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

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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. Internal distance from the bottom of the apertural cavity to the broken moulds of Salterella sp. are a common component of the result- anterior margin. The line has a width of approximately 10 μm ing acid-resistant residues, and these fossils occur together with and can be followed for at least one quarter of the circumference silicified shells of the same organism. The sparse fauna accompa- of the apertural cavity (0.4 mm). nying Salterella sp. in the residue of sample ICS-1421 includes No comparable structures have been found in other specimens the molluscs Pelagiella sp. and Yochelcionella sp., echinoderm and it is therefore not possible to assess the significance of the sclerites, paterinid and lingulid brachiopods, silicified trilobite structure. If interpreted as an original feature of the salterellid and hyolith remains, and species of the problematic genera Trip- shell, it would correspond to a shallow depression on the internal licatella, Hyolithellus and Chancelloria. Sample ICS-1422 con- surface of the apertural cavity. This depression could represent a tains silicified trilobite remains and abundant specimens of the site of attachment for the soft parts of the organism to the shell. mollusc Fordilla sp. in addition to Salterella sp. Internal silicification: Types of preservation The internal surface of the apertural cavity of 61 specimens of Salterella sp. from the residue of sample ICS-1422 is preserved Silicified conchs of Salterella: by silicification (Fig. 1F–H). In this case, however, silicification Ten specimens of Salterella sp. from the residue of sample ICS- does not affect the outer surface of the fossil, and the result- 1421 have the external shell surface preserved by silicification ing specimens can be regarded as partial internal moulds. Most (Fig. 1A, B). All specimens are less than 1.5 mm in length. The specimens preserve only parts of the apertural cavity, preferen- rate of expansion is uniform in all specimens and the diameter tially the outer portion. The inner part of the apertural cavity of the aperture is approximately one half the assumed original and the central canal is usually only preserved in co-occurring length of the conch. Most silicified specimens are empty or filled internal moulds formed either by matrix or by diagenetic phos- with matrix, but at least two specimens have a partly preserved phate. Specimens range from 0.5 to 3 mm in width. The height silicified shell enclosing a phosphatic steinkern (Fig. 1B). The of available moulds varies significantly (compare Figs. 1F and external surface has an ornamentation of well defined transverse 1G). This variation probably reflects variable depth of the origi- annulations. Individual annulations are continuous around the nal apertural cavity, but mineral matter filling parts of the ap- shell and average 30 μm in width. Post mortem stacking of Salt- ertural cavity prior to the precipitation of silica could possibly GFF 125 (2003) Skovsted: Unusually preserved Salterella from the Lower Cambrian Forteau Formation of Newfoundland 19

Fig. 1. Silicified and phosphatised specimens of Salterella sp. from the Forteau Formation of western Newfoundland. SEM micrographs. All scale bars equal 200 μm. A. PMU Ca1. Almost complete, silicified conch with prominent transverse growth-annulations, B. PMU Ca2. Partial silicified conch enclosing a phosphatic mould of the apertural cavity, C. PMU Ca3. Oblique anterior view of external mould exhibiting transverse annulations preserved inside a phosphatic internal mould of a different specimen, D. PMU Ca4. Phosphatic internal mould showing the smooth wall of the apertural cavity and the rugose central canal. The arrow indicates the position of a raised linear marking on the surface of the mould, E. PMU Ca5. Phosphatic internal mould of apertural cavity and central canal. The apertural part of the steinkern is partly covered with coarsely crystalline silica, F. PMU Ca6. Coarsely silicified internal surface of apertural margin and deep apertural cavity, G. PMU Ca7. Oblique view of silicified internal surface of apertural margin and shallow apertural cavity, H. PMU Ca8. Oblique view of internal mould of apertural cavity, partly coated with silicified shell material in the apertural region. 20 Skovsted: Unusually preserved Salterella from the Lower Cambrian Forteau Formation of Newfoundland GFF 125 (2003)

Fig. 2. Longitudinal thin sections of Salterella maccullochi (Murchison 1859) from the Hyolithus Creek Formation of Kap Weber, North-East Greenland. A. MGUH 15920, I18, B. MGUH 15918, I14 (previously ﬁgured in Peel & Yochelson 1982 and Peel & Berg-Madsen 1988).

