FURROWS AND FIRMGROUNDS: EVIDENCE FOR PREDATION AND IMPLICATIONS FOR PALEOZOIC SUBSTRATE EVOLUTION IN RUSOPHYCUS “HUNTING BURROWS” FROM THE SILURIAN OF EAST-CENTRAL NEW YORK

The Silurian-aged Herkimer Formation of east-central New York contains abundant exceptionally preserved composite Rusophycus-teichichnid burrows. We suggest that the most likely interpretation of these composite trace fossils is as structures formed by trilobites entering the sediment in search of prey. Parallel alignment of the paired traces, asymmetrical configuration of the teichichnid along the longitudinal axis of the associated Rusophycus and depth correlation all suggest that this relationship was predatory. Moreover, candidate rusophycid tracemakers known from correlative deposits belong to trilobite groups which may have had macrophagous predatory lifestyles.

Sectioned material suggests that these Rusophycus may have been open at the sediment-water interface. Undisrupted laminae dispersed throughout the sandy infill of certain slabbed Rusophycus indicate that sand sedimentation post-dated formation of the burrows. Additionally, the relatively crisp preservation of both Rusophycus and teichichnids, along with the preservation of such delicate morphological details as scratch marks, suggest that the sediment must have been relatively firm at the time the traces were formed.

The formation and preservation of Rusophycus in cohesive sediments located very close to the sediment-water interface hold profound implications for the manner in which we consider Paleozoic substrates and their temporal and spatial evolution. Moreover, the preservation of shallow-tier trace fossils and delicate morphological structures as late as the Silurian, when the size, complexity and extent of burrowing had developed well beyond that recorded in lowermost Cambrian strata, is cause for further consideration and may provide some indication that firmgrounds were indeed the norm and not the exception in Lower Paleozoic shelfal siliciclastic sediments. The morphology and taphonomy of ichnological associations may, in the context of sedimentological relationships, prove a powerful proxy for tracking substrate conditions through both space and time.