SAMPLING THE SEAFLOOR: UTILIZING THE SURFICIAL FOSSIL RECORD TO QUANTIFY TAPHONOMIC DIFFERENCES BETWEEN ECHINOIDS AND MOLLUSKS

Modern seafloors act as reservoirs for the surficial fossil record of marine paleocommunities. Sampling and interpreting this record provides insight into pre-anthropogenic conditions and community structure. However, proper interpretation requires accounting for taphonomic differences across taxa. Here, we comparatively assess temporal resolution and taphonomic pathways for irregular echinoids and conchiferan mollusks. Echinoids are assumed to be underrepresented within the fossil record relative to mollusks due to skeleton of the former containing a greater number of elements and a higher reliance on soft tissue supports. To evaluate if such intrinsic factors matter, we empirically tested the following hypotheses: (1) Whole echinoid tests will exhibit a higher fragmentation rate relative to whole mollusk shells; (2) Due to a lower reliance on soft-tissues than whole tests, echinoid fragments will show a similar relative preservation rate to mollusk fragments; and (3) Time averaging is related to preservation potential, thus whole echinoid tests will be time averaged at a lower scale than whole mollusk shells, whereas fragments of both taxa will time-average similarly, at a higher scale than whole remains. To test these hypotheses, we sampled shallow marine ecosystems across several localities along the gulf coast of Florida (Carrabelle; Cedar Key; Florida Keys) and the Caribbean (Curaçao). Skeletal material was retained from these samples, and relative proportions of live, whole, and fragmentary material were estimated from the measured mass of sympatric echinoids and mollusks. The results indicate that echinoid tests seem to exhibit a higher fragmentation rate than whole mollusk shells (0.4% whole echinoids vs 17.82% whole mollusks), while fragments of both taxa seem to preserve similarly well (92.2% echinoids vs 81.5% mollusks). Differences in the magnitude of time averaging, measured as interquartile range [IQR] of radiocarbon dates, was consistent with prior predictions: decadal time-averaging for whole echinoids [IQR = 27], centennial for whole mollusks [IQR = 720], and millennial for both echinoid and mollusk fragments [IQR = 2214; 1540]. Such results quantify a taphonomic bias between the clades, highlighting the importance of intrinsic factors in terms of preferential preservation and temporal resolution of the resultant fossil record.