Pycnoria magna Cooper, 1989

Feldman, Howard R., Schemm-Gregory, Mena, Ahmad, Fayez & Wilson, Mark A., 2012, Jurassic rhynchonellide brachiopods from the Jordan Valley, Acta Palaeontologica Polonica 57 (1), pp. 191-204 : 200-201

Pycnoria magna Cooper, 1989

Figs. 3F, 6 .

1989 Pycnoria magna sp. nov.; Cooper 1989: 53–54, pl. 12: 11–36, pl. 18: 26–36, figs. 29, 30.

1991 Pycnoria magna Cooper ; Feldman et al. 1991: 15, figs. 11A–C.

Material.—Six articulated specimens: AMNH FI− 72402, AMNH FI− 72404, AMNH FI− 72403 (sectioned), bed S−7, AMNH FI− 72400, bed S−20, Mughanniyya Formation (Callovian, upper Middle Jurassic), Tel el Dhahab section, AMNH FI− 72401, bed 25, Mughanniyya Formation Formation (Callovian, upper Middle Jurassic), Arda section; AMNH FI− 7299, bed 21, Hamam Formation (Bathonian, Middle Jurassic), Arda section, Jordan.

Description.—The shells are medium to large ( Table 6) with rounded sides and an obtuse apical angle. Maximum width is at or slightly anterior to midvalve; outline subpentagonal, dorsal valve deeper and more convex than ventral valve. Lateral commissure straight, anterior commissure uniplicate. Beak short, erect to incurved; ventral interarea low, foramen hypothyrid, deltidial plates small, conjunct. Costae coarse and angular in cross−section numbering about 13–17. Some shells ( AMNH FI− 72399, AMNH FI− 72400, AMNH FI−72401) have a relatively long linguiform extension.

Ventral valve flat in lateral profile but slightly convex in umbonal region; in anterior profile valve is shallowly concave. Low sulcus originates about one−third of distance from beak, deepening anteriorly; sulcus generally bears 3 costae and widens narrowly in some specimens but widely in others. Flanks narrow, steep bearing 4 costae.

Dorsal valve evenly convex in lateral profile, strongly to moderately domed in anterior view, with a well−defined arcuate median fold somewhat to strongly elevated above steep flanks. Fold narrow, originating at about midlength bearing 3–4 costae, usually 4. Flanks bear 5 costae.

Dental plates thick, long, and slightly divergent (less than 5 °) from valve floor, shell relatively thin, median septum low, very weak. Strong, swollen slightly subquadrate hinge teeth fit into large sockets, crura raduliform, septalium appears to be narrow and short; crural bases weak. One tooth is somewhat corrugated. The general transverse outlines of Cooper’s (1989) shells are similar to the Jordanian shells, the height of which is similar as is the presence of the median septum.

Discussion.—These specimens of Pycnoria magna compare favourably with the specimens described by Feldman et al. (1991) in that they also have 13 strong, angular, deeply incised costae with a well−defined arcuate median dorsal fold and a comparatively narrow sulcus. The Jordanian shells display more variability in that some have a more highly developed median dorsal fold and a slightly wider and more broadly arcuate anterior commissure, similar to that of Strongyloria circularis Cooper, 1989 . Cooper’s (1989) species of Pycnoria magna ( USNM 303631; 243375) differs in that they have very fine concentric lines crossing the costae, a feature that may well have been removed here by exfoliation. Pycnoria differs from common rhynchonellides such as Somalirhynchia and Daghanirhynchia in its more solid and compact form as noted by Cooper (1989). We are in agreement with him that it differs from Globirhynchia triangulata in its stronger costation, narrower fold and deeper sulcus. Burmirhynchia is generally smaller with fewer costae (see Buckman 1918).

Stratigraphic and geographic range.—Bathonian to Callovian; Saudi Arabia, Jordan.

Figures

Fig. 6. Serial sections of rhynchonellide brachiopod Pycnoria magna Cooper, 1989. AMNH FI−72403. Bed 25, Hamam Formation, Arda, Jordan. Numbers describe sectional distance from apex. Abbreviations: c, crus; cb, crural base; dms, dorsal median septum; dp, dental plate; isr, inner socket ridge; osr, outer socket ridge; pf, pedicle foramen; s, socket; t, tooth.