BURROWS OF ARCHANODON (UNIONOIDA, BIVALVIA): DISTRIBUTION IN SPACE AND TIME

The earliest known freshwater bivalves belong to the unionoid genus Archanodon. A. catskillensis (Vanuxem), from Middle and Upper Devonian fluvial rocks of New York’s Catskill region, is the best preserved and best understood of the four provisional Archanodon species. Co-occurrences of A. catskillensis body fossils and burrows at several sites in New York and Pennsylvania indicate that these bivalves constructed unlined, tubular burrows, oval in cross-section, and often having internal menisci. A. catskillensis burrows are up to 1 m in length, and are usually oriented perpendicular to bedding, although some burrows have inclined or horizontal segments, especially near their origins. Some burrows appear to represent escape from anastrophic burial, but most are best interpreted as burrows built by animals moving periodically upward to maintain their preferred living depth in the face of normal sediment accumulation. Horizontal burrows probably represent animals avoiding advancing bedforms or re-burrowing after exhumation. Because the animals commonly lived in multi-individual banks their burrows tend to occur in clusters. Burrows co-occurring with animals are paradigms that help identify Archanodon burrows in cases where Archanodon body fossils do not occur. Applying paradigm criteria to such burrow sites indicates that some, but not all large, meniscate burrows in Catskill fluviolacustrine facies were probably made by Archanodon. Burrows attributable to other species of Archanodon occur in the Devonian of Wales, Ireland, and possibly in Nova Scotia, New Brunswick, and the Gaspé region of Québec. Taken together, burrow and body fossil occurrences for all four Archanodon species indicate that the genus arose in the Early Devonian on the continent of Baltica, and that it migrated westward to the Laurentian side of Laurussia as the suturing of Baltica, Laurentia, and Avalon pushed suitable habitats westward with the developing Acadian Orogen. Extinction of the genus in the Late Carboniferous is associated with increasing ecologic isolation and fragmentation deriving from the block and basin topography of the region at this time, and from the advent of new competitors and durophagous predators.