2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 10
Presentation Time: 1:30 PM-5:30 PM


PRICE-LLOYD, Nathan and TWITCHETT, Richard John, Earth Sciences, Univ of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, United Kingdom, np9473@bris.ac.uk

Fossil taxa in the immediate aftermath of mass extinction events are much smaller than in the pre-extinction fauna. This 'Lilliput effect' is widespread and has been documented in the aftermath of many extinction events and affects many different groups of organisms. The cause(s) of this phenomenon remain unknown. Results of statistical studies show that there is no selection for smaller organisms during mass extinction events: the larger taxa should be there, but are not found as fossils, and size decrease within lineages is often temporary. Twitchett (2001) suggested that the Lilliput effect is due to a temporary decrease in food supply, and knock-on reduction in the biomass of higher trophic levels. Under such conditions, larger taxa become rarer, and are less likely to enter the fossil record, whereas taxa that undergo a reduction in body size maintain population densities and are more likely to enter the fossil record.

We present the first quantitative study of within-lineage size decrease, and subsequent increase, through the Permian-Triassic (P-Tr) mass extinction interval. Four unrelated taxa were studied from P-Tr sections of northern Italy. The mean size of Late Permian Bellerophon is 17 mm (range 9-40 mm), which falls to 5 mm (range 3-8 mm) in the Early Griesbachian (parvus Zone). Modest size increase took place in the later Griesbachian (carinata Zone) before final disappearance of the taxon. Late Paleozoic Lingula are typically 10+ mm long, whereas parvus Zone examples are just 3 mm long (range 1.5-4 mm). Pre-event size is reached in the carinata Zone (mean 11 mm, range 6-13 mm). The bivalves Unionites and Claraia are also very small in the Early Griesbachian (mean lengths 4 and 7 mm respectively, ranges 1-8 and 1-21 mm). Both taxa increase in size in the carinata Zone (means 18 and 24 mm, ranges 6-32 and 10-50 mm). All differences are statistically significant. Size decrease and subsequent increase closely mirrors the fluctuating d13C curve, which may be a proxy for productivity levels. Changes are unrelated to benthic oxygen levels. Growth line analysis reveals that the size changes result from changes in both rates of growth and longevity of the animals.

Twitchett, RJ. 2001. Incompleteness of the Permian-Triassic fossil record: a consequence of productivity decline? Geological Journal, 36: 341-353.