PALEOENVIRONMENTAL ANALYSIS OF LATE DEVONIAN TETRAPOD AND FISH ASSEMBLAGES FROM CATSKILL FORMATION SITES IN NORTH-CENTRAL PENNSYLVANIA

Outcrops of the Upper Devonian Catskill Formation in north-central Pennsylvania yield diverse fossil fauna and flora that provide significant insight on the radiation of animals in freshwater and terrestrial settings. Several fossil sites discovered recently in the Steam Valley, Trout Run, and Mill Creek areas of Lycoming County yield disarticulated spines, scales, and dermal bone fragments of placoderms (Bothriolepis, Turrisaspis), an acanthodian (Gyracanthus), sarcopterygian fishes (Holoptychius, Eusthenodon, a megalicthyid and rhizodontid), and an early tetrapod, as well as Archanodon bivalves. Fossil remains occur in vertically stacked, ~1- to >5-m-thick upward-fining successions bounded by scour surfaces with relief up to a few meters. Broadly lenticular sandstones are interpreted as the deposits of meandering stream channels and point bars, whereas siltstone/claystone intervals are interpreted as floodplain/overbank deposits with abundant evidence for pedogenic overprinting (mudcracks, root casts, irregular mottles/haloes, caliche nodules, pedogenic slickensides) and minor ponded water (laminated green-gray shales/siltstones). Near Mill Creek and Trout Run, strata contain highly fragmentary vertebrate remains at the bases of sandstone/granule conglomerate beds with planar/trough/epsilon cross-stratification and in-situ tree-stump casts. Transported fossil fragments, caliche nodules, and mudstone rip-up clasts accumulated as bedload lags on the lower reaches of point bars and channel bottoms. At Steam Valley, strata exhibit lag deposits as well as less fragmented fossil concentrations in flat-bottomed, horizontally stratified sandstones capped by mudcracks and rootlets. These deposits are interpreted as temporary, aqueous habitats (i.e. crevasse channels/splays) that desiccated shortly following deposition. Depositional facies at Trout Run and Steam Valley sites are comparable with those observed in broadly coeval strata at Red Hill, a well-studied site ~50 km to the west, that yields a diverse fossil fish and early tetrapod assemblage. These heterogeneous, shallow, non-marine aquatic habitats were a crucible for experimentation in limb mobility and other specialization within sarcopterygian fish and early tetrapods during Late Devonian time.