Nothorthis, Ulrich & Cooper, 1938

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 99-102

Nothorthis ? sp. 2

Pl. 26, Figs. 1 –7; Table 34

Material. One complete specimen and 44 dorsal and 156 ventral valves from the samples containing trilobites A84136 , A84181 and A84186 and from samples F3797, JH-15, JH-35, JH-45, JH-52, JH-54, JH-56, JH-60, JH- 129, JH-131, JH-136. The figured specimens are TSGF 16883–16885, TSGF 17066 and TSGF 17068.

Description. Shell small, semicircular to transversely subangular, and planoconvex to ventribiconvex. L/W ratio 0.53–0.85. Cardinal extremities 50–90˚. Anterior commissure weakly unisulcate to rectimarginate. Largest specimen 2.6 mm wide. Larval shell 0.13 mm long. Ornamentation unequally ramicostellate; 2–5 costellae per 0.2 mm at 0.2-mm valve length and 5–9 costellae per mm at 1-mm valve length. Anterolateral and median costae often most strongly developed. Dense, fine filae. Ornamentation variably impressed on valve floor but impressed at least in distal two-thirds of valve floor.

Dorsal valve planar or slightly convex, with broad, shallow angular sulcus beginning just after larval shell and fading out distally. Interarea short (about 22% of valve length), anacline. Notothyrium open. Brachiophores high, rather short, diverging at about 130˚ in anterolateral direction. Brachiophore plates slightly converging on notothyrial platform. Notothyrial platform low but distinctly raised above valve floor and median ridge. Cardinal process simple, very thin. Fulcral plates present. Median ridge low, poorly developed. Dorsal muscle field generally obscure but otherwise with posterior adductor scars located posterolaterally to anterior scars.

Ventral valve moderately convex to subpyramidal with apex close to umbo. Interarea high, slightly concave and catacline to steeply apsacline. Interarea 33% as long as valve length. Delthyrium V-shaped and open. Teeth triangular, supported by recessive dental plates. Muscle field transversely subangular, raised on broad, well-defined callus. Callus generally with straight anterior margin, extending to 20–41% of valve length. Diductor scars cordate, slightly longer then adductor scars. Pedicle callist present. Bases of vascula lateralia weakly impressed, slightly diverging from adductor scars.

Remarks. No satisfactory exteriors were available for illustration. Because of the lack of well-preserved exteriors, the generic assignment is slightly tentative and we chose not to erect a new binomen though there is little doubt the species is new to science. Among the species assigned to Nothorthis , N. algainensis ( Severgina, 1967) from the Tremadocian of the Altai Mountains is subangular with a weak dorsal sulcus and moderately apsacline ventral interarea. N. asiatica Severgina, 1984 from the Tremadocian of the Altai Mountains is about four times as large, slightly biconvex, and widest at about midvalve length. The type species, N. delicatula Ulrich & Cooper, 1938 , is distinguished by the interior impression of the ornamentation, which is restricted to the distal half of the valve floor, and by a broad, distinct dorsal median ridge proximally. N. fascicostata Xu & Liu, 1984 from the Floian of SW China generally has obtuse cardinal extremities, much larger shells and less differentiated costellae. N. marginata Benedetto et al., 2003 from the Floian of Argentina has a distinct marginal rim on the valve floor, higher L/W ratio, and moderately apsacline ventral interarea. N. neichiaensis ( Chang, 1934) from the Floian to Dapingian of SW China has distinctly obtuse cardinal extremities and subequal costellae. N. penetrabilis Rubel, 1961 from the Dapingian of Estonia is subangular, with its greatest width near mid-valve and its ventral apex well anterior to the umbo. N. pennsylvanica Ulrich & Cooper, 1938 from the Darriwilian of Pennsylvania and Ireland has a moderately apsacline ventral interarea, subequal costellae, and a weak dorsal sulcus. N. perplexa Xu & Liu, 1984 from the Floian of SW China is widest well anterior to the hinge axis, and its notothyrial platform is very poorly developed. N. rude Xu & Liu, 1984 from the Floian of SW China has obtuse to rectangular cardinal extremities and a moderate, angular dorsal sulcus. N. sella Severgina, 1984 from the Tremadocian of the Altai Mountains is about eight times as large, subangular, and with moderately apsacline ventral interarea. N. tangshanensis Wang & Xu in Liu et al., 1983 from the Lower Ordovician of eastern China has nearly rectangular cardinal extremities and a moderate dorsal sulcus. N. tarda Cooper, 1956 from the Sandbian of Alabama has an L/W ratio normally close to 0.85, a maximum valve width located near mid-valve length, a moderately apsacline interarea, and a ventral apex located well anterior to the umbo. N. tarda lata Williams, 1962 from the Darriwilian of Scotland is slightly biconvex, and its maximum width is located well anterior to the hinge line. N. termalis ( Herrera & Benedetto, 1989) from the Floian of Argentina is subquadrate with a higher L/W ratio, and the apex of the ventral valve is well anterior to the umbo. N. tianjingshanensis Fu, 1982 from the Lower Ordovician of China is slightly biconvex and less transverse in outline. N. transversa Cooper, 1956 from the Sandbian of Alabama has an apex located well anterior to the umbo, a maximum width well anterior to the hinge line, and a markedly more strongly developed proximal part of the dorsal median ridge.

Occurrence. 74–75, 84, 87, 90 and 93–99 m above base of Olenidsletta Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland, Spitsbergen.

Plate 26

Nothorthis ? sp. 2

Valhallfonna Formation, Olenidsletta Member.

1. TSGF16883 , mould of dorsal valve interior. 90 m above base. Coll. J. Hansen, 20.07.2008, sample JH-45 .

2. TSGF16885 , mould of dorsal valve interior. 90 m above base. Coll. J. Hansen, 20.07.2008, sample JH-45 .

3–4. TSGF17067 , mould of ventral valve interior, oblique anterolateral view. 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

5. TSGF17068 , mould of ventral valve interior. 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136. 6 . TSGF17066 , mould of ventral valve interior. 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136. 7 . TSGF16884 , mould of ventral valve interior. 90 m above base. Coll. J. Hansen, 20.07.2008, sample JH-45 .

Orthidiella longwelli Ulrich & Cooper, 1936

Valhallfonna Formation, Profilbekken Member, 67 m above base. Coll. J. Hansen, 24.07.2008, sample JH-95.

8. TSGF16894, dorsal valve exterior.

9. TSGF16892, ventral valve exterior.

10–11. TSGF16893, dorsal valve interior, oblique anterolateral view.

12. TSGF16891, ventral valve interior.

Orthid sp.

Valhallfonna Formation, Profilbekken Member.

13–14. TSGF17069 , mould of dorsal valve interior, oblique anterolateral view. 21 m above base. Coll. J. Hansen, 19.07.2008, sample JH-23 .

15. TSGF17070 , mould of ventral valve interior. 21 m above base. Coll. J. Hansen, 19.07.2008, sample JH-23 .

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.