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

The Catskill Formation of north-central Pennsylvania contains diverse assemblages of Late Devonian continental organisms. Four vertebrate fossil sites were recently discovered in northern Lycoming County. The Steam Valley East and Mill Creek sites have yielded mostly Bothriolepis (Placoderm; Antiarchi) and Holoptychius (Sarcopterygii; Porolepiformes) fossils, with phyllolepid placoderm material also being found at Steam Valley East. The Trout Run north site produced an acanthodian (Gyracanthus), two genera of sarcopterygian fishes (a megalicthyid and Holoptychius) and an isolated left femur of an early tetrapod (cf. Whatcheeriidae). Steam Valley West produced two genera of placoderms (Turrisaspis and Phyllolepis) and four genera of sarcopterygians (a megalicthyid, Holoptychius, Eusthenodon, and a rhizodontid). Most fossils are preserved as disarticulated spines, scales, and dermal bone fragments within basal lags associated with fluvial channel-bar complexes and temporary channels (i.e. crevasse splays) that meandered across vegetated floodplains. Fossil sites in these nearly flat-lying fluvial strata occur in multiple stratigraphic horizons spanning >180m of delta plain sedimentation (Sherman Creek Member) that transitions upsection into alluvial plain facies (Duncannon Member), although the duration of time sampled is not well constrained. Traditionally, Catskill vertebrate faunas have been viewed as occurring within two distinct assemblages: a Bothriolepis and Holoptychius dominated assemblage characteristic of the delta plain succession and a more diverse fauna of placoderms, acanthodians, sarcopterygians, and early tetrapods from more proximal, alluvial plain successions. Our data indicate a more complex scenario. Bothriolepis-Holoptychius types of assemblages occur in both lower delta plain (Sherman Creek Member) and alluvial plain (Duncannon Member) settings. More diverse assemblages of vertebrates are also common in some alluvial plain settings and vary significantly between sites, indicating the fossil assemblages reflect paleoecological variations among paleo-subenvironments (channel-bars vs. floodplains) and/or taphonomic processes.