ABUNDANT AND DIVERSE ECHINOID POPULATIONS IN THE NORTHERN GULF OF MEXICO, FLORIDA: PALEOECOLOGICAL AND TAPHONOMIC IMPLICATIONS

Modern echinoid communities are a valuable source of information regarding the ecology and taphonomy of this paleontologically important group. Here, we report on recent echinoid surveys from the nearshore sandy habitats of Alligator Bay (northeastern Gulf of Mexico, Florida) and explore the ecological, paleoecological, and taphonomic implications of the study system.

Our study sites, here listed as CB3 and CB4, are located ~7 km east of Dog Island and situated ~1.5 km apart. They represent comparable shallow habitats (~ 6 m) with a soft, siliciclastic sandy bottom inhabited by exceptionally abundant mellitid echinoids (150/m² and 245/m² in CB3 and CB4, respectively). The majority of these echinoids belonged to a single size class (test diameters of 20 to 25 mm) of the taxon Mellita tenuis, but also included various size classes of Encope aberrans, E. michelini and Leodia sexiesperforata. Complete tests of dead echinoids were also observed, albeit at lower densities (1.5/m² for CB3 and 5/m² for CB4) than in the case of the living fauna. Dead specimens were smaller (5-10 mm) than live specimens. Mellita tenuis was dominant in the death assemblage, but E. aberrans, and E. michelini were also present.

Echinoid communities consisting of multiple species and size classes are unusual, for present-day echinoid populations typically occur as monospecific populations dominated by a single age/size cohort. Our data indicate that the species diversity in the live assemblage is reflected in the sympatric dead material. However, significant differences between the live and dead material is present in terms of density per m² and average body size. The size offset may reflect cumulative effects of progressive mortality following the spawning event of the abundant juvenile M. tenuis cohort. Taphonomic biases may also play a role, due to the seemingly preferential preservation of small tests (older, larger specimens are represented strictly as fragments). These results are consistent with recent work suggesting preferential preservation of small tests and high live-dead fidelity due to the short post-mortem survival and limited time-averaging of echinoid tests. This study supports the hypotheses that the fossil record of sand dollars may be characterized by high temporal resolution and compositional fidelity.