Argopecten eccentricus lacabrensis 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. 49, pl. 5, figs. 6-22]
2011 Argopecten eccentricus lacabrensis Waller, 2011
T. R. Waller, 2011, plate 5.
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«Diagnosis.— Argopecten eccentricus having a posterior auricle with a deeply sigmoidal posterior margin and auricular costae that extend to this margin.
Description.— Shell not known to exceed 24 mm Ht, with length greater than height in mid- to late-ontogeny. Dorsalventral profi le of RV commonly "humped," with change from low relative convexity in early ontogeny to higher relative convexity in late ontogeny occurring at 10-12 mm Ht; LV more convex than RV in early ontogeny, becoming less convex than RV later. Disks with 20-22 interlocking radial ribs; rib crests narrower than interspaces on both valves in early ontogeny, on RV becoming approximately equal or slightly wider than interspaces in late ontogeny; ribs of RV sharply rounded in profile in early ontogeny, gradually becoming steep-sided trapezoidal with gently rounded, flattened, or slightly depressed or grooved crests; rib profi les of LV high and sharply rounded in early ontogeny, becoming trigonal with rounded or slightly flattened crests in later ontogeny. Commarginal lamellae commonly arcuate in low-convexity stage of early ontogeny, less commonly persisting throughout ontogeny; lamellate phases rare in stratigraphically lower forms, more common in higher forms. Right anterior auricle relatively long and narrow with deep byssal notch in early ontogeny, becoming relatively shorter with 3 or 4 radial costae in later ontogeny but remaining longer than posterior auricle. Right posterior auricle with moderately sinuate to nearly straight posterior margin with overall trend of margin forming angle with dorsal margin that ranges from slightly acute to slightly obtuse; costae of this auricle uniformly developed and persisting to posterior margin in stratigraphically lower forms but more weakly developed in stratigraphically higher forms, where dorsal costae fade out but ventral costae remain weak to margin. Disk flanks of LV possessing 1-3 fine, commonly scabrous, radial costellae in early ontogeny, fading out at 2-4 mm from beak measured along disk flank; disk flanks of RV lower than on LV, with radial costellae in early ontogeny fading earlier in ontogeny than on LV. Hinge dentition as in Argopecten eccentricus eccentricus. Etymology.— Named after Arroyo La Cabra, which enters the left bank of the Río Gurabo at locality NMB 15913 (Saunders et al., 1986: text-fig. 4). This locality in the Cercado Formation yielded Argopecten eccentricus lacabrensis n. ssp. in evolutionary stage I (see below), as did localities on the Río Gurabo immediately upstream and downstream from this site. Type material and measurements.— Holotype: NMB G17502, 1 RV, 19.5 mm Ht, 21.2 mm L, 6.6 mm cvx (Pl. 5, Figs 6-10). Type locality.— NMB 15909, Cercado Formation, 111-113 m above base of section, Río Gurabo. Other material.— 10,488 specimens from 50 localities in the northern Dominican Republic (Table 7). Except for 4 juvenile articulated DV, all of these are single valves. Specimens from localities NMB 15873, 15896, and 15909 are illustrated on Pl. 5, Figs 11-22. Remarks.-- Argopecten eccentricus lacabrensis n. ssp., as discussed above under A. e. eccentricus, is regarded as the ancestral segment of a single lineage, the descendant segments being A. e. eccentricus followed by A. e. caimiticus. Argopecten e. lacabrensis n. ssp. can be subdivided into two evolutionary stages, indicated as A. e. lacabrensis-I and A. e. lacabrensis-II (abbreviated EL-I and EL-II), which diff er in shape and costation of the right posterior auricle and in width of disk ribs relative to width of interspaces. In EL-I, the posterior margin of the right posterior auricle is distinctly sigmoidal with an overall trend that is approximately perpendicular to the hinge or even slightly acute (trace growth lines on posterior auricle in Pl. 5, Figs 6, 11). In EL-II, this margin is straighter with only a very shallow sinus at > 15 mm Ht and with an overall trend that forms an obtuse angle with the dorsal margin (Pl. 5, Figs 15, 19-20). In EL-I, the costae of the