EXCEPTIONAL PRESERVATION AND ACTIVE TAPHONOMIC ALTERATION OF THE LATE MIOCENE BRITTLE STAR, OPHIURA MARYLANDICA

The Calvert Cliffs in southern Maryland comprise the most complete geologic record of Miocene nearshore sediments along the east coast of North America. These sediments preserve a series of transgressive-regressive cycles within an overall shallowing trend that spans from approximately 20 to 7.5 million years ago. Ophiuroids are only known to occur at the southernmost end of the Calvert Cliffs in the late Miocene Little Cove Point Member of the St. Marys Formation and are specifically associated with lag deposits that sporadically occur within dense shell beds. Only a single species is known to occur, Ophiura marylandica, with more than 100 individuals recovered to date. These individuals often preserve as articulated, unaltered body fossils within bivalve and gastropod shells, which seemingly serve to protect the delicate skeletal elements; although, the brittle stars are not exclusively found within mollusk shells. A secondary preservation state also occurs within the Little Cover Point Member, in which the brittle star body fossils have been entirely replaced with ironstone. This taphonomic alteration is the result of an active process, in which fluid flows through the overlying Pleistocene iron-rich unconsolidated sands and percolates through the underlying Miocene siliciclastics. As this iron saturated fluid flows through the Miocene strata, it alters the underlying clays, silts, and sands to ironstone. In doing so, many of the Miocene invertebrate body fossils are altered to molds and casts. This unique taphonomic alteration has been continuously occurring since the deposition of the iron-rich Pleistocene sands and continues to occur to this day.