INVERTEBRATE TRACE FOSSILS IN SEMI-ARID TO ARID BRAIDED-EPHEMERAL RIVER DEPOSITS OF THE MISSISSIPPIAN MIDDLE MEMBER OF THE MAUCH CHUNK FORMATION, EASTERN PENNSYLVANIA, USA

The rapid diversification of terrestrial ecosystems was initiated in the Silurian and subsequently continued through the Mississippian. Detailed collection within rocky outcrops of the middle member of the Mississippian Mauch Chunk Formation of eastern Pennsylvania has yielded a number of important vertebrate and invertebrate trace fossils, indicating the presence of a significant terrestrial biotic diversity during the Early Carboniferous.

This report illustrates the diverse invertebrate trace fossil assemblage associated with the ephemeral-river systems of the Mauch Chunk. The Mauch Chunk invertebrate ichnoassemblage consists of backfilled burrows of deposit feeders, both meniscate (Taenidium) and non-meniscate (Planolites) that typically crosscut bedding; arthropod trackways (Diplichnites, Paleohelcura, Kouphichnium and Stialla); striated trails (Cruziana) and resting traces (Rusophycus); surface or shallow subsurface grazing trails or burrows (Gordia, Helminthoidichnites). Tetrapod footprints and fish swimming traces (Undichna) are also part of this ichnocoenosis.

The Mauch Chunk invertebrate ichnocoenosis corresponds well to the Scoyenia ichnofacies in consisting of simple burrows, trackways, striate and bilobate trails and pits and simple meniscate burrows. Typically, trace fossils of the Scoyenia ichnofacies indicate opportunistic behaviors in subaerial environments that are temporarily or periodically inundated.Indeed, the paleoenvironmental setting of these traces is a semi-arid to possibly arid, braided-ephemeral river system with extensive over-bank deposits. The invertebrate traces were developed both as infaunal burrows in the tops and within channel sandstones and as surface and grazing traces on in-channel mudstone-draped bedforms and in proximal overbank deposits. The in-channel mudstone-draped bedforms display both vertebrate and invertebrate trace fossils. Emergence features, such as extensive rills and raindrop impressions, are developed on the mudstone-draped foresets.