Protoskenidioides promontorium, Hansen & Holmer, 2011

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 89-92

Protoskenidioides promontorium sp. nov.

Pl. 23, Figs. 16–19; Pl. 24, Figs. 1–3 ; Table 30

Derivation of name. Latin ‘ promontorium ’, a ridge; refers to the low dorsal median ridge that never develops into a septum.

Holotype. Pl. 23, Fig. 17; TSGF16870 , dorsal valve; 90 m above base of Olenidsletta Member, Valhallfonna Formation, sample JH-45; Profilstranda (N79°50.894’, E017°41.818’), Basissletta, Ny Friesland, Spitsbergen.

Material. 10 dorsal and 56 ventral valves from the samples containing the trilobites A84360 a and from samples F*-scratch, F3749, F3797, JH-14, JH-45, JH-52, JH-129 and JH-136. The paratypes are TSGF16866–16869 , TSGF16871 and TSGF17065 .

Diagnosis. Protoskenidioides with a low dorsal median ridge and moderately subpyramidal ventral valve; thin cardinal process slightly widening posteriorly and developing very early in ontogeny; steep, free spondylium about 20% as wide as valve; and normally costate ornamentation.

Description. Shell mucronate, semicircular. Largest measured specimen 1.3 mm long. L/W ratio 0.45–0.69. Anterior commissure rectimarginate to unisulcate. Cardinal extremities diverging at about 45–60°. Ornamentation generally costate, but median costa in ventral valve and corresponding intercosta in dorsal valve may split into three in large specimens. Two slightly rounded costae within 0.2 mm at 0.2-mm valve length; 3–7 costae within 1 mm at 1-mm valve length; 11–17 costae or costellae along valve margin. Midline defined by broad costa in ventral valve and corresponding intercosta in dorsal valve. Costae weakly to moderately impressed on valve floor.

Dorsal valve slightly convex, with moderate or shallow, narrow, angular sulcus defined by broad intercosta. Fulcral plates well defined. Notothyrial platform low, slightly narrower than long, and divided by thin cardinal process. Cardinal process slightly widening posteriorly. Front of notothyrial platform slightly raised above median ridge, evenly rounded or bilobed. Growth lines on notothyrial platform indicate that at least most specimens have a bilobed notothyrial platform early in ontogeny. Cardinal process separated from median ridge by brachiophore supports. Median ridge broad, low and angular. Adductor scars generally obscure but occasionally confined by angular ridge. Posterior adductor scars subangular and located posterolaterally to anterior scars when impressed.

Plate 24

Protoskenidioides promontorium sp. nov.

Valhallfonna Formation, Olenidsletta Member, 90 m above base. Coll. J. Hansen, 20.07.2008, sample JH-45.

1. TSGF16868 , paratype, mould of ventral interior .

2. TSGF16869 , paratype, mould of ventral interior .

3. TSGF16866 , paratype, mould of ventral interior .

Trondorthis ? sp.

Valhallfonna Formation, Olenidsletta Member, 80 m above base. Coll. G. Vallance & R. Fortey, 1967, sample F4743/F4750. 4. F4743/F4750a, ventral valve exterior.

5–6. F4743/F4750b, ventral valve exterior and interior.

7. F4743/4750c, ventral valve exterior.

Anomalorthis rossi sp. nov.

8. TSGF16769, ventral valve exterior. Valhallfonna Formation, Profilbekken Member, 59 m above base. Coll. J. Hansen, 24.07.2008, sample JH-87.

9. F4804, ventral valve interior. Valhallfonna Formation, Profilbekken Member, 65 m above base. Coll. G. Vallance & R. Fortey, 1967, sample F4804.

10. TSGF16882, dorsal valve interior, oblique anterolateral view. Valhallfonna Formation, Profilbekken Member, 83 m above base. Coll. J. Hansen, 01.08.2008, sample JH-161.

11. USNM 160805 , holotype, dorsal valve interior. Antelope Valley Limestone , 1,272 feet below top. Sample D 1583. Photographed by Suzanne McIntire .

12. USNM 160806 b, paratype, ventral valve interior. Antelope Valley Limestone , 1,272 feet below top. Sample D 1583. Photographed by Suzanne McIntire .

Hesperonomia sp.

13. TSGF17085, ventral valve exterior. Kirtonryggen Formation, basal portion of Nordporten Member. Coll. J. Hansen, 22.07.2008, sample JH-64.

Archaeorthis groenlandica Poulsen, 1937

Kirtonryggen Formation, Nordporten Member.

14. TSGF17086 , dorsal valve exterior. Upper portion of member. Coll. J. Hansen, 17.07.2008, sample JH-34. 15 . TSGF17087 , dorsal valve exterior. Basal portion of member. Coll. J. Hansen, 22.07.2008, sample JH-63 .

Ventral valve moderately high, subpyramidal, with umbo normally constituting apex. Interarea catacline to slightly procline. Delthyrium open, moderately wide, U- or V-shaped. Teeth small. Free spondylium very short, about 20% as long as valve, with rounded anterior margin. Larger specimens occasionally developing a weak ridge in angle between valve floor and spondylium, but never a supporting septum. Muscle scars obscure.

Remarks. The features of this species appear intermediate between Protoskenidioides and Skenidioides . The cardinal process is not continuous with the median ridge, suggesting a placement in Protoskenidioides , but all specimens in which the trait can be observed have a fully developed cardinal process like that of Skenidioides . However, some specimens have a bilobed notothyrial platform, and at least most specimens show signs of having had a bilobed platform at an early ontogenetic stage. Therefore, we assign the species to Protoskenidioides . P. hibernicus Williams & Curry, 1985 from the lower Darriwilian of Ireland has a strongly convex ventral valve, a faint cardinal process, and a median septum developing into a prominent spine anteriorly. P. huanghuaensis Zeng in Zeng et al., 1983 from the Darriwilian of China is distinctly costellate and has a poorly developed or absent cardinal process. P. minor Xu & Liu, 1984 from the Floian of SW China has a much longer and lower free spondylium and is costellate. The type species, P. revelata Williams, 1974 , has no cardinal process and no contact between the notothyrial platform and median ridge in specimens less than 1.4 mm long, and the L/W ratio of the shell is smaller. P. weixinensis Zhan & Jin, 2005 from the Darriwilian of South China has a well developed median septum, costellate ornamentation, and an L/W ratio of 0.71–0.85.

Occurrence. 80, 84, 90, 94 and 97–99 m above base of Olenidsletta Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland, Spitsbergen.

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.

FIGURE 2 Stratigraphical ranges and diversity of brachiopod species at Basissletta, Spitsbergen, and relative sea levels and percentages of linguliform brachiopods in the total brachiopod fauna. White shading in the last graph indicates stratigraphical intervals without brachiopods. Thicknesses of units are according to Fortey & Bruton (1973). Lithological log and sea level curve are from Hansen & Holmer (2010).

FIGURE 3 Rose histograms for the umbonal orientation of the large brachiopod Ectenoglossa? oviforma sp. nov. found in the Olenidsletta Member, Valhallfonna Formation. a) Histogram of 58 specimens from 88 m above the base of the member. b) Histogram of 13 specimens from 94 m above the base of the member.