Clavitella curvata, Skovsted & Peel, 2007

Skovsted, Christian B. & Peel, John S., 2007, Small shelly fossils from the argillaceous facies of the Lower Cambrian Forteau Formation of western Newfoundland, Acta Palaeontologica Polonica 52 (4), pp. 729-748 : 743-746

Clavitella curvata sp. nov.

Fig. 7A–C.

Holotype: Phosphatic internal mould NFM−715 ( Fig. 7B) from sample JSP1982−01.

Derivation of the name: For the curved shape of the fossils.

Additional material.—NFM−714, NFM−716, and six additional specimens from sample JSP1982−01.

Diagnosis.—Same as for the genus.

Description.—Gently curved, cone−shaped fossils (1.0– 1.6 mm long) with circular cross section (0.4–0.8 mm in diameter). For most of its length the cone tapers slowly, adapically, while its curvature frequently increases slightly towards the modified apex ( Fig. 7A). The apex is reduced to a narrow spine, formed as a continuation of the supra−apical, convex, wall of the fossil (as seen in lateral view), while the sub−apical, concave side is constricted, apparently by some kind of thickening or septation within the original shell interior ( Fig. 7A, B 1, C 1 ). The spine−like apical extension sometimes tapers to a bluntly rounded point ( Fig. 7B 2 ), but is usually equidimensioned through much of its length (up to 0.4 mm long, about 0.1 mm in diameter; Fig. 7A). In one specimen it expands slightly at about mid−length ( Fig. 7C 2 ). The sub−apical thickening usually produces a gently rounded (but sometimes irregular) termination to the conical part of the internal mould and is continuous with, or forms a small angle to, the sub−apical wall, resulting in a sigmoidal profile ( Fig. 7A). The adapical termination lies in a single plane, representing a simple circular aperture. The surface of the internal mould is usually smooth, although faint furrows may be present close to the junction of the cone and the apical extension ( Fig. 7B 2 ). However, several specimens are partly covered by diagenetic minerals ( Fig. 7C 1 ).

Discussion.—These unusual fossils are all internal moulds formed by diagenetic infilling of calcium phosphate into open cavities within the original, presumably calcareous, shell. The characteristic constriction of the apex probably reflects secondary secretion of shell material in the apical part of the cone; the apical spine at the supra−apical margin probably represents a narrow, tubular cavity that penetrated or was overlain by the secondary deposits. The nature of the secondary deposits is not clear. In most specimens there is no indication of annulations or other types of ornamentation on the surface of the apical extension, and the structure appears to reflect a simple tubular continuation of the internal cavity through a solid deposit of secondary shell material. In a single specimen, the apical extension has a distinct swelling at about mid−length ( Fig. 7C), perhaps indicating that the apex was divided into distinct compartments by the repeated formation of septa. However, the lack of diagenetic phosphatic infilling of the chambers between septa indicates that these space, if present, were probably completely sealed off by shell material from the tubular extension of the cavity.

The morphology of Clavitella curvata is at least superficially reminiscent of the internal cavity of the problematic Salterella Billings, 1861 . Salterella is present in the Forteau Formation of Labrador ( Billings 1861) and western Newfoundland ( Peel and Berg−Madsen 1988; Skovsted 2003), and it is very common in the residue that yielded all known specimens of Clavitella . The shell of Salterella consists of a simple cone of calcium carbonate with a circular cross−section, partly filled with laminated deposits composed of alternating calcareous and agglutinated layers. The laminated deposits surround a straight central canal and a cone−shaped apertural cavity ( Yochelson 1977). Moulds of the cone are usually smooth, while those of the central canal are coarsely annulated (see Fig. 7H, I). Clavitella differ from internal moulds of Salterella in that the apical extension lies along the supra−apical margin, and by the lack of regular annulation on the apical spine. However, cone−shaped fossils referred to Salterella pulchella ( Billings, 1861) are gently curved in similar fashion to the present material ( Lochman 1956). This taxon was originally described from the Forteau Formation of Labrador ( Billings 1861), but has also been reported from Quebec, Vermont, and New York ( Walcott, 1886: Lochman 1956). Its internal morphology is poorly known.

