James W. Hagadorn

Rare helical spheroidal fossils from the Doushantuo Lagerstatte: Ediacaran animal embryos come of age?

A small quantity of helically coiled spheroidal fossils has been recovered from acid digestion of phosphorite samples from the Ediacaran Doushantuo Formation, South China. These fossils consist of an internal body enclosed in a sculptured envelope that is very similar to that of Doushantuo animal eggs and blastula embryos such as Megasphaera ornata . A hallmark of these fossils is a three-dimensional spiral structure, which always consists of three clockwise coils, and occurs on both the envelope and the internal body. The spiral structure consists of a spiral tunnel or canal flanked by two raised levees, and it is punctured by a series of holes. Some specimens show evidence of uncoiling, invagination along the spiral structure, or bipectinate furrowing on the band between canals. A possible ontogenetic link between these helical spheroidal fossils and Megasphaera ornata is suggested by similar size, similar envelope sculptures, and co-occurrence. We tentatively interpret these fossils as postblastula embryos related to Megasphaera ornata . Thus, they may represent the most advanced embryonic fossils so far known from the Ediacaran, although their adult morphologies and phylogenetic affinity remain unknown.

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.

STRANDED IN UPSTATE NEW YORK: CAMBRIAN SCYPHOMEDUSAE FROM THE POTSDAM SANDSTONE

Abstract The Cambrian portion of the Potsdam Sandstone contains a suite of scyphomedusae impressions in fine-grained to medium-grained quartz arenites that outcrop on the periphery of the Adirondack Mountains, New York. The fossils are similar taphonomically and morphologically to coeval scyphomedusae from the Elk Mound Group of Wisconsin and were likely stranded on a sand flat. Soft-tissue preservation in such sandstones is rare, except in Ediacaran Konservat-Lagerstätten. Although subtidal facies are abundant and continental facies are present in the Potsdam, soft-bodied fossils are found only in emergent coastal facies. These units are characterized by microbial structures including domal sand buildups, sand shadows, and breached ripples and by such horizontal trace fossils as Climactichnites and Protichnites. Domal sand buildups mantle some medusa carcasses and suggest that carcasses were exposed at the sediment-water or sediment-air interface for significant intervals of time prior to burial. It is unknown if microbial binding mediated preservation of these carcasses, but evidence for rapid flow regime changes in the section suggest stranded medusae were resistant to the upper-flow regime deposition that buried them. In many Laurentian Cambrian sandstones, microbial binding is common, and metazoan bioturbation is minimal in intertidal and emergent facies. The Potsdam Sandstone, thus, exemplifies how the Ediacara-style taphonomic window persisted in emergent Cambrian settings. This preservational regime may persist because bioturbating metazoans did not fully colonize tidal flats until the Middle Ordovician, which allowed soft-bodied animal tracks and carcasses to be preserved without scavenging or disturbance.

Hermit arthropods 500 million years ago

Cambrian intertidal sandstones of North America record early excursions of large animals onto tidal flats, where continuous microbial films served as preservational agents for surface tracks. Whereas biomineralized fossils are rare in such lithofacies, trace fossils from the Late Cambrian Elk Mound Group of Wisconsin illustrate how some arthropods might have managed to withstand the vicissitudes of subaerial exposure—by using foreign shells like hermit crabs. This behavior is suggested by trackways (Protichnites eremita isp. nov.), which have “tail” impressions that are obliquely segmented and always shingled to the left side. These anomalous impressions are best explained by a dextrally coiled shell intermittently touching the sediment. However, unlike in modern hermit crabs, this shell was too small to house the whole animal. It probably served only to provide a humid chamber that reduced desiccation of the animal9s abdominal gills. The dorsal flexure of the tail, in connection with dextral shell coiling, resulted in left-hand shingling of the touch marks.