Changes of community structure in response to competition usually take place on timescales that are much too short to be visible in the geological record. Here we report the notable exception of a benthic marine community in the wake of the end-Permian mass extinction, which is associated with the microbial limestone facies of the earliest Triassic of South China. The newly reported fauna is well preserved and extraordinarily rich (30 benthic macroinvertebrate species, including the new species Astartella? stefaniae (Bivalvia) and Eucochlis obliquecostata (Gastropoda)) and stems from an environmentally stable setting providing favourable conditions for benthic organisms. Whereas changes in the taxonomic composition are negligible over the observed time interval of 10-100 ka, three ecological stages are identified, in which relative abundances of initially rare species continuously increased at the cost of previously dominant species. Concomitant with the changes of dominant species is an increase in faunal evenness and hetero-

New investigations in the Nanpanjiang Basin indicate that the onset of the iconic microbialites associated with the Paleozoic-Mesozoic boundary was Early Triassic in age. Bathymetry (water agitation, oxygenation, light penetration) and clastic load are shown to have exerted a direct control on the growth of microbialites. Carbonate supersaturation is also required for the deposition of the microbialites. Bathymetric control is further corroborated by the topographic inheritance of a latest Permian pull-apart basin into Early Triassic times, with a distribution of basal Early Triassic microbialites (BETM) restricted to uplifted blocks. This control is also reflected by the accumulation of carbonaceous black shales in adjacent troughs. The geographically most extensive Nanpanjiang BETM bloomed on a large NW-SE trending uplifted block exceeding 12,000 km2 (Luolou Platform) centered on northwestern Guangxi. Post-Triassic displacements along the Youjiang Fault obscure the paleogeographic relation of BETM exposed west of this fault. Triassic foraminifers occur in the basal most BETM episode, which is locally bracketed by high-energy grainstones made of reworked Permian foraminifers. Therefore, the Permian-Triassic boundary (PTB) is within the unconformity that separates the Late Permian Heshan Fm. from the basal most BETM. Where accommodation space was sufficient, up to five event surfaces are associated with the unconformity. Microfacies analysis supports chemical dissolution but did not reveal evidence for subaerial erosion, although intercalated grainstone made of Permian foraminifers indicate reworking. Chemical dissolution and mechanical erosion both conceivably contributed to the genesis of the unconformity. The upward shift from tabulated to domical microbial build-ups is accompanied by accumulation of coquinoid lenses between domes, which indicates deepening of the Luolou Platform BETM. The main drowning resulting from both regional tectonic subsidence and a global sea-level rise led to the cessation of the BETM that were buried under predominant fine siliciclastics. Any concomitant change in sea water chemistry appears unlikely.

New high-resolution U-Pb dates indicate a duration of 89 ± 38 kyr for the Permian hiatus and of 14 ± 57 kyr for the overlying Triassic microbial limestone in shallow water settings of the Nanpanjiang Basin, South China. The age and duration of the hiatus coincides with the Permian-Triassic boundary (PTB) and the extinction interval in the Meishan Global Stratotype Section and Point, and strongly supports a glacio-eustatic regression, which best explains the genesis of the worldwide hiatus straddling the PTB in shallow water records. In adjacent deep marine troughs, rates of sediment accumulation display a six-fold decrease across the PTB compatible with a dryer and cooler climate as indicated by terrestrial plants. Our model of the Permian-Triassic boundary mass extinction (PTBME) hinges on the synchronicity of the hiatus with the onset of the Siberian Traps volcanism. This early eruptive phase released sulfur-rich volatiles into the stratosphere, thus simultaneously eliciting a short-lived ice age responsible for the global regression and a brief but intense acidification. Abrupt cooling, shrunk habitats on shelves and acidification may all have synergistically triggered the PTBME. Subsequently, the build-up of volcanic CO2 induced a transient cool climate whose early phase saw the deposition of the microbial limestone.

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