The apical extension of Clavitella is also superficially similar to the infilled siphuncle of nautiloid cephalopods. The earliest known cephalopods are the plectronoceratids that appear in the Upper Cambrian (e.g., Chen and Teichert 1983; Peel 1991). These early cephalopods, exemplified by Plectronoceras Ulrich and Foerste, 1933 , have an eccentrically placed siphon adjacent to the sub−apical wall. Clavitella differs from early cephalopods by the supra−apical position of the apical spine. Furthermore, there is no strong evidence that the shell was actually divided into compartments by septa. The apical part of the shell may have been filled with massive deposits of secondary shell material. The small size and Early Cambrian age of the present fossils is also difficult to reconcile with the size and age of the earliest undoubted the manner of sponge−spicules), allowing new shell material cephalopods. to be deposited simultaneously on all surfaces.

The simple curved conchs of the hyolith Petrasotheca The triangular shape of Sphenopteron is also vaguely remiminuta Landing and Bartowski, 1996 from the Taconic Al− niscent of some hyolith opercula, and the ventral projections lochthon of New York resemble the fossils described above could be compared to hyolith clavicles. However, Sphenoin both size and curvature. However, the apices of the two pteron differ from typical hyolith opercula in lacking increconchs of P. minuta illustrated by Landing and Bartowski mental growth. Sphenopteron also shows some resemblance (1996: fig. 6.10–11) appear to be simple cones. No opercula to certain echinoderm ossicles, but does not preserve any trace identical to those of P. minuta were found in the samples of the stereome ultrastructure that is usually preserved in from western Newfoundland. In general, Clavitella is similar co−occurring echinoderm plates (see Fig. 3E–J ). to the conchs of certain orthothecid hyoliths with circular Sphenopteron boomerang gen. et sp. nov. cross−section (e.g., Conotheca Missarzhevsky, 1969 ). The Fig. 7D–G. fossils also occur in western Newfoundland together with

Holotype: Phosphatised specimen, NFM−717 ( Fig. 7D), from sample several circular or sub−circular hyolith (or hyolith−like) oper− JSP1982−01. cula without a known associated conch (e.g., Allatheca sp. ,

Derivation of the name: From the boomerang−shape of the fossil in dor− Triplicatella peltata , Cassitella baculata , and Operculum sal view. A). The apical parts of hyolith conchs are also known to be

Diagnosis.—Same as for genus. frequently constricted by septa, but these septa are not known to over grow an extension of the main shell cavity, as Additional material.—NFM−718, NFM−719, NFM−720, and seen in Clavitella . six additional specimens from sample JSP1982−01.

Description.—Roughly triangular or wedge−shaped fossils, Ocurrence.—Lower Forteau Formation of the Bonne Bay

in plan view, formed by two elongated, wing−like structures Region, western Newfoundland.

or flanks. The (arbitrarily defined) dorsal surface is shal−

lowly concave ( Fig. 7E), and the ventral surface is convex

Phylum, class, order, and family uncertain ( Fig. 7F, G 1 ). In dorsal outline bilateral symmetry is evi−

Genus Sphenopteron nov. dent, and the two straight, leading edges of the wings meet at an angle of 65–90 °, forming a rounded, “anterior”, termi− Derivation of the name: From Greek sphenos, wedge and pteron, wing.

nation. The “posterior” margin is slightly curved towards Type species: Sphenopteron boomerang sp. nov. (by monotypy). the anterior. Viewed from the anterior ( Fig. 7D), the lateral Diagnosis.—Deltoid fossils with a rounded anterior (arbi− wings are inclined at about 30 ° above the horizontal plane trarily defined) corner and two trailing flanks. The dorsal (ar− and the curved posterior margin is elevated above the rest of bitrarily defined) surface is concave, while the ventral sur− the dorsal surface. The ventral surfaces of the lateral wings face is convex and crested with one inclined blade−like pro− are convex and crested with blade−like projections (Fig. jection on each flank. 7G 1). The projections are elongated towards the posterior and overhang the posterolateral slope of the wings (Fig. Discussion.—The deltoid fossils described above are difficult to interpret biologically. The generally flattened shape of 7G2). Both dorsal and ventral surfaces lack any type of distinct ornamentation. the (presumed) dorsal surface suggests that Sphenopteron boomerang may have functioned as dermal sclerites on some Discussion.—See generic discussion above. larger animal, with the blade−like projections on the ventral surface providing sites of attachment for soft tissues. The tri− Phylum, class, and order uncertain angular shape with a posterior embayment is not incompati− Family Salterellidae Walcott, 1886 ble with this interpretation as the sclerites, if arranged in al− Genus Salterella Billings, 1861 ternating rows, could form an imbricating pattern. However,

