OCCURRENCE AND SIGNIFICANCE OF THE GHOST CRAB OCYPODE QUADRATA (FABRICIUS, 1787) FROM THE UPPER PLEISTOCENE TO HOLOCENE? ANASTASIA FORMATION OF FLORIDA

Whole-body decapod crustaceans are rare in Quaternary deposits of the southeastern United States because they possess a relatively thin exoskeleton that is easily destroyed by biologic and/or geologic processes. Typically, only the most heavily mineralized parts (chelae and fingers) are preserved. Occasionally, however, conditions permit exceptional preservation of intact specimens. Along east coast beaches in Brevard County, Florida, over 500 nearly complete specimens of the ghost crab Ocypode quadrata were collected from the Upper Pleistocene to Holocene? Anastasia Formation. The low degree of disarticulation and the posture of the crabs indicate that they died while in their burrows, either by winterkill or by burrow collapse. Fossils of O. quadrata were found in a range of conditions: from those with a clearly crab-like form, bearing a loose, friable matrix of shell-hash with little cementation, no visible abrasion, no calcite infilling, and no attached Recent epibionts; to those that were barely recognizable as crabs, with a thick layer of heavily cemented matrix, a highly sand-abraded (polished) surface, some voids filled with calcite-cemented grains, and some with Recent epibionts in exposed cavities. We suspect that fossil crabs found on Brevard County beaches derive from two sources. The unabraded crabs, with friable matrix, have recently been washed from the sand dunes or upper beach escarpment by storm waves while the abraded crabs have been cast upon the beach from the nearshore subtidal zone by storm waves. The Anastasia Formation, a coquinoid limestone with interbedded quartz sands, is the youngest lithified marine deposit along Florida’s coast. Based on radiometric dating (U²³⁴/Th²³⁰), the principal time of deposition of the unit is thought to have occurred approximately 110,000 years ago. However, the barrier island-sand dune system along the central east coast of Florida is believed to have formed within the last 7,000 years as the rate of sea level rise slowed. The shoreline and dune system has since receded to its present location. The abraded crabs possibly could have accumulated over the past 110,000 years, but the unabraded specimens, derived from the sand dunes or upper beach escarpment, indicate that portions of the Anastasia Formation are considerably younger in age.