Occurrence of the Atlantic ghost crab Ocypode quadrata from the upper Pleistocene to Holocene Anastasia Formation of Florida

Abstract

More than 500 nearly complete specimens of the Atlantic ghost crab Ocypode quadrata (Fabricius) were collected from the upper Pleistocene to Holocene Anastasia Formation along beaches in Brevard County, Florida. Such whole-body decapod crustaceans are rare in Quaternary deposits of the southeastern United States. The low degree of disarticulation and the posture of the crabs indicate that they died while in their burrows, probably by winterkill. Fossil O. quadrata were found in two conditions: 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 fossil or Recent epibionts; and 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 or calcite crystals, and some with Recent epibionts in exposed cavities. The nonabraded crabs with friable matrix have been recently exhumed from a poorly consolidated part of the Anastasia Formation or from the sand dunes or upper beach escarpment by storm waves, whereas the abraded crabs have been cast upon the beach from the nearshore subtidal zone by storm waves. Deposition of the Anastasia Formation is thought to have occurred approximately 110,000 YBP. The barrier-island-sand-dune system along the central East Coast of Florida is believed to have formed within the last 7000 yr. The abraded fossil crabs could have accumulated over the past 110,000 yr, but the nonabraded specimens, if derived from the existing beach, are considerably younger.