Ptilorhynchia obscuricostata Dagys, 1968

Sandy, Michael R., Hryniewicz, Krzysztof, Hammer, Øyvind, Nakrem, Hans Arne & Little, Crispin T. S., 2014, Brachiopods from Late Jurassic-Early Cretaceous hydrocarbon seep deposits, central Spitsbergen, Svalbard, Zootaxa 3884 (6), pp. 501-532 : 513-515

Ptilorhynchia obscuricostata Dagys, 1968

Fig. 6.9–6.17

1968 Ptilorhynchia obscuricostata new species —Dagys, p. 63, pl. 5, fig. 6, 7, text-fig. 42.

2011 v. partim ‘ Lacunosella ’ sp.—Hammer et al., p. 21, table 2.

Material and occurrence. Seep 3 (PMO 224.915), seep 9 (PMO 224.881, PMO 224.880), seep 12 (PMO 224.884).

Description. Wide triangular outline. Biconvex profile. Broad flat-topped uniplication of the anterior commissure. Smooth shelled with weak or incipient costae developed at the anterior margin, primarily in the fold/ sulcus of the anterior commissure. A few growth lines are present. Median septum and dental lamellae are visible. Other internal structures have not been investigated.

Discussion. The generally smooth and uniplicate form with the development of costae only towards the margin of the valves was one reason why this material was tentatively referred to ‘ Lacunosella ’ ( Hammer et al. 2011), due to its resemblance to L. groenlandica Owen (1976 ; and in Harper et al. 2005). However, the smooth shell, presence of costae at the valve margins, and broad uniplication of the anterior commissure are also characteristic of Ptilorhynchia obscuricostata ( Dagys 1968) . Even in the absence of serial sections of the present material, some useful observations can be made. The Spitsbergen specimens do not appear to show the massive shell-thickening of the ventral umbo that Owen observed in serial sections of L. groenlandica ( Owen 1976, fig. 3). The presence of distinct dental lamellae in the umbo of the ventral valve is consistent with Ptilorhynchia , but of course not diagnostic of this genus as they are present in many rhynchonellide genera. In L. groenlandica Owen dental lamellae are massively overgrown ( Owen 1976, fig. 3). A median septum is present in the dorsal valve of the Spitsbergen material whereas Lacunosella lacks a median septum (e.g. Owen 1976, fig. 3; Manceñido & Owen 2002, p. 1208).

Stratigraphic and geographic distribution. Upper Volgian–uppermost Ryazanian, Spitsbergen and Lower Volgian of northern Siberia ( Dagys 1968).

Figures

FIGURE 6. 1–4. Ptilorhynchia aff. plumasensis Crickmay, USGS Mesozoic Locality 32032 (=80Ace119), Karluk Quadrangle, Alaska, Shelikof Formation, Callovian, Middle Jurassic, dorsal, ventral, lateral, and anterior views. 5–8. Ptilorhynchia jeletzkyi Owen, USGS Mesozoic locality 14496, co–occurs with Buchia mosquensis (von Buch), probably Naknek series, Kimmeridgian–Tithonian(?), Upper Jurassic, McCarthy Quadrangle, Alaska. Collector, Fred H. Moffit, 1928. Dorsal, ventral, lateral, and anterior views.9–17. Ptilorhynchia obscuricostata Dagys. 9–12. PMO 224.880, seep 9, dorsal, ventral, lateral, and anterior views. 13–17. PMO 224.881, seep 9, dorsal, ventral, lateral, anterior and posterior views. Scale bar = 1 cm.