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176-9 Rock Versus Shell: Ecological Variation in Living and Dead Benthic Assemblages From Lithic and Biogenic Gravel Substrates of the San Juan Islands

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

Presenting Author:

Broc Kokesh

Authors:

Kokesh, Broc S¹, Samuels-Fair, Maya², Stowe, Kayli A³, Yohler, Ryan M⁴, Kahn, Leah⁵, Finnegan, Seth⁶

(1) University of California, Berkeley, Berkeley, CA, USA, (2) University of California, Berkeley, Berkeley, CA, USA, (3) University of California, Berkeley, Berkeley, CA, USA, (4) University of California, Berkeley, Berkeley, CA, USA, (5) Stanford University, Stanford, CA, USA, (6) University of California, Berkeley, Berkeley, CA, USA,

Abstract:

Taphonomic and ecological interpretations of marine fossil assemblages are often tested by assessments of modern benthic shellfish communities (living assemblages) and their persistent skeletal remains (death assemblages). Such live-dead comparisons have a long history of use in soft-sediment coastal seabeds and carbonate reef systems that dominate modern continental shelves. However, temperate hardgrounds and gravels (>2 mm clasts) receive comparatively less focus due to their scarcity in modern nearshore seabeds. Nevertheless, these habitats are perhaps the best available modern analogues for dense shellbed deposits of the Paleozoic for their unique epibenthic faunal assemblages (mollusks, brachiopods, bryozoans, regular echinoids, etc.).

Seabeds from the San Juan Islands of Washington State are exceptionally variable in both depth (intertidal to 200 m) and substrate (muds to cobbles) while harboring a single regional benthic fauna, making this location an ideal testing ground for the influences of seabed character on community composition and preservational bias. We took dredge samples from two locations in the San Juan Channel, one dominated by lithic gravels, and another dominated by similarly sized shell hash. Living and death assemblages consisting of mollusks and brachiopods were sorted from bulk sediments by separating intact shells that could be both identified and counted as individuals.

Death assemblages had consistently higher abundances (4-10x) and richness (1.5-2.5x) compared to living assemblages, but also similar measures of evenness and rarefied diversity. Live-dead comparisons within each seabed indicated high to moderate agreement, while comparisons of assemblages between substrates (i.e., live-live and dead-dead) were notably more mismatched. While only a few taxa were particularly dominant, their distributions notably varied among substrates: (1) motile scallops were abundant both dead and alive everywhere, (2) lithic substrates harbored high proportions of infaunal Venus clams and epifaunal mussels, and (3) shelly substrates harbored higher proportions of epifaunal brachiopods, limpet gastropods, and rock oysters.

While it is well understood that benthic communities vary along gradients of water depth and sediment grain size in temperate coastal habitats, these preliminary results further suggest that finer-scale sorting within a regional fauna may be influenced by the intrinsic physical properties and (a)biogenic sources of substrate materials.

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

Rock Versus Shell: Ecological Variation in Living and Dead Benthic Assemblages From Lithic and Biogenic Gravel Substrates of the San Juan Islands

Description

Session Format: Oral

Presentation Date: 10/21/2025

Presentation Start Time: 10:15 AM

Presentation Room: HGCC, 304B

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