Incompleteness of the Permian-Triassic fossil record: a consequence of productivity decline?

Analysis of published data shows that, for most animal groups, the fossil r ecord in the immediate aftermath of the end-Permian biotic crisis is less c omplete than during the Late Permian or Middle Triassic. Completeness is me asured by the Simple Completeness Metric. The interval of poor quality foss il record spans the entire Lower Triassic and may have serious consequences for our perception of the magnitude of the end-Permian event. A model is p resented which seeks to explain this phenomenon. There is abundant evidence that levels of primary productivity were severely reduced in the very late st Permian. In response to this, animal biomass must also have been reduced . The biomass of a particular taxon is the product of the size of individua l organisms multiplied by the number of individuals. Those taxa that reduce d population size, but maintained original body size, would tend not to be preserved (apparent extinction) and would also be more prone to 'real' exti nction. Those taxa that retained large population sizes, but reduced body s ize, would resist extinction and would also maintain their presence in the fossil record. One testable prediction is that taxa present in the fossil r ecord in the immediate aftermath of the end-Permian crisis will have smalle r body size than their pre-event relatives, regardless of their initial siz e. Anecdotal evidence supports this prediction. Such a biomass reduction mo del may also be applicable to other mass extinction events. Copyright (C) 2 001 John Wiley & Sons, Ltd.