FUNNEL-SHAPED TRACE FOSSILS FROM THE UPPER DEVONIAN OF NORTH-CENTRAL PENNSYLVANIA: PRESERVATION AND TRACEMAKER BEHAVIOR

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. Burrows are typically about 0.5–1.0 centimeters in diameter and about 2–3 centimeters deep. They are tentatively attributed to the ichnogenus Rosselia. The lower portions of the burrows taper gently and are preserved in full relief within sandstone bedding planes. Sectioned specimens display a sandy central tube surrounded by somewhat muddier sediment. The upper portions of the burrows are often preserved as hypichnial casts that widen more broadly, giving an overall funnel shape to the burrows. They appear to have been passively filled when buried by sandy sediment. 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, separated by gaps of consistent dimensions. These linear arrays reflect the living social organization of the 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, if the tracemakers reproduced asexually (e.g., some form of splitting or budding), the linear groups could have been formed through a series of asexual reproduction events. The linear groups could also have been aligned with waves or currents, or they might have improved the efficiency of upward migration following burial in a setting with frequent sedimentation events.