Eotebenna danica, Peel & Berg-Madsen, 2023

Peel, John S. & Berg-Madsen, Vivianne, 2023, Muscle attachment scars in helcionelloids from Denmark cast light on mollusc evolution in the Cambrian, Acta Palaeontologica Polonica 68 (4), pp. 625-638 : 629

Eotebenna danica sp. nov.

Figs. 4 , 5 .

1985 rostroconch sp. II; Berg-Madsen 1985: fig. 5E.

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Etymology: From the occurrence of the type suite on the Danish island of Bornholm ( Fig. 1 ).

Type material: Holotype: MGUH 34280 ( Fig. 4D ) . Paratypes: MGUH 34277–34279 , 34281 , 34282 . All phosphatized internal moulds from the type locality and horizon .

Type locality: The rivulet Øleå, southern Bornholm, Denmark ( Fig. 1C ) . Type horizon: Andrarum Limestone, Guzhangian, Miaolingian, middle Cambrian.

Material.— In addition to the type suite from southern Bornholm , rare specimens were collected from the lower Drumian (Miaolingian, middle Cambrian), on the western slope of Kinnekulle in southern Sweden ( Fig. 1D ), one of which is figured ( Fig. 5A ; MGUH 34283 ) .

Diagnosis.—Laterally compressed species of the univalve Eotebenna ( Fig. 4D 2 ) in which height (as oriented in Fig. 4D 1 ) is about two thirds of length. Apex overhanging the concave sub-apical surface and slightly closer to the sub-apical margin. Aperture oval, joined to the narrow, inverted teardrop-shaped opening (snorkel) at the sub-apical margin by a narrow slit. Ornamentation of growth lines crossed by fine radial lines. Multiple small swellings on the internal mould, interpreted as muscle scars on the apex, occur principally on the supra-apical (dorsal) surface.

Description.—In lateral view ( Fig. 4D 1 ), in which the upright orientation follows Peel (1991b: fig. 30) and Parkhaev (2001: fig. 2), the height of the shell is about two thirds of its overall length. The overturned apex forms the tip of an isosceles triangle, representing the main part of the body cavity, the sides of which diverge at about 50° to the shallowly convex, basal, apertural margin. The supra-apical surface is slightly sigmoidal, initially shallowly convex but becoming shallowly concave; the dorsum is uniformly rounded. The sub-apical surface is concave, ultimately rising to a higher level than the apex as it forms the upper margin of a sail-like extension of the main part of the body cavity (left side of Fig. 4D 1 ), the junction with which may be marked by a radial fold. The supra-apical surface terminates at the inverted teardrop-shaped aperture of the snorkel ( Fig. 4D 2 ), which is connected to the ovoid aperture by a thin slit around the margin. The entire margin, from tip of the snorkel to the supra-apical margin, is almost semicircular. Ornamentation consists of comarginal growth lines, which may be rugose near the aperture, that cross closely spaced fine growth lines. The apex of the internal mould retains an irregular pattern of small (up to 20 µm in diameter) swellings, mainly on the supra-apical surface, interpreted as muscle attachment scars (see below).

Remarks.— Eotebenna danica sp. nov. differs from Eotebenna papilio Runnegar and Jell, 1976 , described from a single specimen from the Coonigan Formation of New South Wales, Australia, in which height and length are equal, in terms of its more elongate form and the lack of widely spaced, prominent comarginal rugae. More closely spaced, weak comarginal rugae are present on the internal moulds of the Scandinavian materal in the later growth stages ( Fig. 4D 1 , D 2 ). In addition, the apex in Eotebenna danica slightly overhangs the sub-apical surface. Eotebenna papilio has widely spaced radial ribs on the shell exterior but radial ornamentation is not seen in the type suite of Eotebenna danica from Bornholm, which may reflect their preservation as internal moulds. Eotebenna papilio is a silica replica of the shell with well-preserved details of the shell exterior ( Runnegar and Jell 1976: 10–14, fig. 11B). Ornamentation of closely spaced growth lines is seen in specimens assigned to Eotebenna danica from the middle Cambrian (Miaolingian) of Kinnekulle in southern Sweden ( Fig. 5A , arrow).