have obscured the original morphology. Approximately 10% of material (Fig. 1H). The central canal is sometimes preserved as a available specimens are oval rather than circular in cross-section. short and sharply pointed spine on the apical end of the fossils. A faint concentric ornamentation on the surface of some specimens (Fig. 1G) could possibly reflect traces of the laminated and agglutinated deposits occupying the posterior part of the conch. Discussion The laminated and agglutinated deposits themselves are not pre- Three species of Salterella, S. maccullochi (Murchison 1859), S. served in any specimens. pulchella Billings, 1861 and S. conulata Clark, 1924 have been The diameter of the moulds increases dramatically and sud- reported to occur in the Forteau Formation (Schuchert & Dun- denly close to the aperture, representing the impression in the bar 1934, Yochelson 1970, Fritz & Yochelson 1988). Indeed, the matrix of the apertural margin. This increase in diameter of the generic name Salterella was first used for specimens from the moulds corresponds to the presumed thickness of the shell (Fig. Forteau Formation of southern Labrador (Billings 1861). The 1F–G), and varies between 20% and 50%, of the diameter of concept of S. maccullochi was greatly expanded by Yochelson the shell interior. There is apparently no correlation between this (1983) and it is both the most common and the best known spe- ratio and the absolute diameter of the moulds. The same pattern cies of the genus (Fritz & Yochelson 1988). No modern study of of apertural widening of the cavity corresponding to the thick- S. pulchella has been published, but the species is supposedly ness of the shell is evident in sectioned specimens of Salterella distinguished from S. maccullochi mainly by the gentle curva- conulata Clark, 1924 described by Yochelson (1970). ture of the conch (Billings 1861; Walcott 1886; Lochman 1956). Steinkerns formed by matrix or diagenetic phosphate from Yochelson (1970) redescribed S. conulata and illustrated thin sample ICS-1422 are generally more coarsely preserved than sections of specimens from a number of localities (including the comparable specimens from sample ICS-1421 and surface de- Bonne Bay area), but little is known about the external morphol- tails are obscured. Available specimens are cone-shaped and ogy of this species. undulating in outline, sometimes partly coated in silicified shell- GFF 125 (2003) Skovsted: Unusually preserved Salterella from the Lower Cambrian Forteau Formation of Newfoundland 21

