SIZE CHANGE IN COLONIAL SCLEROBIONTS AT THE K–T MASS EXTINCTION

A widely observed phenomenon following mass extinction events is a decrease in organism size. This trend, the Lilliput Effect, has been observed in a range of taxa across all mass extinction events and implies that organisms may respond in a predictable manner to biotic crises. Although post-extinction size decrease appears to be common in the fossil record, the process is still poorly understood and has recently come under some dispute. Further study of its existence and influence are required. Bryozoan zooids are ideal fossil tools from which to measure size variation over time as once they are budded their size remains fixed. A number of environmental variables have been shown to influence living bryozoan zooid size, particularly seawater temperature, and fossil zooid size variation can therefore indicate past environmental changes. Analysis of zooid size change also allows insight into the response of colonial animals to mass extinction events in comparison to that of unitary animals shown in previous Lilliput studies. The sizes of encrusting cheilostome bryozoan zooids were compared across the K-T boundary of south-eastern USA. Conflicting variation in size over the mass extinction event between different species indicates a differential response between species and suggests that the Lilliput Effect does not occur in all organisms at this time.