TAPHONOMIC CONTROL OF SHALLOW SUBTIDAL FOSSIL ABUNDANCE

Species abundance in the fossil record is a reflection of original ecologic abundance tempered by taphonomic processes. Estimating the extent to which taphonomy has altered original abundance is difficult, and most work has focused on actualistic studies of Recent life and death assemblages. Because trilobites had an exoskeleton formed of sclerites with different shapes and sizes, fossil trilobite assemblages can reflect relative amounts of taphonomic sorting. Differences in the ratio of number of heads (cranidia) to tails (pygidia) can occur only because of differences in taphonomic regime, since regardless of age, environment, or taxon, they entered the fossil record in unity.

The Lower Ordovician Red Canyon Member, House Formation, western Utah, records deposition in a shallow subtidal setting between storm and fair weather wave base. The type section of the member contains prolific silicified faunas, permitting large sample sizes. Forty-two horizons were sampled in a 69.5 m interval; over two tonnes of rock were digested in acid. Trilobite cranidia and pygidia were identified to species level and their number recorded in each collection. A total of 17,409 sclerites were counted, representing 13,950 individuals.

Head:tail ratios vary dramatically through the horizons, from a low of exactly 1:1 to an extreme of 50.6:1. Evenness is effectively uncorrelated with this bias. Coupled with striking heterogeneity in the abundance of specific taxa from horizon to horizon, this suggests that evenness is extremely difficult to interpret in process terms and may reflect little of interest about fossil assemblages.

Four different trilobite shape classes were identified, differing in the relative size of the pygidium versus the cranidium. Critically, variation in the sclerite ratios of these shape categories shows little or no significant correlation in pairwise comparisons. Hence, different shape classes were affected in different ways by different taphonomic regimes. The extreme range of taphonomic overprint and lack of correlation in the response of differently shaped trilobites to this bias indicate that abundance patterns in the Red Canyon Member, and the shallow marine record in general, may largely be under taphonomic control.