Ambigostrea sechura
Creators: name: DeVries, Thomas J.
Publisher: name: Zenodo
Year: 2018
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Bivalvia
Ostreida
Ostreidae
Ambigostrea
Ambigostrea sechura

description: Ambigostrea sechura (Olsson, 1944)
(Figure 2 (f – s))
Ostrea (Lopha) sechura Olsson 1944, 41, pl. 1, figs. 7, 8.
‘Ostrea’ [sic] sechura Olsson, 1944. Alleman 1995, 68, figs. 5, 6.
? [sic] Ambigostrea sechura Olsson, 1944. Dhondt and Jaillard 2005, 339, pl. 1, figs 1, 2.
Remarks
Ostrea (Lopha) sechura Olsson, 1944; was assigned to Ambigostrea Malchus, 1990; by Dhondt and Jaillard (2005); but with some hesitation for want of a study of shell microstructure of the Peruvian specimens. An examination of nearly 100 Caballas Formation specimens supports the assignment to Ambigostrea based on shell morphology and the presence of a simple foliated microstructure.
Full-sized left valves of Caballas Formation oysters exhibit the distinctive characters seen on left valves of Ambigostrea sechura: a sickle shape; strong radiating chevronshaped ribs, which are few in number; and posterior and anterior margins crossed transversely away from the commissural plane by closely spaced corrugations, usually near the umbones but in large specimens extending as far as the hooked ventral margin. The corrugations are produced by the stacking of closely spaced marginal crenulations in successively older shell layers. The marginal crenulations are produced near the umbones by the overlap of chomata with the margin and farther from the umbones by marginal crenulations that are several times smaller and at oblique or right angles to the radiating chevron-shaped ribs.
With more material available than was known to Olsson (1944), further description of the left valve is possible. The adductor muscle scar has a skewed U-shape, with the posterior edge extending farther towards the hinge than the anterior edge. The entire ventral edge of the adductor scar is raised above the floor of the valve and bordered by a myophoric buttress. The chevron-shaped radial ribs begin directly beyond the ventral edge of the attachment scar. The transverse corrugations appear on juveniles, but only close to the umbones. Chomata extend for variable distances ventrally on the posterior and/or anterior margins.
The right valve of Ambigostrea sechura is usually but not invariably less convex than the left valve and usually but not invariably has weaker radiating ribs. The adductor scar is U-shaped and swollen above the floor of the valve. Transverse corrugations are developed to the extent permitted by the shell thickness and appear on juvenile specimens near the umbones and on larger specimens at least as far ventrally as the adductor scar.
Specimens of the Argentinian Maastrichtian oyster, Ambigostrea clarae (Ihering, 1907), exhibit the sickle shape, comma-shaped posterior adductor muscle scar, transverse corrugations, and bifurcating radial chevron-shaped ribs on right and left valves seen on specimens of A. sechura (Casadío 1998). The chomata of A. clarae, however, are always restricted to the dorsal half of the posterior and anterior margins, which on some specimens of A. sechura (Figure 2 (h)) is not the case, and the radial ribs are more numerous (8 – 13 ribs) than on specimens of A. sechura (5 – 10 ribs).
A few oyster specimens from the Tortuga and La Mesa formations were assigned by Dhondt and Jaillard (2005) to the finely ribbed Ambigostrea villei (Coquand, 1862), a Campanian – Maastrichtian species known throughout North Africa and the Middle East (Malchus 1990). The specimens from the Caballas Formation, however, closely resemble the coarsely ribbed specimens from the Tortuga and La Mesa formations, i.e. specimens of A. sechura.
Species of Cubitostrea Sacco, 1897, a genus that ranges from the Maastrichtian to late Miocene in South America (Griffin et al. 2005), may exhibit the sickle-shaped, strongly ribbed form of Ambigostrea sechura, and some species, e.g. C. cubitus (Deshayes, 1824 – 1837), the type species of Cubitostrea, and C. primordialis Griffin, Casadío, and Parras, 2005, a Maastrichtian species from Argentina, also exhibit closely spaced transverse corrugations like those of A. sechura. The right valve of Cubitostrea taxa, however, is entirely smooth (Griffin et al. 2005), lacking the chevron-shaped radiating ribs seen on right valves of A. sechura.
Specimens of Ambigostrea sechura differ from equally large specimens of the upper lower Eocene Ostrea buski Woods, 1922, from northern Peru (Woods 1922, figs. 3 – 5) and other specimens of O. buski examined in Olsson ’ s undescribed material at the Paleontological Research Institution and the Smithsonian National Museum of Natural History. Large specimens of O. buski are fan-shaped; usually have smaller, more numerous chevron-shaped radiating ribs (15 – 20 ribs, compared with 5 – 10 ribs on specimens of A. sechura); have anterior and posterior margins near the umbones of both valves scored by deeply scooped chomata; and lack the transverse corrugations seen on most specimens of A. sechura.
