Eurysites rotundus 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 : 194-196

Eurysites rotundus Cooper, 1989

Fig. 3A.

1989 Eurysites rotundus sp. nov.; Cooper 1989: 32–33, pl. 8: 27–49, pl.

17: 1–11, fig. 17.

Material.—Three articulated specimens: AMNH FI− 72366, AMNH FI− 72367, AMNH FI− 72368, bed S−7, Mughanniyya Formation, Tel el Dhahab section, Jordan.

Description.—Shell medium size ( Table 1), somewhat worn, outline roundly triangular and almost pyriform but slightly rounded at margins, length almost equal to width; strongly dorsibiconvex; anterior commissure uniplicate, lateral commissure straight but rises slightly posteriorly. Ventral beak straight, long, and pointed; costae high, subangular, numbering about 14.

Ventral valve broadly convex in anterior view, less convex in lateral view. Ventral interarea appears high but partially obscured as is the pedicle foramen. Umbo convex, swollen, becoming flatter anteriorly; sulcus low bearing 4 costae, flanks slightly curved with 10 costae.

Dorsal valve with weakly defined fold rising slightly above flanks begins just past midlength becoming more distinct near anterior commissure. The fold bears 5 costae, with 8 costae on each dorsal flank; flanks rounded when viewed anteriorly. In lateral view convexity of dorsal valve is at maximum at midvalve; sides steep.

http://dx.doi.org/10.4202/app.2010.0092

Due to a paucity of material we did not section any specimens. However, Cooper (1989) illustrates transverse serial sections which show relatively thick dental plates that disappear before midvalve. His dorsal valves have a very short median septum or ridge with thick outer hinge plates; the crura are raduliform with well developed bases.

Discussion.— Eurysites rotundus differs from E. transversus (see Cooper 1989: fig. 18; pl. 8: 50–59; pl. 9: 1–16; pl. 17: 12) in its triangular shape and more swollen dorsal valves that are distinctly more convex in lateral view and less elliptical outline. On our specimen of E. rotundus there are no fila visible possibly due to the somewhat weathered shell surface. Cooper (1989) noted that on his specimens of E. rotundus from Saudi Arabia (housed in the USNM) there were fine concentric fila in the interspaces between the costae. E. rotundus is similar to Flabellirhynchia Buckman, 1918 from the Bajocian of Great Britain in its elongated beak and outline. It resembles Flabellirhynchia delicata ( Buckman 1918: pl. 19: a–c) in general outline but differs in its lower fold, thicker and stubbier anterior commissure and longer beak.

Remarks.—The genera Cymatorhynchia and Flabellirhynchia were introduced by Buckman (1918) in a paper that has been dated originally as 1917, however, the footnote to Corrigenda sheet from the Geological Survey of India states: “Print off orders regarding this memoir were given to the press in September, 1917, but owing to shortage of paper, copies were not actually issued till July, 1918”; Shi and Grant (1993: 144). As not all following authors noticed the actual publication date Cymatorhynchia and Flabellirhynchia are cited in some papers as 1918 and elsewhere as 1917.

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