinction and so by comparing this background signal with extinction losses at times of mass extinction, the enhanced risk due to factors unique to this interval will be studied.

Work performed

To find out the critical factors that allow species to survive during times of extreme stress, Dr. Song has compiled a fossil database at high temporal resolution for the interval from the latest Permian to the start of the Jurassic. This database consists of over 50,000 fossil occurrences derived from ~1500 literature sources. Based on the database, we evaluate the magnitude of the background extinction rates in the Early Triassic and shown that the late Smithian extinction was the third biggest crisis in the past 250 million years (Myr) based on the generic extinction rates metric. Analysis of the fossil database has discovered the surprising result that the ecological structure in Triassic oceans changed at a pace that was an order magnitude slower than the recovery of diversity. This important result has shown the top-down rebuilding of marine ecosystems was still underway in the latest Triassic, ~50 Myr after the mass extinction, and contrasts with the ~5 Myr recovery required for biodiversity recovery. It also shows that trends implicated in the end-Triassic mass extinction (~50 myrs after the end-Permian extinction) were actually part of the long-term changes still underway after the earlier extinction. Previously late Triassic diversity/ecology trends have been explained as a precursor to the end Triassic mass extinction when in fact, in context, they are seen to be a legacy of the preceding mass extinction at the end of the Permian.

Two manuscripts have been written about these important discoveries:-
(1) Song H.J. and P.B. Wignall, The late Smithian extinction was the third biggest crisis of the past 250 million years, submitted to Geology.
(2) Song H., P.B. Wignall, and A. Dunhill, Prolonged top-down recovery of marine ecosystems in Triassic oceans. In preparation for submission to SCIENCE.

Final results

The compilation of a marine invertebrate generic ranges database has revealed several major events and trends that were not hitherto clearly shown or expected. These include the quantification of extinction rates in the late Smithian Substage (in the Early Triassic, 248 million years ago) - previously only identified as a crisis - and elevates it to a major mass extinction with a magnitude the third highest in the past 250 million years. Analysis of the database has also revealed a disconnect between ecological recovery from the Permo-Triassic mass extinction and the recovery of biodiversity; the former is seen to be much slower than the latter with the result that nekton-rich, benthos-poor communities dominated Triassic oceans for tens of millions of years. This has indirect impact for understanding the long-term impact of the ongoing sixth mass extinction, although this is changing marine ecosystems in distinct way by eliminating the top of the pelagic food chain and doing less damage to the seabed communities.