A UNIQUE PRESERVATIONAL MODE: DOLOMITIC CASTS FROM THE EARLY MIOCENE ARCADIA FORMATION IN SARASOTA COUNTY, FLORIDA

Dolomitization of carbonate sediments generally preserves only organisms that secrete low-Mg calcite skeletons or those that rapidly transformed to low-Mg calcite early in diagenesis, such as red coralline algae, foraminifers, and echinoids, often with an altered microstructure. Mollusk shells, as well as other faunal elements that are either high-Mg calcite and/or aragonite, are generally destroyed in the dolomitization process, although, in rare cases, these organisms are preserved as 'ghosts'. Here we present the only documented case of preservation through the formation of dolomitic casts. The casts were collected from a variably-indurated, dolomitic layer that contains common molluscan casts and far fewer molluscan molds together with sparse marine vertebrate remains and abundant phosphate pebbles that form a pavement, suggesting transport from their initial site(s) of deposition or burial. This exposure of the Arcadia Formation contained at least 50 invertebrate taxa, many of which are preserved as dolomitic casts, including Lirophora sp., Panchione sp., Mercenaria sp., Carditamera sp. A, Carditamera sp. B, Glycymeris sp., Americardia sp., Turritella sp., and Orthaulax sp. A limited number of dolomitized steinkerns, including representatives of Lirophora sp. and other rarer bivalves from this locality, such as Eucrassatella sp., Dinocardium sp., and a lucinid, and the echinoid Agassizia sp., are also present in this unit. With the exception of mild distortion in some valves and some fragmented specimens, these molluscan casts are nearly perfect replicas of the original aragonitic shells, and preserve such details as the hinge structure, pallial line and sinus, muscle scars, and even the presence of encrusting bryozoans. However, the casts can be distinguished from an original shell by both their yellowish brown color and their lack of ultrastructural details. We suggest that the casts were formed due to infiltration of very fine-grained dolomitic mud, produced in an environment similar to the Coorong of Australia, into an underlying micritic unit; the latter was dissolved at some later point, whereas the more resistant dolomitic casts remained. Their presence in Florida, however, suggests that other apparently barren dolomites may contain previously undetected fossils.