UNUSUALLY ABUNDANT AND DIVERSE SAND DOLLAR (MELLITIDAE) POPULATIONS WITHIN THE NORTHERN GULF OF MEXICO: PALEOECOLOGIC AND TAPHONOMIC IMPLICATIONS

The shallow (<9 m depth) sublittoral zone of the northern Gulf of Mexico, near Carrabelle, Florida, harbors mellitid echinoid populations that are not only unusually dense (study maximum = 245 tests/m²), but also diverse in both taxonomic composition and size/age class. While such densities have been previously observed, they were almost always monospecific and restricted to single size/age classes. Thus, the Carrabelle populations and the sympatric death assemblages present an opportunity to elucidate the taphonomic patterns of multi-species, multi-cohort echinoid communities. We studied these populations over nine months, at three separate sites covering a 4.8 km section of seafloor. Samplings involved SCUBA surveys where we collected live and dead specimens, as well as sediment samples, along a 20 m benthic transect. Sediment samples were sieved through 1 mm mesh to retain live and dead material otherwise overlooked by hand collection. Initial surveys revealed the highest density populations, dominated by juvenile Mellita tenuis and Encope spp. (5-30 mm diameter) with less common M. tenuis and Leodia sexiesperforata adults/subadults (60-120 mm dia.). Subsequent sampling events indicated that both Mellita and Encope juveniles remained present at all sites as they approached sub adult sizes (55-70 mm dia.), while L. sexiesperforata was occasionally observed in various life stages (35-100 mm dia.). Population densities did decline severely throughout the growth period (final event max. = 16 tests/m²), possibly reflecting the cumulative mortality of the juvenile populations. In the death assemblage, complete tests were relatively rare (initial event max. = 84 tests/m²; final event max. = 1.6 tests/m²) but were representative of the living biodiversity and size/age shifts. The tight tracking of the living assemblage by the death assemblage suggests minimal taphonomic inertia from previous cohorts, and thus a rapid post-mortem destruction of complete tests, corroborating the hypothesis that irregular echinoid tests have limited fossilization potential under typical depositional conditions. However, the high live/dead fidelity of echinoid biodiversity and size class, suggests that they may provide highly-resolved (non-time-averaged) paleoecological data when complete tests are preserved.