right posterior auricle are uniformly developed and tend to persist to the posterior margin of the auricle even in the largest specimens (Pl. 5, Figs 6, 11, 13). In EL-II, these auricular costae are evenly developed in early ontogeny, but then either all of the costae or only the dorsal costae fade out before reaching the posterior margin, generally at a radial distance from the beak of > 2.5 or 3 mm, whereas the ventral costae, although weak, persist to the margin (Pl. 5, Figs 15, 19-20). Specimens of Argopecten eccentricus lacabrensis n. ssp. also vary in the degree of development of commarginal lamellae. Lamellate phases (Pl. 5, Figs 19-21) are sculptural ontogenetic phases dominated by projecting commarginal lamellae in interspaces and over the ribs. Although these phases are transient during ontogeny, they are more common in EL-II than in ELI. In some small specimens, the entire surface can be lamellate (Pl. 5, Figs 20-21), but in some larger specimens, the lamellate phase terminates and later growth is non-lamellate (Pl. 5, Fig. 19) like that of most members of the subspecies. Th e two evolutionary stages also diff er in the presence and extent of the disk-flank costae on the left valve in early ontogeny, those of EL-II disappearing earlier compared to EL-I. Samples from the Río Gurabo ca. 118-125 m above the base of the section contain specimens that are transitional between the two evolutionary stages (Text-fig. 3, Table 7). At NMB 15903 (120 m above base of section), for example, there is variation in almost every character, including strength of costae on the right posterior auricle and depth of the right posterior auricular sinus, which in some specimens becomes obsolete early in ontogeny. All of the stratigraphically highest representatives of EL-II in the Río Gurabo (155-208 m above base of section) have a very extensive re-entry of foliated-calcite on valve interiors extending ventrally well over halfway to the pallial line, commonly to the level of the bottom of the nonstriate adductor scar on the right valve and the bottom of the undiff erentiated adductor scar of the left valve (Pl. 5, Fig. 22). In the Cercado Formation on the Río Cana, both highcalcite and low-calcite forms of EL-II are present. Comparisons.— Argopecten eccentricus lacabrensis n. ssp. differs from A. e. eccentricus in having a more left-convex early growth stage in which the right anterior auricle is relatively larger, narrower ribs on the right disk, and, in the stratigraphically lower form, a right posterior auricle on which the radial costae are undiff erentiated and persist to a posterior auricular margin and a deeper posterior auricular sinus. Argopecten e. lacabrensis n. ssp. diff ers from A. levicostatus (Toula, 1909) of the Gatun and Chagres formations of Panamá (Woodring, 1982: 596) in having smaller auricles, straighter posterior auricular margins that form a less acute angle with the dorsal margin, and lower convexity of the right valve in early ontogeny. Evolution.— See under preceding remarks and under Argopecten eccentricus eccentricus. Occurrence.— The occurrences of the two evolutionary stages of Argopecten eccentricus lacabrensis n. ssp. are shown in Table 7. EL-I is known only from the lower Cercado Formation in the Río Gurabo section between 59 and ca. 120 m above the base of the section. EL-II is known from the Río Cana, Río Gurabo, and in an area ca. 18 km southwest of Santiago de los Caballeros (locality TU 1407). In the Río Gurabo section, EL-II succeeds EL-I, extending from ca. 120 m to 226 m above the base of the section, disappearing where the shallow-water Cercado sediments are gradually replaced by the deep-water Gurabo facies. Within this stratigraphic range of EL-II in the Río Gurabo section, the upper 75 m contains only the variant of EL-II that has very extensive foliated- calcite on the valve interior, extending to the level of the base of the adductor scar and nearly reaching the pallial line. This variant is the only one present in the Río Cana section and in the Santiago area at TU 1407 (Table 7). Distribution.— Argopecten eccentricus lacabrensis n. ssp. has not been recognized outside of the Dominican Republic». THOMAS R. WALLER, 2011
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