176-12 Low Taphonomic Inertia in Dense Populations of Mellitid Echinoids from the Northern Gulf of Mexico Suggests Fast Skeletal Destruction Rates and Low Potential for Time Averaging

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

Presenting Author:

Luis Torres

Authors:

Torres, Luis¹, Jang, Hyungjoo², Liyanagedara, Pamod Heshan³, Hernandez, Enrique Raul⁴, Portell, Roger W.⁵, Kowalewski, Michal J.⁶

(1) University of Florida, Gainesville, FL, USA, (2) University of Florida, Gainesville, FL, USA, (3) University of Florida, Gainesville, FL, USA, (4) University of Florida, Gainesville, FL, USA, (5) Florida Museum Natural History, Invertebrate Paleontology, Gainesville, FL, USA, (6) University of Florida, Gainesville, FL, USA; Florida Museum of Natural History, Gainesville, FL, USA,

Abstract:

Unusually diverse and abundant populations of mellitid echinoids were observed along large areas (>5 km²) of shallow marine habitat (<8 m depth) in the northern Gulf of Mexico. These high-density populations were dominated by recently settled juveniles of multiple taxa, presenting a unique opportunity to explore taphonomic inertia, preservation potential, and anticipated fidelity of sand dollar fossil records formed under high-energy settings with low-to-moderate sedimentation rates. Initial SCUBA-based samplings of the live populations revealed a predominance of Mellita tenuis (Test Length [TL]: 15-30 mm), similarly sized Encope spp., larger M. tenuis (TL: 40-60 mm), minute nondiagnostic mellitids (TL: <15 mm), and one Leodia sexiesperforata (TL: 120 mm), with taxon-pooled, sample-level densities ranging from 113 to 650/m². Preliminary time series based on five sampling events, interspersed at three-month intervals over the course of a year (11/2023-11/2024), revealed drastic reductions in these densities over time, with the initial >100 specimens/m² densities diminishing to ~5 specimens/m² in final samplings. These diminishments were coincident with increases in average body size, suggesting a cumulative mortality of juvenile specimens as they matured, resulting in significant contributions of skeletal material to the currently forming death assemblages. Whole tests within these death assemblages recorded similar shifts in density and size, while also maintaining comparable taxonomic incidence and relative abundance, though the minute nondiagnostic mellitids (likely young juveniles of the observed species) were overrepresented. Concordant density declines and similar shifts in size frequency distributions for both the live and dead assemblages suggest that the tests of newly dead mellitids tend to disintegrate rapidly (days to months) under base conditions, thus preventing the formation of time-averaged accumulations. This low taphonomic inertia indicates a low preservation potential amongst mellitids, with most specimens undergoing rapid disarticulation post-mortem. In contrast to sympatric mollusks, with scarce live specimens and an overwhelming dominance of skeletal remains, sand dollar assemblages are dominated by live individuals with whole dead tests being relatively rare. This is consistent with previous studies, where fossil records formed under high-energy settings with low-to-moderate sedimentation rates were found to be unconducive to sand dollar fossilization. However, rapid burial events may preserve such sand dollar assemblages and provide highly resolved fossil records (with sub annual time averaging) that archive faithful snapshots of mellitid echinoid communities.

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

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.