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

Abstract

The rapid diversification of terrestrial ecosystems was initiated in the Silurian and subsequently continued through the Mississippian. Detailed collecting of the middle member of the Mississippian Mauch Chunk Formation in 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 cross-cut bedding; arthropod trackways ( Diplichnites, Diplopodichnus, Stiaria, Kouphichnium and Stialla); striated trails ( Cruziana) and resting traces ( Rusophycus); and surface or shallow subsurface grazing trails or burrows ( Gordia). Tetrapod footprints, mostly made by temnospondyl amphibians, are also part of this ichnoassemblage. The Mauch Chunk invertebrate ichnoassemblage 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 palaeoenvironmental setting of these traces is a semi-arid to possibly arid, braided-ephemeral-river system with extensive overbank 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. The Mauch Chunk invertebrate ichnofossil assemblage resembles other Devonian through Triassic ichnoassemblages from ephemeral fluvial red beds, confirming that the terrestrial invertebrate fauna had already fully colonized fluvial environments by Mississippian time. Furthermore, the development of the Scoyenia ichnoguild in the Mauch Chunk Formation, in the form of abundant traces of deposit feeders of a shallow to intermediate depth infauna (esp. Taenidium), is unusually early, when compared to the previously known Palaeozoic record.