176-15 A user’s guide to acquiring reliable historical baseline and trend data from dead-shell assemblages

Session: Laws of the Grave: Advances in Taphonomy Across the Paleontologic Record

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The very young fossil record represented by death assemblages (live-dead analysis) and easily-cored Holocene-sediments is increasingly appreciated by biologists as a valuable source of ecological insights, but practical advice is difficult for any newcomer to find. Our “user’s guide”, with much advice learned the hard way, is motivated by inquiries from biologists and uses molluscan assemblages from level-bottom, soft-sedimentary subtidal seabeds as the exemplar system, with asides on other groups and settings. We summarize what the taphonomic and CPB communities have learned about what matters most – eg., the fundamental differences between time-averaged and snapshot information -- and what turns out to matter far less than we feared (postmortem between-habitat transport), as well as some common pitfalls (under-estimating how laborious and complicated live-dead analysis actually is) and constraints (e.g., mesh size matters, and the same size needs to be used for both live and dead). Our advice is organized by workflow, i.e., issues that arise and consequential decisions that are made (I) before the start of fieldwork (these are strategic, including to start with live-dead rather than with cores; pilot work to determine the abundance of living assemblages); (II) during fieldwork (changes made under duress, but includes sieving method, which is commonly too harsh or becomes harsh -- the effects cannot be ‘undone’); (III) during lab processing (aggressive, thorough sorting for individuals is essential – split samples if you are overwhelmed, and quantify your “sorting efficiency” the way agencies do; for your lab team, be scrupulous about chain-of-custody protocols and use paper lab-sheets for tallies); and (IV) during data analysis (to halve or halve-not your bivalve counts?; perils of truncating rather than transforming data), which might then segue into “second-order analyses” such as shell age-dating and sediment coring (addressed by other guides). The workflows of monitoring agencies are typically too packed and closely regulated by licensing authorities (e.g., EPA, State Water Board) to add paleoecological analysis. However, agencies can be enthusiastic about collaborations that support it, and academic biologists are keen to try it themselves, motivating our guide.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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