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Discoidal macrofossils reported herein from the lower Cambrian Zabriskie Quartzite (Great Basin, western United States) record the oldest Phanerozoic medusozoan body fossils, as well as the oldest medusozoan stranding event on record. Moreover, these fossils provide evidence of a significant shift in the taphonomic mode characteristic of preservation of nonmineralized taxa in coarse-grained siliciclastic successions near the onset of the Phanerozoic. Taphonomic and sedimentological evidence recorded by these and younger examples of stranded Cambrian medusae is consistent in suggesting that several of the requirements for preservation of these fossils were holdovers from the Ediacaran Period, including the presence of microbial mats and a lack of carcass disturbance by scavenging and/or bioturbating taxa. To shed further light upon the taphonomic factors necessary for the preservation of Cambrian medusae, we compared the biostratinomy and sedimentology of Cambrian medusa strandings to those of Ediacara Biota assemblages from lithologically similar successions. We find key secular disparities in the taphonomic histories of these two types of fossil assemblage. Inconsistencies between the preservational styles characteristic of fossil assemblages preserved in sandstone lithofacies on each side of the Precambrian–Cambrian boundary are explained by a considerable change in the preferred depositional setting in which these macrofossil assemblages are preserved. Thus, rather than documenting a single taphonomic continuum through the Precambrian–Cambrian transition, the Zabriskie and younger medusozoan body fossil assemblages record the advent of an entirely new, yet still very rarely exploited, taphonomic window exclusive to the Cambrian Period.

Zoophycos-group burrows are prevalent elements of the post-Cambrian trace fossil record. Here we report the oldest specimens of Zoophycos from Lower Cambrian strata of the Lower Member Wood Canyon Formation in southeastern California. In addition to these being the oldest examples of this well-known trace fossil, the discovery of these specimens also reveals the presence of deposit feeding considerably earlier than has been suggested for the advent of this feeding style. This type of activity may have had a significant impact on sediment mixing during the Precambrian–Cambrian transition, though the rarity and shallow tier position of these specimens suggests otherwise.

Garson, D.E., Gaines, R.R., Droser, M.L., Liddell, W.D. & Sappenfield, A. 2011: Dynamicpalaeoredox and exceptional preservation in the Cambrian Spence Shale of Utah. Le-thaia, DOI:10.1111⁄j.1502-3931.2011.00266.xBurgess Shale-type faunas provide a unique glimpse into the diversification of metazoanlife during the Cambrian. Although anoxia has long been thought to be a pre-requisitefor this particular type of soft-bodied preservation, the palaeoenvironmental conditionsthat regulated extraordinary preservation have not been fully constrained. In particular,the necessity of bottom water anoxia, long considered a pre-requisite, has been the sub-ject of recent debate. In this study, we apply a micro-stratigraphical, ichnologicalapproachtodeterminebottom wateroxygen conditionsunder,which BurgessShale-typebiotas were preserved in the Middle Cambrian Spence Shale of Utah. Mudstones of theSpence Shale are characterizedby fine scale (mm-cm)alternation between laminated andbioturbated intervals, suggesting high-frequency fluctuations in bottom water oxygena-tion. Whilst background oxygen levels were not high enough to support continuousinfaunal activity, brief intervals of improved bottom water oxygen conditions punctuatethe succession. A diverse skeletonized benthic fauna, including various polymerid trilo-bites, hyolithids, brachiopods and ctenocystoids suggests that complex dysoxic benthiccommunity was established during times when bottom water oxygen conditions werepermissive. Burgess Shale-type preservation within the Spence Shale is largely confinedto non-bioturbated horizons, suggesting that benthic anoxia prevailed in intervals, wherethese fossils were preserved. However, some soft-bodied fossils are found within weaklyto moderately bioturbated intervals (Ichnofabric Index 2 and 3). This suggests that Bur-gess Shale-type preservation is strongly favoured by bottom water anoxia, but may notrequire it in all cases. h Anoxia, Burgess Shale, Burgess Shale type-preservation, LangstonFormation, Spence Shale Member,Utah.Daniel E. Garson [daniel.garson@gmail.com], Department of Earth Sciences, University ofCalifornia, Riverside, CA 92521, USA; Robert R. Gaines [Robert.Gaines@pomona.edu],Geology Department, Pomona College, 185 E. Sixth St., Claremont, CA 91711, USA; MaryL. Droser [mary.droser@ucr.edu], Department of Earth Sciences, University of California,Riverside, CA 92521, USA; W. David Liddel [dave.liddell@usu.edu], Department ofGeology, Utah State University, Logan, UT 84322-4505, USA; Aaron Sappenfield[aaron.sappenfield@email.ucr.edu], Department of Earth Sciences, University of California,Riverside, CA 92521, USA; manuscript received on 14 September 2010; manuscript acceptedon 04 February 2011.

Ediacaran trace fossils are becoming an increasingly less common component of the total Precambrian fossil record as structures previously interpreted as trace fossils are reinterpreted as body fossils by utilizing qualitative criteria. Two morphotypes, Form E and Form F of Glaessner (1969), interpreted as trace fossils from the Ediacara Member of the Rawnsley Quartzite in South Australia are shown here to be body fossils of a single, previously unidentified tubular constructional morphology formally described herein as Somatohelix sinuosus n. gen. n. sp. S. sinuosus is 2-7 mm wide and 3-14 cm long and is preserved as sinusoidal casts and molds on the base of beds. Well-preserved examples of this fossil preserve distinct body fossil traits such as folding, current alignment, and potential attachment to holdfasts. Nearly 200 specimens of this fossil have been documented from reconstructed bedding surfaces within the Ediacara Member. When viewed in isolated hand sample, many of these specimens resemble ichnofossils. However, the ability to view large quantities of reassembled and successive bedding surfaces within specific outcrops of the Ediacara Member provides a new perspective, revealing that isolated specimens of rectilinear grooves on bed bases are not trace fossils but are poorly preserved specimens of S. sinuosus. Variation in the quality and style of preservation of S. sinuosus on a single surface and the few distinct characteristics preserved within this relatively indistinct fossil also provides the necessary data required to define a taphonomic gradient for this fossil. Armed with this information, structures which have been problematic in the past can now be confidently identified as S. sinuosus based on morphological criteria. This suggests that the original organism that produced this fossil was a widespread and abundant component of the Ediacaran ecosystem.