A MISSISSIPPIAN VERTEBRATE BURROW?

A burrow-like specimen was recovered from the upper member of the Mississippian Mauch Chunk Formation in eastern Pennsylvania. Facies analysis of the upper member of the Mauch Chunk Formation indicates deposition in an ephemeral braided stream setting. Justification as a burrow is not definite, however evidence supporting and refuting the burrow interpretation is presented.

The burrow-like feature is housed in a mudstone and is filled by a normally graded conglomerate to sandstone. Laterally along the basal bedding plane, the conglomerate preserves rill features that were developed on the mudstone. The structure is characterized by a flared opening leading into a narrower linear tunnel that pinches and swells in diameter and is ovate in cross section. This tunnel abruptly bends 40° into an inflated ovate chamber-like feature, ~51 to 60 cm in diameter and maximum height of approximately 20 cm. The ovate-chamber is partially breached at the top by the overlying conglomerate. The flared opening is stratigraphically higher than the elevation at the base of the chamber.

Observations that are consistent with a burrow include the following: 1) bedding plane view geometries, abrupt angle changes and inflated termination, 2) width to height ratios of the terminal chamber, 3) graded fill indicating the feature was open during sedimentation, and 4) associated trackways, Palaeosauropus primaevus, recovered from this outcrop. P. primaevus, an amphibian track, was of sufficient size to create a large burrow. The observations that are inconsistent with a burrow are: 1) absence of an entombed fossil and 2) breaching/erosion of the upper reaches of the feature possibly indicating an “unusual” erosional feature.

If the feature is a burrow, then the age of burrowing by vertebrates is extended into the Mississippain and the potential maker was an amphibian.