Ostracodes as indicators of brackish water environments in the Catskill Magnafacies (Devonian) of New York State

Leperditiid and beyrichiid ostracodes in marine margin deposits of the Devo nian Catskill magnafacies of New York State document brackish conditions in what were previously considered freshwater floodplain deposits. The very l ow diversity ostracode fauna (two species) includes an abundance of anomalo usly thin-shelled individuals that belong to the genera Leperditia? and Wel leria. This ostracode fauna is one of the oldest reported from brackish wat er in the stratigraphic record, and it documents the early ecological radia tion of ostracodes into waters of reduced salinity. Associated fossils, inc luding the bivalve Archanodon and the conchostracan Euestheria, have been c onsidered to be freshwater forms, partly because of their occurrence in the se rocks that were assumed to be: fluvial in origin. Nearly everywhere, how ever, the ostracodes are associated with cosmine-bearing fish fragments, li ngulid brachiopods, and the feeding trace Spirophyton, which have a marine or brackish-water affiliation. The presence of open carapaces on bedding pl anes as well as adults and several instar stages of each ostracode species indicate lack of postmortem transport. At least one ostracode-bearing mudst one contains the mineral amesite; associated calcareous mudrocks interbedde d with these units contain symmetrical ripple marks, mudcracks, and locally intense manganese-oxide staining. Collectively, these features indicate ex treme environmental conditions and suggest highly variable salinities in sh allow pools or embayments along the paleoshoreline. The presence of brackis h-water faunas, perhaps estuarine in origin, indicates an aspect of the div ersity of Devonian paleoenvironments along this portion of the Catskill del taic complex not previously recognized. The deposits are widespread, althou gh not ubiquitous, within thick sequences of fine-grained flood-basin depos its. Because mast previous paleoecological studies of fossils from these ro cks assumed the sediments to be of freshwater origin, the new interpretatio n of brackish-water communities presented here could have important implica tions for understanding the evolution of early terrestrial organisms. For e xample, some animals may have evolved in brackish estuarine environments ra ther than in freshwater rivers as assumed in previous studies. Recognition of this brackish facies elsewhere in the Catskill succession will not only aid future identification of paleoshoreline position but may also help clar ify whether early terrestrial organisms made the transition to land via bra ckish or freshwater environments. (C) 1999 Elsevier Science B.V. All rights reserved.