Conservation Paleobiology and Taphonomy: Differential Preservation and Time-Averaging of Echinoids and Mollusks

Abstract

Fossils from surficial death assemblages and shallow cores are an important archive used in the field of conservation paleobiology. Understanding the taphonomic filters and time-averaging that affect modern biomineralized taxa is crucial for using their fossil record as a source of geohistorical data. Through comparative analysis of the live-dead patterns of echinoids and mollusks, we aim to assess multiple hypotheses regarding differences between the two taxa in terms of preservation potential and fossil record resolution. In this pilot study, six sediment samples (five surface [0-10cm] and one subsurface [30-40cm]) were collected in a shallow, subtidal habitat off the coast of Cedar Key, Florida. These samples were sieved for mollusks and echinoids. Specimens were segregated by taxa and classified into live, whole dead, and fragments. Results were consistent among surface samples, which showed that, by weight, an average of 8% of all mollusk specimens and 55% of all echinoid specimens were live collected. Fragmentation rate by weight was notable higher for echinoids than for mollusks: 77% for mollusks and >99% for echinoids. The subsurface sample lacked complete echinoid tests, but complete mollusks shells, mollusk fragments and echinoid fragments remained common. These results support the hypothesis that echinoid tests degrade more quickly than mollusk shells, making them less likely to be preserved as part of time-averaged assemblages formed under low net-accumulation rates. Echinoid fragments do not seem to follow this pattern, and instead seem to preserve similarly to mollusks. The results suggest that geohistorical records provided by echinoids and mollusks may be fundamentally distinct from one another in terms of temporal resolution and completeness of the fossil record.