ESCAPE BURROWING OF MODERN FRESHWATER BIVALVES AS A PARADIGM FOR ESCAPE CAPABILITIES OF ARCHANODON CATSKILLENSIS (DEVONIAN), THE OLDEST KNOWN FRESHWATER BIVALVE

Little is known about the extinct, sporadically occurring and generally poorly preserved Archanodon catskillensis, the oldest known freshwater unionoid bivalve. The vertical burrows associated with Archanodon, often one meter in length, are thought to be escape burrows generated upon catastrophic burial. To illuminate the nature of Archanodon burrows, the escape burrowing capabilities (escape potential, burrowing rate, burrowing dynamics as a function of shell morphology, sediment grain size, and burial depth) of two contemporary Archanodon analogues are assessed: the two native freshwater unionoid bivalves Elliptio complanata and Pyganodon cataracta. Pyganodon’s escape burrowing behavior is of particular interest as this animal is the best morphological analogue currently available for Archanodon based on shell size, shell shape and hinge structure. For comparative purposes, the escape behavior of the invasive venerid Corbicula fluminea is assessed as well. Burial trials reveal that Archanodon’s analogues are poor escape burrowers relative to Corbicula. Corbicula’s escape rate in fine sand is six times higher than that of Pyganodon and nearly three times higher than that of Elliptio. The probability of a successful escape in fine sand is more likely than in coarse sand for all three species, although escape rates in coarse sand are 50% lower. However, both unionoid species rarely escape a burial depth of more than 10 cm. Thus the upward burrowing capabilities of these animals, particularly Pyganodon, demonstrate that Archanodon’s burrows are likely a result of episodic sedimentation rather than a single catastrophic sedimentation event.