Úna C. Farrell

Beyond Beecher's Trilobite Bed: Widespread pyritization of soft tissues in the Late Ordovician Taconic foreland basin

Pyritization of soft tissues is extremely rare. Pyritized fossils have been discovered at six new localities spanning 54 km of outcrop of the Ordovician Lorraine Group of New York State, suggesting that soft-tissue pyritization is widespread in the Taconic basin. Notable new taxa with soft-tissue preservation include ostracods and other arthropods. Such fossils are rare and occur within 4-9-cm-thick mudstones representing single rapid depositional events. High ratios of reactive iron to total iron and high values of δ 34 S, together with a near-absence of disarticulated and fragmented skeletal material, suggest that organisms in these pyritic horizons were buried rapidly and underwent bacterial sulfate reduction in porewaters rich in highly reactive iron and low in organic carbon. These conditions facilitated iron sulfi de precipitation within and on decaying carcasses. Such conditions occur repeatedly in some fi ne- grained distal turbiditic facies of the Taconic foreland basin. Pyritized soft-bodied fossils await discovery elsewhere in the Lorraine Group.

Exceptionally preserved 450-million-year-old ordovician ostracods with brood care.

Ostracod crustaceans are the most abundant fossil arthropods and are characterized by a long stratigraphic range. However, their soft parts are very rarely preserved, and the presence of ostracods in rocks older than the Silurian period [1-5] was hitherto based on the occurrence of their supposed shells. Pyritized ostracods that preserve limbs and in situ embryos, including an egg within an ovary and possible hatched individuals, are here described from rocks of the Upper Ordovician Katian Stage Lorraine Group of New York State, including examples from the famous Beechers Trilobite Bed [6, 7]. This discovery extends our knowledge of the paleobiology of ostracods by some 25 million years and provides the first unequivocal demonstration of ostracods in the Ordovician period, including the oldest known myodocope, Luprisca incuba gen. et sp. nov. It also provides conclusive evidence of a developmental brood-care strategy conserved within Ostracoda for at least 450 million years.

Paleoredox and pyritization of soft-bodied fossils in the ordovician frankfort shale of New York

Multiple beds in the Frankfort Shale (Upper Ordovician, New York State), including the original “Beechers Trilobite Bed,” yield fossils with pyritized soft-tissues. A bed-by-bed geochemical and sedimentological analysis was carried out to test previous models of soft-tissue pyritization by investigating environmental, depositional and diagenetic conditions in beds with and without soft-tissue preservation. Highly-reactive iron (FeHR), total iron (FeT), δ34S, organic carbon and redox-sensitive trace elements were measured. In particular, the partitioning of highly-reactive iron between iron-carbonates (Fe-carb), iron-oxides (Fe-ox), magnetite (Fe-mag), and pyrite (FeP) was examined. Overall, the multi-proxy sedimentary geochemical data suggest that the succession containing pyritized trilobite beds was deposited under a dysoxic water-column, in agreement with the paleontological data. The data do not exclude brief episodes of water-column anoxia characterized by a ferruginous rather than an euxinic state. However, the highest FeHR/FeT values and redox-sensitive trace element enrichments occur in siltstone portions of turbidite beds and in concretions, suggesting that subsequent diagenesis had a significant effect on the distribution of redox-sensitive elements in this succession. Moderately high FeHR/FeT and FeP/FeHR, low organic carbon, enriched δ34S, and the frequent presence of iron-rich carbonate concretions in beds with soft tissue preservation confirm that pyritization was favored where pore-waters were iron-dominated in sediments relatively poor in organic carbon.

A pyritized polychaete from the Devonian of Ontario

A polychaete from the Middle Devonian Arkona Shale at Hungry Hollow, Arkona, Ontario is preserved in three dimensions in pyrite. The prostomium bears a single median antenna, a pair of lateral antennae and a pair of ventral palps. It is assumed to be fused to a reduced peristomium. The anteriormost three pairs of trunk appendages are modified as tentacular cirri, the third long and biramous. The remainder of the finely annulated trunk bears at least 21 similar biramous parapodia, some of which preserve evidence of chaetae. The postsegmental pygidium is very small and may bear up to two pairs of cirri. The polychaete, Arkonips topororum, falls within the Palpata, Aciculata, among the crown group Phyllodocida. Its remarkable preservation highlights the potential of the Arkona Shale to yield other examples of soft-tissue preservation.

Paleoecology of the olenid Trilobite Triarthrus: New evidence from Beecher's Trilobite Bed and other sites of pyritization

ABSTRACT Olenid trilobites are characteristic of low-oxygen environments in the early Paleozoic, and researchers have proposed that olenids may have harbored chemoautotrophic symbionts, allowing them to live in borderline sulfidic environments. Beds with soft-tissue preservation at the Beechers Trilobite Bed site in the Frankfort Shale and the Martin Quarry in the Whetstone Gulf Formation (both Ordovician, New York State) are dominated by the olenid Triarthrus. A bed-by-bed analysis of the sedimentology, taphonomy, paleoecology, and ichnology demonstrates that the exceptionally preserved organisms did not undergo extensive transport, and that the intervals bearing Triarthrus accumulated predominantly in the lower part of the dysaerobic zone. These intervals contain a low-diversity benthic fauna occurring in relatively low abundance, and consisting primarily of small brachiopods and trilobites. The taphonomy, in particular localized pyritization, the associated fauna, and the distribution of Triarthrus elsewhere in the Taconic foreland basin demonstrate that the environments in which Triarthrus lived were not sulfidic, and that these trilobites were unlikely to have adopted a chemoautotrophic mode of life.