The Late Triassic Crisis and the Lazarus Effect: Evidence from the Bivalve Fossil Record of Northwest Europe

The quality of the Triassic – Jurassic bivalve fossil record in northwest Europe has been measured using the Simple Completeness Metric (SCM). The SCM has been applied to the fossil record of total bivalve diversity and to the records of different ecological guilds. Quantitative paleoecological analyses of the Late Triassic biotic crisis in southwest UK have demonstrated that a significant bio-event took place during the interval spanning the upper Westbury Formation to lower Lilstock Formation (Late Rhaetian). SCM data from this present study indicate that this event is unlikely to be an artefact of reduced fossil record completeness, at least among the Bivalvia. The ‘Pre-Planorbis Beds' of the lower Lias Group, however, witness a precipitous decline in the completeness of most bivalve guilds and emigration of taxa due to localized marine anoxia is a likely cause. Neither variation in lithofacies, shell mineralogy, sedimentary rock outcrop area, nor sequence architecture can convincingly explain the observed patterns of completeness. Our SCM data reveal that the Early Jurassic fossil record of infaunal suspension-feeding bivalves is significantly poorer than that of epifaunal bivalves. Any differences in the apparent Rhaetian extinction rates between these two guilds should therefore be viewed with caution. Analyses of selectivity during the Late Triassic mass extinction based on studies of global databases appear robust in light of our SCM data. Nevertheless, future investigations of the Triassic – Jurassic benthic marine ecosystem undertaken at a finer-resolution, may need to account for the poor quality of the Early Jurassic fossil records of certain ecological guilds, such as the infaunal suspension-feeding taxa.