Depositional setting, taphonomy and geochronology of new fossil sites in the Catskill Formation (Upper Devonian) of north-central Pennsylvania, USA, including a new early tetrapod fossil

Abstract

Late Devonian terrestrial floodplain environments were important sites of plant and animal diversification and evolution. Recently discovered fossil sites in the Catskill Formation in north-central Pennsylvania yield important insights into Late Devonian terrestrial environments and ecologies. The fossil sites yield the progymnosperm plant Archaeopteris ssp., Archanodon bivalves, as well as abundant placoderms, acanthodians, and sarcopterygians including a new specimen of an early tetrapod. Most vertebrate fossils occur as abraded and disarticulated teeth, scales, and dermal bone fragments within basal lags of fluvial channel-bar complexes and articulated skull material, well-preserved scales, bones, and teeth in shallow temporary channels that meandered across vegetated floodplains. Fossil sites in these nearly flat-lying fluvial strata occur in multiple stratigraphic horizons spanning hundreds of meters of stratigraphy that represent the Sherman Creek Member and overlying Duncannon Member. The strata coarsen upsection and record progressively larger channel-bar complexes and better-developed paleosols. Applied detrital muscovite 40Ar/39Ar geochronology supplies the first quantitative age data from the sampled strata and provides information on erosion rate parameters and channel network morphology. Detrital ages indicate rapid uplift, erosion, and deposition of sediments in the Catskill fluvial system during Late Devonian time. Detrital ages also indicate temporal variation and lack of depositional linkages among Late Devonian fossil sites. Variations in the fossil assemblages between sites reflect paleoecological variations among paleo-subenvironments (channel-bars vs. floodplains) and/or taphonomic processes. Vertebrate assemblages and fluvial lithofacies documented at the recently discovered sites are broadly comparable with those in slightly younger strata at Red Hill, a well-studied site ~50 km to the west that yields a diverse vertebrate assemblage, including several taxa of early tetrapods. These heterogeneous, shallow, non-marine aquatic habitats were a crucible for experimentation in limb mobility and other specializations among sarcopterygians during Late Devonian time. Integration of paleontological, sedimentological, and geochronological data offers new insights into Late Devonian fluvial ecosystem evolution.