Large vertebrate burrow from the Upper Mississippian Mauch Chunk Formation, eastern Pennsylvania, USA

Abstract

A burrow of probable amphibian origin was discovered in the upper member of the Mississippian Mauch Chunk Formation in eastern Pennsylvania. Facies analysis of the upper member indicates that deposition occurred in an ephemeral–braided stream setting. The burrow is housed in a mudstone and is filled by two graded beds of conglomerate to sandstone. It is characterized by a flared opening leading into a narrower slightly helical tunnel that ends in an inflated ovate chamber approximately 51 to 60 cm in diameter with a maximum height of 20 cm. The flared opening is stratigraphically higher than the elevation at the base of the chamber. The tunnel has a semicircular base. The geometries, abrupt angle changes, inflated termination, width-to-height ratios of the terminal chamber, and graded fill indicate that the structure was an open void prior to sedimentation. The geometry and size of this structure are incompatible with known invertebrate burrows and erosional features. Palaeosauropus primaevus, an amphibian footprint ichnotaxon, recovered from the Mauch Chunk Formation, was made by an amphibian of sufficient size to create such a large burrow. This type of burrowing was most likely a response to seasonal droughts as local water sources evaporated.