DEVONIAN ARCHANODONT UNIONOIDS FROM THE CATSKILL MOUNTAINS OF NEW YORK: IMPLICATIONS FOR THE PALEOECOLOGY AND BIOGEOGRAPHY OF THE FIRST FRESHWATER BIVALVES

The earliest known freshwater bivalves belong to the unionoid genus Archanodon. Study of Archanodon shell impressions, trace fossils, and associated depositional environments from localities in Middle and Upper Devonian rocks of the Catskill Mountains suggest that these animals inhabited freshwater floodplain and brackish estuarine paleoenvironments. Morphological comparison with modern bivalves, particularly other unionoids, suggests that Archanodon were semi-infaunal suspension feeders. Their main nutrient resource was undoubtedly current-borne particulate matter deriving from forests spreading over continental landscapes during the Devonian. Archanodon is the most abundant benthic suspension feeder in Catskill freshwater environments. Growth of individuals in Archanodon populations appears to have been resource limited. The biogeographic range of Archanodon is restricted to the region of the Acadian and Caledonian Highlands of the Old Red Continent. The earliest known evidence of Archanodon are meniscate burrows in the Lower Devonian of Wales. From the European side of the Old Red Continent Archanodon spread toward North America with the advance of suitable habitats as the suturing of North America, Baltica, and Avalonia progressed southwestward during the Acadian Orogeny. Archanodon abundance and habitat diversity appear to diminish in the Carboniferous. The extinction of Archanodon in the late Carboniferous or early Permian may be related to the proliferation of non-archanodont freshwater bivalves and freshwater durophagous predators which occurred at this time.