Apheoorthis, Ulrich & Cooper, 1936

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 105-106

Apheoorthis ? sp.

Pl. 27, Figs. 2–3

Material. Three ventral valves (JH-64, JH-70) and one fragment (JH-70).

Description. Shell transversely subangular with maximum valve width slightly anterior to nearly rectangular cardinal extremities. Anterior commissure rectimarginate. Largest valve 6.8 mm long. Ornamentation unequicostellate with rather coarse costellae separated by numerous fine costellae; 5–7 low, rounded, well-defined costellae per mm at 1-mm valve length. Coarse costellae changing distally into broad, poorly defined radial undulations covered by fine costellae. Low filae and infrequent strong growth lines also present. Finer costellae impressed on distal 35% of valve floor. Strong costellae impressed throughout valve floor.

Ventral valve slightly convex to resupinate and deepest at umbo. Ventral interarea planar and apsacline, tending toward catacline, about 20% as long as valve. Valve interior not exposed, but wetting the largest specimen provided some information. Muscle field raised above valve floor, especially in front. One broad central and two narrow lateral ridges, all very short, extend slightly anterior to muscle field. Muscle field about 25% as long as valve. Tracks of dental plates bordering muscle field first diverge, then begin to converge and finally diverge slightly again.

Remarks. The external features of the specimens from Spitsbergen show strong resemblance to those of species within the genus Apheoorthis . However, the assignment is tentative as long as dorsal valves and more information about the interiors are unavailable. The Spitsbergen specimens is most closely comparable with the upper Tremadocian A. melita ( Hall & Whitfield, 1877) but is distinguished by a markedly higher density of coarse costellae and lack of an anterior ventral sulcus.

Occurrence. Lower and middle parts of Nordporten Member, Kirtonryggen Formation, Basissletta (N79°51.227’, E017°41.332’ and N79°51.478’, E017°41.068’) in northeastern Ny Friesland , Spitsbergen .

Figures

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.