TEMPORAL CHANGES IN LATEST DEVONIAN VERTEBRATE FOSSIL ASSEMBLAGES IN NORTH-CENTRAL PENNSYLVANIA: A CONSEQUENCE OF MOUNTAIN BUILDING, GLACIATION, AND ENVIRONMENTAL CHANGE?

Sedimentary strata exposed in north-central Pennsylvania record vertebrate communities in terrestrial environments coeval with mountain building and glaciation during the Mid-Paleozoic. Roadcut outcrops near Blossburg in Tioga County record upsection changes in vertebrate fossil assemblages coeval with changes in fluvial style. Diverse, abundant fossil remains in Catskill Formation (Upper Devonian) strata occur in lenticular red sandstones and tabular mudrocks interpreted as fluvial channel bodies and floodplain/overbank deposits. Fossil taxa are preserved as fragmented, disarticulated remains in channel lags and less fragmented, partly articulated remains along channel margins. Identification of fossil remains from several stratigraphic horizons within the Catskill Formation documents two genera of placoderms (Bothriolepis, Phyllolepis), an unidentified acanthodian, several taxa of sarcopterygian fishes (including lungfish, Holoptychius, Sauripterus, a tristichopterid, and cosmine-covered fragments of an osteolepid), a palaeoniscid actinopterygian, Archanodon bivalves, and unidentifiable floral remains. These fossiliferous strata transition upsection into gray sandstone and mudrock of the Huntley Mountain Formation (latest Devonian-Early Mississippian). Sedimentological features in this younger succession indicate coarser grained, lower-sinuosity channel-bar complexes and less abundant muddy floodplain deposits relative to the underlying Catskill Formation. Only a few vertebrate fossil fragments have been recovered from sparse basal-channel lags in the Huntley Mountain Formation, both locally and regionally. The abrupt upsection decrease in fossil abundance/diversity may be explained by differences in sampling and taphonomy. Alternatively, we hypothesize that changes to the local habitat, driven by regional tectonics and climate cooling/glaciation, negatively impacted organisms that thrived in Catskill fluvial environments. Our detrital geochronologic data imply rapid exhumation/erosion of the latest Devonian Acadian Mountains and recent studies of coeval strata (Spechty Kopf Formation) provide widespread stratigraphic evidence for basinward progradation of glaciers and proglacial rivers that prompted faunal migration/extinction.