Thin sections of Salterella from samples ICS-1421 and ICS- The two collections of Salterella sp. described above differ 1422 could not be produced due to lack of material. However, not only in preservation, but also to some degree in morphology. Yochelson has illustrated sectioned specimens of S. conulata Depth of the apertural cavity is widely variable in specimens from the same formation and general area of Newfoundland (Yo- from the stratigraphically lower sample, ICS-1422, while inter- chelson 1970, pl. 2, figs 1–4 and pl.3, figs 5–6). The material was nal moulds from the stratigraphically higher sample, ICS-1421, collected in 1910 by C. Schuchert from the Forteau Formation are quite uniform in this character. In addition, the characteristic of the Bonne Bay area (Schuchert & Dunbar1934), but the strati- apertural widening of specimens from ICS-1422 is not recog- graphic position of samples is not quite clear (Yochelson 1970). nised in specimens from ICS-1421. However, these differences Sectioned specimens of S. maccullochi from the Hyolithus Creek could perhaps be explained by differences in preservation. Formation of Kap Weber, North-East Greenland were studied for Acid isolated specimens from Newfoundland can be compared comparison with the Newfoundland material (Fig. 2). Specimens to published pictures of thin sections of S. conulata (Yochelson from the same thin sections were illustrated and briefly described 1970). The depth of the apertural cavity in this species appears by Peel & Yochelson (1982) as well as Peel & Berg-Madsen to be highly variable (compare Yochelson 1970, pl. 2, fig. 4 to (1988). In thin section the Greenland specimens preserve the fa- pl. 6 fig. 4), and the species further shows wide expansion of the miliar calcitic and agglutinated shell. The conchs are evenly ex- internal cavity at the aperture (Yochelson 1970, pl. 4, figs 6–7, pl. panding and the agglutinated deposits fill about two thirds of the 6, figs 3–6). It is recognised that all specimens from Newfound- conchs. The apertural cavity is lined with predominately calcitic land could be compared to variously preserved S. conulata. In laminae and has a smooth but slightly undulating outline compa- sample ICS-1422 the preferentially preserved aperture shows the rable to that of the phosphatic internal moulds from sample ICS same characteristic expansion as in S. conulata (compare Fig. 1G 1421 of Newfoundland. Individual shell laminae lining the aper- to Yochelson 1970, pl. 2, figs 2–3 and Figs. 1F, H to Yochelson tural cavity appear to be continous from the central canal to the 1970, pl. 4, figs 6–7, pl. 6, figs 3–6), while steinkerns formed by actual aperture. There are no examples of externally formed shell phosphate or matrix could represent more complete moulds of the layers or of expanded apertures in the Greenland material. The apertural cavity (compare Fig. 1H to Yochelson 1970, pl. 4, fig. central canal have a rugged outline and the agglutinated laminae 5). Internal moulds from sample ICS-1421 could be compared are inserted at a relatively high angle (see also Fritz & Yochelson to the narrow central canal and the inner parts of the apertural 1988, pl. 3, fig. 1). This feature led Peel & Berg-Madsen (1988) cavity in S. conulata (compare Fig. 1C–E to Yochelson 1970, to correlate the rugged surface of the apical spines of problem- pl. 4, fig. 7). However, the more simple morphology of these atic internal moulds from the Lower Cambrian of Newfoundland specimens could likewise be compared to the apertural cavity and the Middle Cambrian of Bornholm with the central canal of and central canal of sectioned specimens of S. maccullochi (Fig. salterellids. The same pattern is evident on the apical spines on 2; Griffin & Yochelson 1975, fig. 1) or S. pulchella (Yochelson Salterella sp. described herein. 1977, pl. 2, fig. 2). No specific assignment of the present mate- Illustrated specimens of Salterella commonly have a length of rial is attempted here due to the lack of published information on 8–9 mm (Yochelson 1977), and specimens over 10 mm long are juvenile morphology and variation in species of Salterella. Fur- known (Fritz & Yochelson 1988) which suggest that most speci- thermore, the morphological contrast between the 3-dimensional mens under consideration here can be regarded as juveniles. The acid-residue remains described here, and specimens described rate of expansion of the silicified conchs from Newfoundland is from thin-sections in most previous studies currently frustrates higher than in most published specimens (Griffin & Yochelson taxonomic comparison. 1975; Yochelson 1977, 1983), but at least in S. conulata the rate Acknowledgements. – I am indebted to A.R. Palmer of the Institute for Cambrian Studies in of expansion is substatially higher in juveniles than in adult spec- Boulder, Colorado for making his collections from Newfoundland available for study. J.S. Peel kindly supplied photographs of sectioned Salterella from North-East Greenland. Finan- imens (Yochelson 1970). Comparison with the apical (juvenile) cial support from the Swedish polar research fund Ymer-80, and from the Swedish Natural part of well preserved S. maccullochi from the Sekwi Formation Science Research Council (NFR) through grants to J.S. Peel is gratefully acknowledged. J.S. Peel, L.E. Holmer and A. Kouchinsky of Uppsala University and two external reviewers, J. of north-west Canada (Fritz & Yochelson 1988) suggests that the Bergström and S. Bengtson (Stockholm) are thanked for valuable comments on the manuscript. Illustrated specimens are housed in the Museum of Evolution, Uppsala University regular ornamentation of transverse growth-increments may also (PMU prefix), or in the Geological Museum, Copenhagen (MGUH prefix). be a feature of a juvenile growth stage. Salterella sp. from ICS-1421 and ICS-1422 exhibits a range of preservational states not previously reported. Some degree of silicification and the formation of phosphatic internal moulds by References infilling of the apertural cavity and central canal are common Billings, E., 1861: On some new or little known species of Lower Silurian fossils from the Potsdam group (Primordial zone). Palaeozoic fossils vol. 1, no. 1, to both samples. Silicification of the outer calcareous shell oc- 1–18. Canadian Geological Survey. Dawson Brothers. Montreal. curs only in ICS-1421, while silicification of the internal surface Blaker, M. R. & Peel, J. S. 1997: Lower Cambrian trilobites from North Green- land. Meddelelser om Grønland, Geoscience 35, 145 pp. of the apertural cavity is common only in ICS-1422. The large Clark, T.H., 1924: The paleontology of the Beekmantown series at Levis, Que- amount of variation among specimens from sample ICS-1422 bec. Bulletins of American Paleontology 10 (41), 135 pp. could reflect differentiation in preservation. Whether individual Fritz, W.H. & Yochelson, E.L., 1988: The status of Salterella as a Lower Cam- brian index fossil. Canadian Journal of Earth Sciences 25, 403–416. laminae in the internal laminated deposits or the actual internal Griffin, W.L. & Yochelson, E.L., 1975: Mineralogy of Early Cambrian Salterella surface of the apertural cavity is preserved is sometimes difficult rugosa from East Greenland. Bulletin of the Geological Society of Denmark to assess. The presence of a siliceous coating on the surface of 24, 221–227. Hagadorn, J.W. & Waggoner, B. 2002: The Early Cambrian problematic fossil steinkerns from sample ICS-1422 suggests that at least in some Volborthella: New insights from the Basin and Range. In F.A. Corsetti (ed.): specimens the shell material lining the apertural cavity was re- Proterozoic.Cambrian of the Great Basin and beyond. Field trip guidebook and volume prepared for the annual Pacific Section SEPM fall field trip, 137– placed by silica after the formation of the internal mould. Lack 152. The Pacific Section SEPM, Fullerton, California. of material for thin sections precludes detailed analysis of the James, N.P. & Debrenne, F., 1980: First regular archaeocyaths from the northern diagenetic history of the Newfoundland Salterella. Appalachians, Forteau Formation, western Newfoundland. 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