Eotebenna viviannae differs from the co-occurring Eotebenna danica by its profoundly extended shell form when viewed in lateral aspect ( Figs. 2A 1 –A 3, 3A 1, B 1 ). Specimens of the former ( Figs. 2 , 3 ) are illustrated with the sub-apical and supra-apical surfaces horizontal but the long axis is sub-vertical in reconstructions by Peel (1991b, 2006). A radial fold separates the main part of the body cavity from the sub-apical sail and snorkel ( Fig. 2A 4 ), as in Eotebenna danica , but is often inconspicuous. The type species of Eotebenna , Eotebenna

Figures

Fig. 4. Helcionelloid mollusc Eotebenna danica sp. nov., internal moulds, Andrarum Limestone, Bornholm, Denmark, Guzhangian, Miaolingian, middle Cambrian. A. MGUH 34277, lateral view (A1) with rectangles indicating location of Fig. 5B1, B2, and B3. Muscle scars at apex illustrated in different orientations (A2–A6), with arrow in A2 indicating slight diagenetic compression or deformation along edge of muscle field.Arrow in A3 indicating imbricated lamellar structure shown in detail in A4. B. MGUH 34278, apex in lateral view. C. MGUH 34279, lateral view. D. MGUH 34280, holotype, lateral (D1) and apico-lateral (D2) views, the latter showing the laterally compressed shell form, with detail of apical muscle scars (D3). E. MGUH 34281, lateral view. F. MGUH 34282, apex in lateral view. pontifex Runnegar and Jell, 1976, from the Currant Bush southern Freuchen Land, North Greenland is more strongly Limestone (Miaolingian) of Queensland, Australia, is also coiled than Eotebenna danica, with a convex supra-apical much more elongate than Eotebenna danica but the massive surface, in lateral view, and the apex strongly overhanging snorkel is circular in cross-section compared to the inverted the sub-apical surface (Peel 1989, 1991b). teardrop-shape in the two Bornholm species. Eotebenna arctica Peel, 1989, from the Henson Gletscher Stratigraphic and geographic range.—Drumian of Sweden Formation (uppermost Series 2, Stage 4, lower Cambrian) of and Guzhangian of Denmark (both middle Cambrian).

Fig. 5. Helcionelloid mollusc Eotebenna danica sp. nov. from Miaolingian, middle Cambrian A. MGUH 34283, internal mould with traces of comarginal ornamentation and rugae (arrow), western slopes of Kinnekulle, southern Sweden, Drumian. B. MGUH 34277, details of shell structure,Andrarum Limestone, Bornholm, Denmark, Guzhangian (general view of the specimen in Fig. 4A). Surface of internal mould (B1) digitally inverted and mirrored here (B2) to depict shell structure on the interior surface of the shell. Detail of imbricate lamellae on internal mould (B3).

Fig. 1. Geological and geographical background. A. Cambrian stratigraphy of southern Bornholm, Denmark (based on Nielsen and Schovsbo 2007). B. Map of the Baltic area showing location of Bornholm, with location of studied locality (asterisk) on the rivulet Øleå (C), and the Lake Vänern area in southern Sweden (D), with collection locality on the western slopes of the hill Kinnekulle (asterisk).

Fig. 2. Helcionelloid mollusc Eotebenna viviannae Peel, 1991a, MGUH 19564, holotype, internal mould, Andrarum Limestone, Bornholm, Denmark, Guzhangian, Miaolingian, middle Cambrian.A1. Oblique lateral view showing margin of shell (arrow) along the narrow slit joining the sub-apical and supra-apical apertures.A2. Oblique apico-lateral view.A3. Lateral view.A4. Oblique view showing inverted teardrop-shaped sub-apical aperture and irregular area (arrow) of possible muscle scar.A5. Lateral view of apex.A6. Oblique lateral view of apex showing radial fibrous structure beneath smooth outer layer.

Fig.3. Helcionelloid mollusc Eotebenna viviannae Peel, 1991a, internal moulds,Andrarum Limestone, Bornholm,Denmark,Guzhangian, Miaolingian, middle Cambrian. A. MGUH 19565, paratype, lateral view (A1) with detail of apex (A2). B. MGUH 34273, lateral (B1) and apico-lateral views showing impression of comarginal ornamentation and cylindrical form of median sub-apical area (B2). C. Specimen lost, lateral view (C1) with detail of apex (C2), arrow locates detail of shell structure (C3). D. MGUH 34274, lateral view with detail of radial fibrous structure and overlying imbricated lamellae (D2), and patch of ornamented outer shell (D1, arrow). E. MGUH 34275, lateral view with detail of apex (E2) with muscle scars; arrows locate possible muscle scar. F. MGUH 34276, lateral view (F1) with detail of possible muscle scar (F2) located by arrows.