Lenticulella amphora ( Krause & Rowell, 1975 )

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 16-19

Lenticulella amphora ( Krause & Rowell, 1975)

Pl. 1, Figs. 1 –12; Table 1

1975 Lingulella amphora new species ―Krause & Rowell, pp. 17–19, Figs. 11–12; Pl. 1, Figs. 11–18.

2001 Lingulella amphora Krause & Rowell ―Robson & Pratt, pp. 254–255, Figs. 14.1–14.8.

Holotype. Conjoined valves ( UKMIP 79823 ); Orthidiella zone (Dapingian) , Antelope Valley Formation; Meiklejohn Peak , Nevada ( Krause & Rowell 1975).

Spitsbergen material. 11 conjoined valves, 205 dorsal and 215 ventral valves and 127 fragments from samples JH-23, JH-25, JH-78, JH-89, JH-184, F5032 and F5316 and from sample with A84208. The figured specimens are TSGF 16906– TSGF 16911.

Description of Spitsbergen specimens. Shell minute, unequivalved, and thin-walled with broad lenticular outline and pointed anterior margin. Shell dorsibiconvex. L/W ratio 1.12–1.50. Posterolateral margins diverge at about 90–130° for dorsal valve and 80–115° for ventral valve. Anterolateral margins diverge at 90–115°. Largest measured specimen 1.5 mm long and 1.1 mm wide. Larval shell lacking ornamentation. Postlarval shell with fine growth lines.

Dorsal valve moderately to strongly convex with more pointed anterior than posterior margin. Valve deepest at 30–45% of valve length and widest at 43–54% of valve length. Larval shell subcircular, 0.15–0.23 mm long. Pseudointerarea orthocline, vestigial and short, about 3% as long as valve. Deep, wide, well-defined median groove bisecting pseudointerarea, 21–53% as wide and 3–15% as long as valve. Median groove slightly raised on platform or plate, usually with concave frontal margin but occasionally straight or even convex. Visceral area obscure; no pitting on valve floor. Limbus absent. Weak median ridge generally present in anterior part of larger valves. Vascula lateralia forming short groove anterolaterally of median groove margins and paralleling valve margin to about mid-valve. Vascula media subparallel or slightly diverging in proximal 65–80% of valve.

Ventral valve gently to moderately convex with proximal part of posterolateral margins slightly concave. Umbo more pointed than anterior valve margin. Valve deepest at 37–47% of valve length and widest at 44–56% of valve length. Larval shell acuminate, subcircular, 0.20–0.26 mm long. Pseudointerarea about 60–75% as wide as valve, orthocline, and distinctly separated from valve floor. Outer propareas thin. Broad, triangular inner propareas markedly declining toward valve floor and poorly defined from pedicle groove. Propareas with slightly convex anterior and inner borders. Inner part of growth lines on propareas slightly curved in posterior direction. Flexure lines strongly developed and nearly straight. Triangular pedicle groove mostly widening at front and 21–28% as wide as valve (about 6% as wide as valve slightly posterior to front). Pedicle groove widening at about 20˚ but appearing markedly wider because weakly defined propareas appear to be part of groove. Pedicle groove shallow, slightly raised above valve floor, 5–15% as long as valve. Visceral area reaching 45% of valve length but generally obscure. Pedicle nerve imprints diverging at about 5–10˚ and reaching about 35% of valve length. Weak median ridge very occasionally present in anterior part of valve. Mantle canal system baculate. Vascula lateralia paralleling valve margin to about midlength of valve and then curving inward.

