ASYMMETRY OF EARLY PALEOZOIC TRILOBITES

Asymmetry in fossils can arise through a variety of biological and geo logical mechanisms. If geological sources of asymmetry can be minimize d or factored out, it might be possible to assess biological sources o f asymmetry. Fluctuating asymmetry (FA), a general measure of developm ental precision, is documented for nine species of lower Paleozoic tri lobites. Taphonomic analyses suggest that the populations studied for each taxon span relatively short time intervals that are approximately equal in duration. Tectonic deformation may have affected the specime ns studied, since deviations from normal distributions are common. Sev eral measures of FA were applied to 3-5 homologous measures in each ta xon. Measurement error was assessed by the analysis of variance (ANOVA ) for repeated measurements of individual specimens and by analysis of the statistical moments of the distributions of asymmetry measures. M easurement error was significantly smaller than the difference between measures taken on each side of a specimen. However, the distribution of differences between sides often deviated from a mean of zero, or wa s skewed or kurtosic. Regression of levels of FA against geologic age revealed no statistically significant changes in levels of asymmetry t hrough time. Geological and taphonomic effects make it difficult to id entify asymmetry due to biological factors. Although fluctuating asymm etry is a function of both intrinsic and extrinsic factors, the result s suggest that early Cambrian trilobites possessed genetic or developm ental mechanisms used to maintain developmental stability comparable t o those of younger trilobites. Although the measures are biased by tim e averaging and deviations from the normal distribution, these data do not lend strong support to 'genomic' hypotheses that have been sugges ted to control the tempo of the Cambrian radiation.