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Unusually Preserved Salterella from the Lower Cambrian Forteau Formation of Newfoundland

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Unusually Preserved Salterella from the Lower Cambrian Forteau Formation of Newfoundland See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/298929612 Unusually preserved Salterella from the Lower Cambrian Forteau Formation of Newfoundland Article in Gff -Uppsala- · March 2003 Impact Factor: 1.31 READS 19 1 author: Christian B. Skovsted Swedish Museum of Natural History 98 PUBLICATIONS 1,239 CITATIONS SEE PROFILE All in-text references underlined in blue are linked to publications on ResearchGate, Available from: Christian B. Skovsted letting you access and read them immediately. Retrieved on: 15 July 2016 GFF volume 125 (2003), pp. 17–22 Article Unusually preserved Salterella from the Lower Cambrian Forteau Formation of Newfoundland CHRISTIAN B. SKOVSTED1 Skovsted, C.B., 2003: Unusually preserved Salterella from the Lower Cambrian Forteau Formation of Newfoundland. GFF, Vol. 125 (Pt. 1, March), pp. 17–22. Stockholm. ISSN 1103-5897. Abstract: Three-dimensionally preserved specimens of the problematic Early Cambrian fossil Salterella are described from acid-resistant residues of limestone samples from the Lower Cambrian Forteau Formation of western Newfoundland. The material exhibits three disparate modes of preservation, complementing the prevailing description of the fossil in the literature from thin- sections. Juvenile specimens of Salterella with the external shell surface replaced by silica show a pronounced ornamentation of transverse annulations. Other specimens are preserved either by infilling of the apertural cavity by diagenetic phosphate, or replacement by silica of originally calcareous shell material. Replacement by silica of the internal lining of the apertural cavity, and infilling of the apertural cavity by secondary phosphate reveal a high degree of variability of the internal morphology of the apertural region of the fossil. Keywords: Agmata, Salterella, Lower Cambrian, Forteau Formation, Newfoundland 1Department of Earth Sciences (Historical Geology and Palaeontology), Uppsala University. Norbyvägen 22, 752 36 Uppsala, Sweden; [email protected]. Manuscript received 22 January 2002. Revised manuscript accepted 21 February 2003. Introduction Fossils attributable to the problematic genus Salterella Billings, from the Lower Cambrian of the Baltic platform (Yochelson et 1861 form a conspicuous element in Early Cambrian faunas al. 1977; Rozanov 1983) and western North America (Lipps & from Laurentia. The conical fossil has a calcareous outer shell Sylvester 1968; Hagadorn & Waggoner 2002), Campitius Firby and an inner structure of alternating laminae formed by calcare- & Durham, 1975 from the Lower Cambrian of California and ous and agglutinated material. A hollow central canal runs from Nevada (Signor et al. 1985), and the Middle Cambrian Ellisell the closed apex to a cone-shaped body-cavity occupying most Peel & Berg-Madsen, 1988 from Bornholm (Peel & Berg-Mad- of the shell-width at the apertural end (Yochelson 1970, 1977, sen 1988). The suprageneric classification of Salterella and relat- Yochelson et al. 1970). The laminated deposits are formed by ed genera has been debated. Referred to the Cephalopoda in the selective agglutination of silt-size sediment grains. Both carbon- early twentieth century (e.g. Clark 1924; Poulsen 1927, 1932; ate (dolomite and calcite) and siliciclastic grains can be selected Teichert 1964), Salterella is now assigned to the family Salterel- depending on the surrounding environment (Griffin & Yochelson lidae of the extinct phylum Agmata (Yochelson 1977; Signor et 1975). The secreted calcareous component was probably calcite al. 1985). (James & Klappa 1983). No associations of Salterella conchs Salterella is commonly studied in thin sections, preferably in with any kind of operculum have been reported. The salterel- cross-sections oriented parallel to the long axis of the conch (Yo- lid appears to have been an opportunistic organism and is com- chelson 1970, 1977, 1983; Peel & Yochelson 1982). This report, monly found in great numbers on discrete bedding planes or in however, concerns silicified specimens and phosphatic internal single beds, while being absent or sparse in surrounding strata moulds of Salterella sp. in acetic acid-resistant residues from (Fritz & Yochelson 1988). the Forteau Formation of western Newfoundland and the three- Fritz & Yochelson (1988) regarded Salterella to be characteris- dimensional morphology of these fossils contributes a further tic of, and a potential index fossil for, the medial part of the Dyer- perspective to the familiar two-dimensional form visible in thin an stage (Bonnia-Olenellus zone of older usage) in the late Early sections. Acid-isolated specimens of Salterella