A 'PLACODERM'-DOMINATED VERTEBRATE FAUNA PRESERVED AT A TRANSITIONAL MARINE-CONTINENTAL EXPOSURE IN NORTH-CENTRAL PENNSYLVANIA

Upper Devonian strata worldwide record the emergence of continental ecosystems and the diversification of vertebrates. Recent prospecting has revealed a highly fossiliferous site in upper Famennian strata (Upper Lock Haven Formation and Lower Catskill Formation) at the Burlington Quarry site in Bradford County, north-central Pennsylvania. Sedimentology, palynology, and ichnology reveal three depositional environments that preserve vertebrates: distal fluvial channel-floodplain, transitional fluvial–marine, and nearshore marine. ‘Placoderms’ (Bothriolepis, Phyllolepis, and pachyosteomorph arthrodires) are the most commonly occurring vertebrates at Burlington with ischnacanthid acanthodians and sarcopterygians (Holoptychius and a tristichopterid) rarely occurring. Bothriolepis, Phyllolepis, ischnacanthids, Holoptychius, and tristichopterids were recovered from distal fluvial strata while a single articulated Phyllolepis, abundant isolated, fragmentary plates of pachyosteomorph arthrodires, and a fully articulated, three-dimensionally preserved posterior head shield of a large unidentified pachyosteomorph arthrodire were identified from transitional and nearshore marine strata. In nearshore marine, transitional, and distal fluvial deposits, vertebrate fossils are mostly preserved as highly concentrated disarticulated and abraded plates, scales, and bone fragments indicating transport in high-energy waterways before deposition. Partly to fully-articulated, unabraded vertebrate remains occurring in distal fluvial strata indicate deposition in floodplains resulting from crevasse-splay or overbank deposition. Isolated and fragmentary plates of pachyosteomorph arthrodires occur sporadically at other transitional and nearshore marine sites in both Catskill Formation and Lock Haven Formation strata in north-central Pennsylvania. However, remains of these types of ‘placoderms’ are abundant at the Burlington site. The coastal paleoenvironments preserved at Burlington may have represented more favorable habitat for these large-bodied ‘placoderms’ during the Late Devonian compared to paleoenvironments preserved at other sites where pachyosteomorph arthrodire remains are uncommon.