Inland from Puerto Caballas, in the lower part of the Los Choros Member of the Paracas Formation (DeVries 2017), specimens are found of an undescribed oyster equally large and similarly sculpted as the more ancient Ambigostrea sechura (Figure 2 (u)). The left valve of the Los Choros Member species, however, is deeply arched and the attachment scar is several centimetres long, rather than less than two centimetres long, which is the case even for large specimens of A. sechura. The posterior adductor muscle scar is circular and barely raised above the floor of the valve. The posterodorsal margin is lined with vermiculate chomata. A right valve (Figure 2 (t)) has a circular adductor scar that is not lifted above the floor of the valve and a posterodorsal margin fringed with vermiculate chomata. These features indicate that the Los Choros Member species should be assigned to Gryphaeidae, rather than Ostreidae (Coan and Valentich- Scott 2012; Kosenko 2017).
Moulds on the small attachment scar on left valves of Caballas Formation specimens of Ambigostrea sechura include specimens of A. sechura, Carolia (Parinomya) parinensis, venerid bivalves of indeterminate genera, Nodifaunus gainesi sp. nov., and plant fragments (Figure 3 (a), 3(b)).
Material
UWBM 107554, left valve, B8769, L 76.8, H (44), W (22); UWBM 107555, right valve, B8769, L (52.2), H (65.4), W (18); UWBM 107556, left valve, B8769, L (32.0), H (57.4), W 10.0; UWBM 107557, left valve, B8769, L 22.2, H 33.3, W 7.7; UWBM 107558, paired valves, B8769, L 40.6, H 44.3, W 21.9 (pair); UWBM 107559, left valve, B8769, L (25), H 37.8, W (15); UWBM 107560, left valve, B8769, L (19.5), H (25.1), W 9.9; UWBM 107561, left valve, B8769, L (21.9), H (29.3), W 9.0; UWBM 107562, paired valves, B8770, L 16.5, H 24.1, W 12.7 (pair); UWBM 107563, right valve, B8770, L 16.7, H 21.1, W 4.4; UWBM 107564, left valve, B8770, L (11), H (15.4); UWBM 107565, left valve, B8772, L 24.6, H 32.4, W 7.8; UWBM 107566, left valve, B8772, L 18.8, H (31.7), W 7.0; UWBM 107567, left valve, B8772, L 23.3, H 27.2, W 10.2; UWBM 107568, paired valves, B8772, L 26.0, H 13.9, W 6.5 (pair); MUSM INV 235, left valve, B8769, L 55.6, H (59.6), W 18.1; MUSM INV 236, left valve, B8769, L 21.6, H 29.1, W 7.3; MUSM INV 237, left valve, B8769, L (22.4), H (20.6), W 8.0; MUSM INV 238, left valve, B8769, L 29.7, H 32.4, W 12.7; MUSM INV 239, paired valves, B8770, L 25.0, H 30.8, W 15.9 (pair); MUSM INV 240, right valve, B8770, L 15.9, H 17.8, W 3.9; MUSM INV 241, paired valves, B8772, L 15.2, H 24.9, W 8.5 (pair); MUSM INV 242, left valve, B8772, L 22.3, H 28.2, W 5.8. Gryphaeid oysters from Los Choros Member, Paracas Formation, B 8768: UWBM 107641, left valve, L 46.6, H 65.2, W 33.6; UWBM 107642, right valve, L 49.0, H 53.6, W 10.8; UWBM 107643, valve uncertain, L (47.9), H (30.4); UWBM 107644, right valve, L (29.0), H 42.6, W (5); UWBM 107645, left valve, L (42.1), H (39.5).
Occurrence
Upper Campanian to Maastrichtian, Tortuga and La Mesa formations, Sechura Basin, northern Peru; Upper Cretaceous, eastern Peru (Cerron et al. 1998); Lower Paleogene, Cuenca Member, Caballas Formation, East Pisco Basin, southern Peru.
description: Published as part of DeVries, Thomas J., 2019, Early Paleogene brackish-water molluscs from the Caballas Formation of the East Pisco Basin (Southern Peru), pp. 1533-1584 in Journal of Natural History 53 (25) on pages 1541-1547, DOI: 10.1080/00222933.2018.1524032, http://zenodo.org/record/3670229