Remarks. This minute species clearly differs from the much larger Lingulella species in its lack of dorsal flexure lines, lack of pits on the valve floor and characteristic lenticular outline, which becomes more distinct with ontogeny. Lenticulella amphora is highly common in a few beds in the lower to middle part of the Profilbekken Member of the Valhallfonna Formation. The Svalbard specimens are closely comparable to the type material from Nevada and the specimens reported from western Newfoundland by Robson & Pratt (2001). They differ from the type specimens in that the median groove usually has a concave instead of a convex front; the mean size is smaller; and the visceral area and vascular imprints are usually obscure. However, the larger specimens from Spitsbergen are the most similar to the type specimens, suggesting that the differences are ontogenetic. The specimens described by Robson & Pratt (2001) appear to differ only in the presence of umbonal muscle scars in the dorsal valve. Based on their similarity, all of these specimens are included in the species Lenticulella amphora .

Krause & Rowell (1975) discussed the similarities between L. amphora and Lingulella sp. 1 described by Wright (1963) from the Ashgillian Portrane Limestone (upper Katian) of Eire. However, like Wright’s (1963) Lingulella sp. 2 , the latter species is distinguished by a triangular muscle plate in the dorsal valve, a trait shared with the similarly shaped Upper Ordovician genus Ovolingula Mergl, 1998 from Bohemia. Both species of Wright (1963) are therefore referred to Ovolingula . Krause & Rowell (1975) discussed the similarities between L. amphora and Lingulella sp. 1 described by Wright (1963) from the Ashgillian Portrane Limestone (upper Katian) of Eire. However, like Wright’s (1963) Lingulella sp. 2 , the latter species is distinguished by a triangular muscle plate in the dorsal valve, a trait shared with the similarly shaped Upper Ordovician genus Ovolingula Mergl, 1998 from Bohemia. Both species of Wright (1963) are therefore referred to Ovolingula .

Plate 1

Lenticulella amphora ( Krause & Rowell, 1975)

Valhallfonna Formation, Profilbekken Member, 21 m above base. Coll. J. Hansen, 19.07.2008, sample JH-23.

1–2. TSGF16906, dorsal valve exterior and posterior view of larval shell.

3. TSGF16908, dorsal valve interior.

4. TSGF16907, oblique anterolateral view of dorsal valve interior.

5–7. TSGF16909, ventral valve exterior, oblique lateral view, and oblique posterior view of larval shell.

8–11. TSGF16911, ventral valve interior, detail of pseudointerarea, oblique lateral view, and oblique lateral view of pseudointerarea.

12. TSGF16910, oblique lateral view of ventral valve interior.

Ectenoglossa ? oviforma sp. nov.

Valhallfonna Formation.

13. TSGF17080 , paratype, dorsal interior. Olenidsletta Member. Coll. J.K. Nielsen, 17.07.2008, sample JH-194 .

14. TSGF16934 , paratype, ventral exterior. Profilbekken Member, 21 m above base. Coll. J. Hansen, 19.07.2008, sample JH- 23 .

15. TSGF17003 , paratype, detail of ventral pseudointerarea. Olenidsletta Member, 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

16. TSGF16933 , paratype, detail of larval shell exterior. Profilbekken Member, 21 m above base. Coll. J. Hansen, 19.07.2008, sample JH-23 .

17. TSGF17004 , paratype, detail of postlarval shell exterior. Olenidsletta Member, 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

18. TSGF17081 , paratype, impression of ventral valve interior. Olenidsletta Member, 89 m above base. Coll. J. Hansen, 20.07.2008, sample JH-48 .

Occurrence. 17, 26, 34, 52, 61 and 63 m above base of Profilbekken Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland, Spitsbergen. Basal part of Antelope Valley Formation (Dapingian), Nevada ( Krause & Rowell 1975). Bed 14 (Dapingian) in Factory Cove Member, Shallow Bay Formation, western Newfoundland ( Robson & Pratt 2001).

Figures

FIGURE 1 Locality maps of the Svalbard Archipelago. a) Locations of sites yielding Lower Palaeozoic fossils mentioned in this paper. b) Geological map of the Basissletta plain, northeastern Ny Friesland; redrawn from Fortey & Bruton (1973). The base map is based on aerial photos made available by the Norwegian Polar Institute and on field observations made by the authors in 2008.