34 Paleontology

Wednesday, 11 March 2026
8:30 AM - 10:00 AM
RCC, 103

Time averaging and fragmentation: comparative radiocarbon dating of irregular echinoids and mollusks from modern seafloors

Paper number: 34-1
Program: Technical Sessions
Time: 8:45 AM – 9:00 AM

Authors

Abstract

Depositional environments affected by low sediment accumulation rates and/or intense sediment mixing may form heavily time-averaged fossil assemblages. However, the scale of time averaging is also potentially dependent on the intrinsic skeletal durability of organisms and may thus vary across taxa. Here we attempt to elucidate time averaging effects on taphonomic biases by radiometrically dating irregular echinoid and mollusk remains sympatrically sourced from shallow, sublittoral seafloors off Cedar Key and Carrabelle, Florida. Irregular echinoid death assemblages were significantly more fragmentary (93.4% by mass) than those of mollusks (74.9% by mass), suggesting comparatively lower preservation potentials among complete echinoid tests relative to complete mollusk shells. Predominance of fragments within the death assemblages of both taxa suggests that fragments have a higher preservation potential than complete skeletons. If intrinsic durability modulates time averaging, we can predict that: (1) complete irregular echinoid tests should be less time-averaged than sympatric complete mollusk shells; (2) echinoid and mollusk fragments should be more time-averaged than complete specimens; (3) echinoid and mollusk fragments should exhibit comparable levels of time averaging. Calibrated radiocarbon dates, derived using a gas integration system (GIS), corroborated all three hypotheses. Nearly all complete echinoid tests were sub-centennial in age (mean = 151 YBP; median = 84 YBP), with one very old specimen of Encope (4920 YBP), and exhibited multi-centennial levels of time averaging (Estimated Time Averaging [ETA] = 980 years). In contrast, complete mollusk shells were time-averaged on a multi-millennial scale (ETA = 7480 years; F = 51.8, p < 0.001, Brown-Forsythe test). Fragments of echinoids (ETA = 7016 years) were time-averaged notably more than their complete counterparts, with this difference being less substantial among mollusks (ETA = 7651 years; p < 0.05 in both taxa). The results suggest that intrinsic durability exerts strong effects on the scale of time averaging. Complete echinoids tests, because of their fragility, exhibit substantially lower time averaging relative to complete mollusk shells. Fossil assemblages with millennial-scale time averaging or greater may therefore be substantially biased in favor of durable taxa. However, echinoid fragments demonstrate greater preservability and form time-averaged assemblages comparable to those represented by mollusks and mollusk fragments. Consequently, abundant echinoid fragments in assemblages with scarce complete tests may indicate highly time-averaged assemblages derived from communities in which echinoids were abundant.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1

doi: 10.1130/abs/2026SE-411814

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