Brachidontes euglyphus (Woods, 1922)

DeVries, Thomas J., 2019, Early Paleogene brackish-water molluscs from the Caballas Formation of the East Pisco Basin (Southern Peru), Journal of Natural History 53 (25), pp. 1533-1584 : 1540-1541

Brachidontes euglyphus (Woods, 1922)

( Figure 2 (d), 2(e))

Mytilus euglyphus Woods 1922 , 63, pl. 1, figs. 6, 7.

Mytilus euglyphus ? [sic] Woods 1922, 63, pl. 1, fig. 8.

Mytilus euglyphus var. negritensis Olsson 1928 , 21, pl. 4, fig. 4.

Remarks

Woods (1922) and Olsson (1928) concurred that small ribbed mytilids from the Eocene Turritella Series of Woods (1922) (=lower lower Eocene Negritos Member of the San Cristobal Formation of Martinez et al. (2005)) have coarser sculpture than those specimens described by Woods (1922) from his Clavilithes Series (Chacra Formation of Martinez et al. (2005)). Ribbed mytilids range through the Salina Member of the San Cristobal Formation and the Pariñas and Chacra formations (Olsson 1928), i.e. the entire lower Eocene. Contrary to Olsson (1928), the Negritos Member ribbed mytilids are not always more strongly angled from the dorsal to ventral margin, as evidenced by Woods ’ s (1922) fig. 8. The Negritos Member specimens are likely ecological variants of the temporally wide-ranging B. euglyphus , which was also much larger in size than previously recognised, judging from a northern Peruvian specimen, nine cm long, in Olsson ’ s undescribed material in the Smithsonian National Museum of Natural History. Volsella cerva (Olsson, 1944) is a radially sculpted late Campanian mytilid from the Tortuga and La Mesa formations in the Sechura Basin of northern Peru ( Olsson 1944; Dhondt and Jaillard 2005) that should be transferred to Brachidontes . The Tortuga Formation species has a broadly rounded posterior umbonal ridge.

Material

UWBM 107551, right valve, mould, B8772, L (11.6), H (30.0), W (7.8).

Occurrence

Lower Eocene, San Cristobal, Pariñas, and Chacra formations, Talara Basin, northern Peru; Lower Paleogene, Cuenca Member, Caballas Formation, East Pisco Basin, southern Peru.

Figures

Figure 2. Bivalves from the Caballas Formation. (a–c) Incanopsis fastigiata sp. nov. (a) UWBM 107752, holotype, B8772, L 17.4 mm. Left valve, exterior. (b) UWBM 107752, cardinal area. (c) UWBM 107753, paratype, B8769, L 14.7 mm. Right valve, broken. (d, e) Brachidontes euglyphus (Woods, 1922). UWBM 107551, B8772, L 30.0 mm. Right valve, mould in part. (d) Exterior. (e) Exterior, anterior view. (f–s) Ambigostrea sechura (Olsson, 1944). (f) MUSM INV 235, B8769, L 55.6 mm. Left valve, exterior. (g) UWBM 107558, B8769, L 40.6 mm. Paired valves, posterior view. (h) MUSM INV 235, interior. (i) UWBM 107562, B8770, L 16.5 mm. Left valve, exterior. (j) UWBM 107562, right valve, exterior. (k) UWBM 107563, B8770, L 16.7 mm. Right valve, exterior. (l) UWBM 107568, B8772, L 26.0 mm. Paired valves, right valve resting on left valve. (m) UWBM 107554, B8769, L 76.8 mm. Left valve, interior. (n) MUSM INV 241, B8772, L 15.2 mm. Paired valves, right valve resting on left valve; with attached oyster spat. (o) UWBM 107565, B8772, L 24.6 mm. Left valve, interior. (p) UWBM 107555, B8769, L 52.2 mm. Right valve, exterior. (q) UWBM 107555, interior. (r) MUSM INV 238, B8769, L 29.7 mm. Left valve, interior. (s) MUSM INV 236, B8769, H 29.1 mm. Left valve, exterior, with twig mould in attachment scar. (t, u) undescribed gryphaeid oyster, Los Choros Member of the Paracas Formation, B8768. (t) UWBM 107642, H 49.0 mm. Right valve, interior showing vermiculate dorsoposterior margin. (u) UWBM 107641, H 65.2 mm (left to right in photo). Left valve, anterior view.