MORPHOLOGY AND PALEOENVIRONMENTAL CONTEXT OF TREPTICHNUS BIFURCUS FROM THE EARLY JURASSIC EAST BERLIN FORMATION, HOLYOKE, MASSACHUSETTS

Invertebrate trace fossils and dinosaur tracks of the Early Jurassic East Berlin Formation are exposed in a small backyard quarry in Holyoke, Massachusetts. We examined a well-exposed bedding plane covered in abundant Treptichnus (“feather-stitch” burrows). The burrows are composed of discrete segments that probed into the sediment, bifurcating to both sides (Treptichnus bifurcus). They are preserved as convex hyporeliefs and concave epireliefs, and they occur in a thin mud horizon bounded by cross-bedded siltstones 2-3 cm thick. The burrows are lined with mud and are filled with silt similar in composition to that found in overlying and underlying layers. Small ripple marks and mudcracks on bedding planes above and below the burrows indicate shallow subaqueous to emergent conditions during deposition at the site; sedimentary observations are consistent with previous research suggesting portions of the East Berlin Formation represent sheet flows on the margin of a playa lake system.

Previously, Treptichnus bifurcus has been reconstructed as a three-dimensional burrow, with horizontal shafts connected to the surface by vertical to sub vertical shafts. To test whether these T. bifurcus have a three-dimensional geometry, we cut thin sections both transverse and parallel to the long axes of the burrows. In cross-section, the burrows are essentially horizontal; there is no evidence that any shafts were oriented vertically. However, we have collected a large number of specimens and will examine additional burrows for vertical structures. Tentatively, we propose that the burrows were feeding traces that exhibited a planar configuration because resources were concentrated within the thin mud layer sandwiched between coarser, cleaner sediments.