ABSTRACT. Aspidella terranovica Billings, 1872 was ®rst described from the late Neoproterozoic Fermeuse Formation (St. John's Group) on the Avalon Peninsula of eastern Newfoundland, approximately 1 km stratigraphically above the famous Ediacaran biota at Mistaken Point, and several kilometres below the base of the Cambrian. Aspidella has been reinterpreted perhaps more than any other Precambrian taxon, and has variously been regarded as a fossil mollusc or medusoid', a gas escape structure, a concretion, or a mechanical suction mark. Our studies indicate that Aspidella includes a wide variety of preservational morphs varying from negative hyporeliefs with a raised rim and ridges radiating from a slit (Aspidella-type preservation), to¯at discs with a central boss and sharp outer ring (Spriggia preservation), to positive hyporeliefs with concentric ornamentation (Ediacaria preservation). Specimens occur in a continuum of sizes, with preservational styles dependent on the size of the specimen and the grain size of the host lithology; the elongation of specimens is tectonic. Aspidella is con®rmed as a body fossil from observations of complex radial and concentric ornamentation, mutually deformed borders in clusters of specimens, and occurrence on the same bedding planes as certain distinctive Ediacaran taxa. Aspidella is indistinguishable from, and has priority over, several of the most common genera of late Neoproterozoic discoidal body fossils worldwide. Similar fossils from Australia are interpreted as holdfasts of frond-like organisms. The density of specimens in the Aspidella beds suggests levels of benthic biomass in the Neoproterozoic that could rival those of modern marine communities. The serial growth forms, Palaeopascichnus, Intrites, Neonereites renarius and Yelovichnus, associated with Aspidella, are interpreted as body fossils of unknown af®nities rather than trace fossils. A new, trilobed, Ediacaran body fossil, Triforillonia costellae gen. et sp. nov., is described from the Aspidella beds of the Fermeuse Formation.T H E discovery and description of Aspidella terranovica Billings, 1872 in what is now known as the Fermeuse Formation of the St. John's Group, from the Avalon Zone of eastern Newfoundland (Text-®gs 1±2), came at a time when geologists were grappling with the relatively new disciplines of stratigraphy and systematic palaeontology. The earliest references to these discoidal impressions realized their utility in establishing stratigraphical relationships on the Avalon Peninsula (Murray 1868, 1873; Billings 1872). It was another matter to explain enormous numbers of fossils of apparently soft-bodied organisms in very old strata. The fact that the Aspidella-bearing formation clearly underlay the local trilobite-bearing`Primordial' strata, with marked unconformity, added to the concern of later commentators.Despite regular reviews of the status of Aspidella (see Hofmann, 1971 and references therein, plus subsequent references by Hsu È (1972), King (1980), Landing et al. (1988), Conway Morris (19...
The Mistaken Point and Trepassey formations (Conception and St. John's groups, respectively) comprise a terminal Neoproterozoic, deep-marine succession of fine-grained turbidites and volcanogenic deposits that are part of the Avalonian Terrane. Debris-flow beds, slumped units, the low dispersion of turbidity-current paleoflow directions, and the absence of wave-generated structures together indicate that the sediment was deposited on a deep-water, southeastfacing slope. Channels were not present in the study area. The upward increase in the abundance of slump structures suggests that these units represent toe-of-slope and mid-slope environments, respectively. These units prograded over basin-floor deposits of the Drook and Briscal formations, which have (axial) paleocurrent directions that are orthogonal to the inferred downslope flow that characterized the overlying deposits. Within the Mistaken Point and Trepassey formations, a diverse assemblage of soft-bodied, non-phototrophic Ediacaran organisms is preserved beneath volcanic ash layers on more than one hundred surfaces. Individual fossiliferous surfaces can be correlated up to several kilometres. The felling orientations of frondose fossils indicate that contour currents, as well as up-and downslope currents of tidal and (or) wind-forced origin, influenced the sea floor in the intervals between event beds when the organisms lived. The contour currents may have been responsible for sustaining the organisms in this deep-water setting. The current-produced inclination of the frondose organisms at the time of ash deposition allowed their preservation by preventing the accumulation of ash beneath them.Résumé : Les formations de Mistaken Point et de Trepassey (respectivement des groupes de Conception et de St. John's) comprennent une succession en eau profonde (Néoprotérozoïque terminal) de dépôts volcanogènes et de turbidites à grain fins qui font partie du terrane d'Avalon. Pris ensemble, les lits de dépôts de coulées de débris, les unités affaissées, la faible dispersion des directions de paléo-écoulements de courants de turbidité et l'absence de structures générées par les vagues indiquent que les sédiments ont été déposés sur une pente en eau profonde qui faisait face au sud-est. La région étudiée ne comportait pas de chenaux. La croissance vers le haut de l'abondance de structures d'affaissement suggère que ces unités représentent respectivement des environnements de pied de talus et de mi-pente. Ces unités se sont accumulées progressivement par-dessus les dépôts de fond du bassin des formations de Drook et de Briscal, lesquelles ont des directions (axiales) de paléocourants qui sont orthogonales à l'écoulement inféré vers le bas de la pente qui caractérise les dépôts sus-jacents. À l'intérieur des formations de Mistaken Point et de Trepassey, un assemblage diversifié de faune d'Ediacara, à corps mou et non phototrophe, est préservé sous des couches de cendre volcanique sur plus d'une centaine de surfaces. Des surfaces fossilifères individuelles peuvent être...
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