Type species: Salterella maccullochi (Murchison, 1859) . the lack of growth−lines or other types of ornamentation may indicate that the structures did not grow by marginal accre− Salterella sp. tion, unless they were completely embedded in soft tissue (in Fig. 7H, I. +

Material.—NFM−721, NFM−722, and thousands of additional specimens from samples ICS 1421, ICS 1422, ICS 1518, ICS 1519, and JSP 1982−01.

Discussion.—Internal moulds and partly preserved conchs of Salterella are extremely numerous in acid residues from the Forteau Formation of western Newfoundland. Internal moulds of the apical cavity and central canal of Salterella were described by Peel and Berg−Madsen (1988), while Skovsted (2003) described internal moulds and associated slicified external shells. Three species of Salterella have been described from this area, S. maccullochi , S. pulchella , and S. clarkii ( Yochelson 1977; Fritz and Yochelson 1989), but specimens from acid residues are not readily identified as to species (see discussion in Skovsted 2003). Occasional specimens with oval rather than circular cross−section are interpreted as tectonically deformed. Salterella was included in the new Phylum Agmata by Yochelson (1977; see also Yochelson and Kisselev 2003), but the status of this grouping has been questioned by other authors (e.g., Hagadorn and Waggoner 1992).

Figures

Fig. 1. Lower Cambrian localities in eastern Laurentia (A), map of Newfoundland indicating the Bonne Bay region (B) and simplified Lower Cambrian stratigraphy in western Newfoundland (C).

Fig. 2. Brachiopods from the Lower Cambrian, Forteau Formation (Western Newfoundland). A, B. Eoobolus priscus (Poulsen, 1932). A. NFM−662, exterior view of incomplete ventral valve. B. NFM−663, ventral valve interior with pseudointerarea. C. Ventral valve of Eothele sp., NFM−664 from ICS 1421. D, E. Acrotretid indet. D. NFM−665, ventral valve exterior. E. NFM−666, dorsal valve exterior. F, G. Micromitra bella (Billings, 1872). F. NFM−667, ventral valve interior. G. NFM−668, dorsal valve interior. H, I. Obolella crassa (Hall, 1847). H. NFM−669, fragmentary ventral valve interior. I. NFM−670, fragmentary dorsal valve interior. All specimens from JSP 1982−01 unless otherwise stated. Scale bars 500 ̊m.

Fig. 3. Trilobites, echinoderms, and sponges from the Lower Cambrian, Forteau Formation (Western Newfoundland). A. Dorypygidae indet., internal mould of fragmentary cephalon, NFM−671 from ICS 1421; in dorsal (A1) and left lateral (A2) views. B. Calodiscus lobatus (Hall, 1847), NFM−672, dorsal view of silicified cephalon. C. Ptychoparioid indet., NFM−673 from ICS 1421, silicified cephalon. D. Olenellid indet., NFM−674 from ICS 1421, dorsal view of silicified juvenile cephalon. E–J. Phosphatised echinoderm ossicles. E. Morphotype 1, NFM−675, dorsal view. F. Morphotype 1, NFM−676, dorsal view. G. Morphotype 1, NFM−677, ventral view. H. Morphotype 2, NFM−678, ventral view. I. Morphotype 2, NFM−679, dorsal view. J. Morphotype 2, NFM−680, oblique lateral view. K. Hexactinellid spicule, NFM−681, scale bar 100 ̊m. All specimens from JSP 1982−01 unless otherwise stated. All scale bars except K 500 ̊m.