ECOSYSTEM REMODELLING AMONG VERTEBRATES AT THE PERMO-TRIASSIC BOUNDARY IN RUSSIA

The mass extinction at the Permian-Triassic boundary (PTB), 251 Myr ago, is accepted as the most profound loss of life on record. Global data compilations indicate a loss of 50% of families or more both in the sea and on land, and these figures scale to a loss of 80-96% of species, based on rarefaction analyses. This level of loss is confirmed by local and regional-scale studies of marine sections, but the terrestrial record has been harder to analyse in such close detail.

The South Urals succession is about 1 km thick, and consists of river-deposited mudstone and sanddstone spanning from Kazanian to Ladinian. A dramatic alluvial-fan conglomerate, with boulders up to 1m across, marks the basalmost Triassic, indicating a surge in orogenic activity in the heart of the Urals, aridification, and stripping of soils, possibly by acid rain following massive eruptions.

A comprehensive survey of 675 specimens of amphibians and reptiles from 289 localities spanning 13 successive geological time zones in the South Urals basin documents the nature of the event in Russia. There was a profound loss of genera and families, and simplification of ecosystems, with the loss of small fish-eaters and insect-eaters, medium and large herbivores, and large carnivores. Faunal dynamics changed too, from high rates of turnover through the Late Permian to greater stability at low diversity through the Early Triassic. Even after 25 Myr of ecosystem rebuilding, some guilds were apparently still absent – small fish-eaters, small insect-eaters, large herbivores, and top carnivores. The Russian series stops at the end of the Ladinian, but vertebrate faunas from other parts of the world show that these remaining guilds were in place by the Carnian, indicating that it took at least 30 Myr for ecosystems to recover to pre-PTB complexity.

These changes in diversity and turnover cannot be explained simply by sampling effects. The richest samples in the Russian succession are those immediately following the PTB, when diversity was lowest. A range of sampling standardization protocols, including rarefaction, suggest that the patterns cannot be explained as geological or sampling artifacts, and that they may be real.