Mimachlamys blowi Waller, 2011
WALLER, T. R. 2011. Neogene
Paleontology of the Northern Dominican Republic. 24. Propeamussiidae and
Pectinidae (Mollusca: Bivalvia: Pectinoidea) of the Cibao Valley. Bulletins of
American Paleontology, 381: 1-197, pls. 1-18. [p. 34, pl. 3, figs. 1-5]
2011 Mimachlamys blowi Waller, 2011
T. R. Waller, 2011, plate 3.
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«Diagnosis.— Mimachlamys with 23-26 simple rounded ribs bearing widely spaced, distally concave scales; secondary costae absent; posterior auricles much smaller than anterior, with posterior auricular margins forming obtuse angle of ca. 130° with dorsal margin.
Description.— Shell reaching ca. 50 mm Ht, height exceeding length, prosocline, convexity moderate with LV more convex than RV, disk gapes absent; disks with 23-26 simple radial ribs, all originating by ca. 2.5 mm Ht from beak; secondary costae absent; ribs nearly semicircular in profi le in early ontogeny, becoming rounded-trigonal in late ontogeny, broader than interspaces on both valves; scales on ribs widely and irregularly spaced, distally concave. Divaricating to diagonal antimarginal microsculpture present in rib interspaces, ending abruptly at bases of rib flanks; ribs lacking microsculpture except for very fine growth lines that pass straight across ribs but form ventrally convex arcs across rib interspaces; raised commarginal lamellae absent. Disk flanks low and steep on RV, anterior flank smooth, posterior flank with few scabrous costellae; disk flanks of LV higher, both bearing scabrous costellae. Posterior auricles only approximately half length of anterior; posterior auricular margins nearly straight, intersecting posterior dorsal margin of auricle at obtuse angle of ca. 130°; total hl approximately half L. Right anterior auricle with 5 strong radial costae bearing blunt scales, broad byssal fasciole, and deep byssal notch floored by strong active ctenolium; left anterior auricle with nearly straight anterior margin forming acute angle with dorsal margin and 10 radial costae of moderate strength bearing delicate, distally concave scales; posterior auricles with 4 or 5 weaker costae and more delicate scales. Hinge dentition and muscle scars not accessible; ribs on shell interior near margin rounded, without internal carinae. Etymology.— Named in honor of the late Warren C. Blow, a dedicated paleontologist who was my assistant for 32 years. Type material.— Holotype, NMB G17497, 1 RV, 41.0 mm Ht, 38.0 mm L (Pl. 3, Fig. 5). Type locality.— Locality NMB 15893, Cercado Formation, ca. 150 m above base of section on Río Gurabo, northern Dominican Republic, as plotted by Saunders et al. (1986: text-fig. 4), associated with poritid corals in a fine gray silt disconformably overlain by a massive conglomerate regarded by Saunders et al. as the base of the Gurabo Formation. Other material.— NMB G17498, a pair of off set valves, the RV incomplete, from the type locality, NMB 15893 (Pl. 3, Figs 1-4). Remarks.— The specimens of Mimachlamys blowi n. sp. are associated with a fine gray silt containing fragments of finely branched poritid corals in a chaotic arrangement. The Mimachlamys specimens appear to be originally paired valves that have become off set and distorted, perhaps due to syndepositional movement as would be expected in soft sediment in a submarine slump. This is precisely the depositional setting of the basal part of the much younger Mao Formation that yielded M. vokesorum, n. sp. (see below). Comparisons.— Mimachlamys blowi n. sp. closely resembles only M. vokesorum n. sp., a new species described below from the Mao Adentro Limestone Member of the Mao Formation. Compared to the Mao species, M. blowi n. sp. has narrower, higher ribs that are less trigonal in profile, a narrower umbonal angle, and less obtuse angles between posterior auricular margins and dorsal margin. Also, unlike M. vokesorum n. sp., M. blowi n. sp. lacks secondary costae along the bases of rib flanks. Mimachlamys landi from the La Cruz Formation, Pliocene, of Cuba has similar scaly ribs and interspace microsculpture but differs in that its ribs are more numerous (>30), finer, and more rounded in profile without any hint of trigonality. Evolution.— Because there are no similar Mimachlamys species known from older deposits in the tropical American region, it is not possible to infer when and where M. blowi n. sp. originated. Based on close morphological resemblance and stratigraphic position, M. blowi n. sp. is probably ancestral to M. vokesorum n. sp. If so, then the more obtuse posterior auricles, broader ribs, and more flaring shape of the latter species represent evolutionary changes, perhaps related to the evolution of greater mobility for predator avoidance or for avoiding local transient environmental changes. The very similar depositional settings of the two species suggest similar habitats, Occurrence and distribution.— Known only from the type locality in the northern Dominican Republic.» THOMAS RICHARD WALLER, 2011
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