FUNNEL-SHAPED TRACE FOSSILS FROM THE MARINE-NONMARINE TRANSITION OF THE UPPER DEVONIAN IN TIOGA COUNTY, PENNSYLVANIA

Funnel-shaped trace fossils from marginal marine facies of the lower Catskill Formation in Tioga County, Pennsylvania, display variable preservation and unusual spatial arrangements on bedding planes. Brachiopod assemblages from underlying marine strata have been dated as early Famennian, and overlying strata have been dated to the early part of the late Famennian using palynology. Burrows are typically about 0.5–1.0 centimeters in diameter and about 2–3 centimeters deep. The lower portions of the burrows taper gently and are preserved in full relief within sandstone bedding planes. The upper portions of the burrows widen more broadly, giving the burrows their funnel shape. Upper portions of the burrows are preserved in full relief or as casts on the bases of beds. Burrows were passively filled when buried by sandy sediment and have mud linings. We interpret the burrows as having a flared opening that was either open to the water column or occupied by the upper portion of the animal’s body.

When buried by thin layers of sand (e.g., a couple millimeters), the animals accommodated by migrating upwards so that the flared, upper portions of the burrows formed cone-in-cone traces. When buried by several centimeters of sand, the animals were still able to migrate upwards, as indicated by alignment of burrows on the bottoms and tops of sandstone slabs. On some bedding planes, burrows are clustered in roughly linear groups that reflect the spatial organization of the living tracemaker population. Comparisons of bottoms and tops of slabs indicate that these groupings were maintained as the animals migrated upwards following burial by up to several centimeters of sediment. We discuss several possible explanations for the formation of these groups, which are not mutually exclusive. For example, in modern habitats subject to tidal currents, some animals form linear groupings or otherwise align themselves to the currents. More speculatively, the groupings might have improved the efficiency of upward migration following burial in a setting with frequent sedimentation events.