TAPHONOMY AND TIME: COMPARATIVE ASSESSMENT OF MOLLUSK AND ECHINOID SKELETAL REMAINS WITH KNOWN RADIOCARBON AGE.

Skeletal remains of biomineralizing invertebrates accumulating on modern seafloors may eventually form time-averaged fossil assemblages. Because these remains may remain on the surface for prolonged periods, their state of preservation may vary as a function of time-since-death, with newly dead specimens expected to be less taphonomically altered than those that died centuries ago. Using a large sample of radiocarbon-dated echinoid and mollusk specimens, we tested the following hypotheses: (1) Taphonomic alteration of skeletal remains increases with their radiocarbon age; and (2) Skeletal specimen mass will inversely scale with radiocarbon age. Data was obtained by taphonomically scoring and weighing specimens (n = 190) gathered from shallow marine environments in Cedar Key, Florida, and comparing them against their radiocarbon ages. The total taphonomic score (TTS) of each specimen was based on the standardized summation of applicable taphonomic attributes for the respective clade (Fragmentation, Encrustation, Bioerosion, Lack of Luster, Staining, Abrasion, Center Breakage and Spine Presence), with scores of 0 and 1 indicating an unaltered or altered status, respectively. Specimen sizes were estimated based on measured mass values [g]. Results show a significant (Spearman) correlation between radiocarbon age and both taphonomic alteration (Mollusk: rho = 0.55, p-value = 6.57x10^-9; Echinoid: rho = 0.59, p-value = 7.19x10^-9) and mass (Mollusk: rho = 0.22, p-value = 2.83x10^-2; Echinoid: rho = 0.52, p-value = 4.77x10^-7). More specifically, these results suggest that taphonomic alteration tends to be more prevalent in specimens with residence times exceeding 1000 years, and thus, fossil assemblages dominated by well-preserved specimens may indicate limited (sub-millennial) time-averaging. Preferential preservation of larger specimens suggests that dissolution processes may have also played a role, possibly due to a greater surface area to volume ratio in smaller specimens. Our results are most applicable to shallow-water, mixed siliciclastic-carbonate settings from warm-temperate climate regimes. Similar studies in other settings should be conducted in the future to assess if the results reported here can be generalized across depositional settings and climate zones.