Paul A. Johnston

Geochemical evidence for oxygenated bottom waters during deposition of fossiliferous strata of the Burgess Shale Formation

Abstract Deciphering the oxygenation potential of bottom waters during deposition of the fossiliferous strata of the Burgess Shale is key to understanding the paleoecology of the organisms preserved as fossils and the processes involved in their preservation. Methods of paleo-redox determination that are based upon trace fossils, organic carbon content, or size analysis of pyrite framboids are problematic when applied to the Burgess Shale. Fortunately, patterns in redox-sensitive trace elements hold great potential for determining paleo-redox conditions for these greenschist-facies rocks because they are independent of both age and metamorphic grade. Four Burgess Shale-Type (BST) fossil-bearing sections were studied; three lie within the Burgess Shale Formation, whereas one lies within the slightly younger Duchesnay unit of the Middle Chancellor Formation. The geochemical proxies of seawater redox conditions that were used are Mo, U/Th, V/Cr, V/(V+Ni), Ni/Co and V/Sc. Each of these redox-sensitive indices suggests that oxygenated conditions prevailed in the overlying seawater during deposition of beds within which BST preservation developed. Non-fossiliferous beds at the base of the Mt. Stephen Trilobite Beds yield three geochemical indices (V/[V+Ni], V/Sc, Ni/Co) that indicate deposition under dysoxic to anoxic conditions. Thus the only beds to have been deposited under anoxic conditions are barren of fossils of soft-bodied taxa, further contradicting the notion that bottom-water anoxia is a factor in BST preservation. The laminated nature of the Burgess Shale and evidence for subsurface anoxia, coupled with evidence for oxygenated bottom waters, presence of bacterial mats, and surficial trace fossils suggest that the Burgess Shale was deposited in the exaerobic zone (oxic–anoxic boundary at the water–sediment interface). Implications of this reinterpretation of the depositional environment of the Burgess Shale include: (1) taphonomic models for BST preservation must account for the presence of oxygen in bottom waters; (2) paleoecological interpretations of the Burgess Shale need to account for a probable autochthonous benthic component in what is a complex assemblage of fossils that includes elements derived from the platform; and (3) the possibility of Burgess Shale communities based upon chemosynthesis and/or bacteria grazing must be considered.

Alleged mosasaur bite marks on Late Cretaceous ammonites are limpet (patellogastropod) home scars

New data contradict the mosasaur-bite hypothesis for the origin of holes seen in Placenticeras ammonites from the Late Cretaceous Pierre Shale and Bearpaw Formation of the western interior of North America. Observations of a limpet-infested Placenticeras ammonite and of several Placenticeras specimens with radular scratch marks reveal that the limpets dwelled on floating ammonites, and their homing activity produced circular depressions. When altered during diagenesis, the depressions mimic tooth punctures. Crushing experiments on fresh Nautilus shells using a mosasaur robot show that mosasaur bites could not have produced holes resembling those in the fossils. Furthermore, sectioning of “bitten” ammonites reveals that sepia are sometimes intact under the holes, an observation irreconcilable with penetration by a tooth. We present an alternative interpretation that the alleged “bite marks” in Placenticeras ammonites are really limpet home scars that were altered after burial. While predation of mosasaurs on ammonites remains a possibility, it would have to be demonstrated by other criteria. Our findings do not support mosasaur-ammonoid coevolution in the Late Cretaceous.

Stratigraphy, Paleoecology, and Origin of Lower Devonian (Emsian) Carbonate Mud Buildups, Hamar Laghdad, Eastern Anti-Atlas, Morocco, Africa

ABSTRACT Recent debates concerning the origin of Emsian carbonate mud buildups exposed at Hamar Laghdad in the eastern Anti-Atlas of Morocco centre around geochemical studies that suggest authigenic or chemosynthetic processes induced precipitation of mud from hydrothermal fluids. Geochemical data alone cannot be used to demonstrate conclusively the origin of the mud comprising the buildups, but an in situ source and early cementation can be inferred from morphology, internal architecture, and sedimentary structures. Morphological types vary from symmetrical mounds up to 12 m high to asymmetrical pinnacles that locally coalesced, forming elongate multicored complexes up to 56 m high. Their steep-sided nature (margins dipping up to 54°) and sporadic occurrence of erect invertebrates suggest localized production of mud rather than hydrodynamic piling or baffling from suspension. Irregular nodular beds that thicken internally imply centralized production and a local source for the mud. Displaced nodules and slump folds indicate early cementation of mud to at least a firm consistency, followed by downslope movement. Abrupt termination of carbonate production during late transgression resulted in drowning of the mud buildups and adjacent platform. Except for a thin condensed bed deposited during maximum flooding, the buildups and platform (transgressive systems tract deposits) remained uncovered for an extended period (upper inversus-laticostatus to lower serotinus zones) before being buried beneath progradational clinoforms of nodular limestone and marlstone (regressive systems tract deposits). Overlying Middle Devonian carbonates contain evidence of hydrothermal seepage, based on the occurrence of solemyid and anomalodesmatan bivalves, which are thought to be chemosymbiotic. These bivalves occur in a mud buildup rooted on an Emsian counterpart, suggesting periodic hydrothermal activity beneath the buildups. A hydrothermal origin for the Hamar Laghdad mud buildups appears to be consistent with other mud buildups of similar age in other parts of northwest Africa. Vast deposits of coeval Minette-type ooidal ironstones, which also have a seep origin in the Tindouf Basin, south of the Anti-Atlas, testify to occasional widespread hydrothermal activity along the northwest margin of Gondwana during much of the Devonian. End_Page 217------------------------

Systematics and ontogeny of a new bivalve, Umburra cinefacta, from the Silurian of Australia: implications for pteriomorphian evolution

A well preserved pteriomorphian bivalve from the Middle Silurian of New South Wales is assigned to a new genus and species, Umburra cinefacta, and a new, monotypic family, Umburridae. The Umburridae are included in the Superorder Eupteriomorphia on the basis of the left-convex, inequivalved shell, but show some prionodont features including a continuous arcoid-like dentition below the ligament, the absence of a byssal notch and a nonalate shell. Dental patterns in U. cinefacta change significantly through ontogeny. These patterns resemble, to varying degrees, the adult and/or juvenile dentitions of a variety of pteriomorphian groups including the Cyrtodontidae, Pterineidae, Rhombopteriidae and early Pectinina as well as the Cycloconchidae (the probable sister group of the Pteriomorphia), allowing recognition of homologous teeth and sockets among these diverse taxa. By outgroup comparison with the Cycloconchidae, the Umburridae are more primitive dentally than are the Cyrtodontidae, a family previously tho...