have previously Cambrian of Laurentia. Peel & Yochelson (1982), however, de- been described only by Peel & Berg-Madsen (1988). Their report scribed an occurrence of Salterella in the Paralleledal Formation concentrated on describing the new salterellid Ellisell yochelsoni (Formation 6 of the Brønlund Fjord Group in Peel & Yochelson Peel & Berg-Madsen, 1988 from the Middle Cambrian of Born- 1982) of North Greenland. Blaker & Peel (1997) interpreted this holm, but they also illustrated and described several phosphatic formation to be of late Bonnia-Olenellus zone age. Salterella it- steinkerns of Salterella cf. S. maccullochi from a level approxi- self does not occur outside Laurentia (Fritz & Yochelson 1988), mately 100 m above the base of the Forteau Formation of Gros but it is considered to be related to Volborthella Schmidt, 1888 Morne, eastern Newfoundland. Similarly preserved material has 18 Skovsted: Unusually preserved Salterella from the Lower Cambrian Forteau Formation of Newfoundland GFF 125 (2003) also been found in Lower Cambrian strata of the taconic alloch- erella specimens inside each other has in four cases resulted in an thon of the United States (J.S. Peel, S. Bengtson, A.R. Palmer external mould inside a phosphatic mould of the apertural cav- pers com. 2002). ity (Fig. 1C). These external moulds show an ornamentation of transverse annulations identical to that of co-occurring silicified specimens. Apparently, the external ornamentation reflects the Salterella from the Forteau Formation of formation of the salterellid shell by successive addition of new growth increments with a slightly flared or thickened brim at the Newfoundland apertural end of the conch. This ornamentations is different from The late Early Cambrian Forteau Formation (Dyeran stage) is a the coarsely rugose ornamentation described for weathered spec- unit of shales and limestones within the autochthonous Cambrian imens of Salterella in older literature (eg. Billings 1861, Clark succession of western Newfoundland and southern Labrador. The 1924). In cross-section most adult Salterella appear smooth (Fig. formation is dominated by carbonates, often with archaeocyath 2) and this led Yochelson (1970) to speculate that the coarse or- build-ups, in southern Labrador and in the western Long Range namentation is formed as the margins of the lamellae are prefer- Mountains of Newfoundland. East and South of the Long Range entially corroded or eroded. Mountains the formation is much thicker and is dominated by shale and siltstone with minor carbonate (James & Debrenne 1980). Phosphatic internal moulds: In 1978 Dr. A.R. Palmer collected fossils from Salterella-rich Most available specimens of Salterella sp. from the acid-treated beds in the Forteau Formation along the north shore of Bonne residue of sample ICS-1421 (72 specimens) have lost not only Bay, in road cuts of Highway 340, in western Newfoundland. the calcareous outer shell, but also the laminated deposits sur- Limestone-scraps accompanying two of the collections were rounding the central canal, leaving a phosphatic internal mould kindly lent to the author for acetic acid preparation in search of of the apertural cavity and central canal. The central canal is rep- Small Shelly Fossils. Sample ICS-1421 was collected from the resented by a coarsely rugose spine and the apertural cavity by a basal layers of a 4 m thick prominent limestone marker bed about simple, smooth cone, slightly undulating in outline (Fig. 1C–E). 180 m above the base of the formation. The locality is in the Gros Rugae on the spine reflect the margins of successive laminations Morne quadrangle at co-ordinates 4. 40,500 E and 54. 85,500 within the conch (cf. Peel & Berg-Madsen 1988). The apertural to 85,900 N, almost due east of Norris Point. The undescribed end of available specimens is always broken and the apertural macro-fauna includes trilobites (species of Wanneria, Bonnia, margins are not delineated. ptychoparioids), brachiopods, hyoliths and the mollusc Pelag- A few specimens are partly coated with coarsely crystalline iella sp., in addition to Salterella sp. Sample ICS-1422 was col- silica (Fig. 1E), but the surface of most moulds is very smooth. lected further north along the highway from calcitic nodules at In a single specimen, a slightly undulating linear marking char- an unresolved level within a 70 m thick siltstone-unit (40 to 110 acterised by a relatively coarse surface-texture is preserved (Fig. m above the base of the formation). Associated macro-fossils 1D). The line is elevated slightly above the surrounding surface include members of the trilobite genera Wanneria and Olenellus? of the mould and runs roughly normal to the long axis of the (A.R. Palmer pers. com. 2002). fossil. This linear feature is situated at about two thirds of the Both samples were digested in 10% acetic acid.
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