Argopecten inaequalis (G. B. Sowerby I, 1850)
SOWERBY I, G. B. 1850. Descriptions of new species of fossil shells found by J. S. Heniker [sic, Heniken] Esq.. In: On Some Tertiary Beds in the Island of San Domingo: From Notes by J. S. Heniker, Esq., with Remarks on the Fossils, by J. C. Moore. Quarterly Journal of the Geological Society of London, 6: 39-53, pls. 9, 10. [p. 52]
1850 Pecten inaequalis G. B. Sowerby I, 1850
Pecten inaequalis Sowb.; H. A. Pilsbry, 1922, Revision of W. M. Gabb's Tertiary Mollusca of Santo Domingo, plate 45, figure 11.
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«Testa suborbicularis, tumidiuscula, costellis radiantibus 19, rotundatis, laevibus, subdistantibus, superne lineis decussantibus hic illic sparsim notatis; interstitiis rotundatis, laevibus, aequidistantibus; alterius valvae costellis angustioribus interstitiis latioribus, Iaevissimis; auribus inaequalibus, radiatim costellatis».
GEORGE BRETTINHAM SOWERBY I, 1850
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«Remarks.— Argopecten inaequalis is not a highly variable species. At the same shell size, minor differences in height versus length, distance of fade-out of auricular costae from the beak, shapes of the posterior auricles, and degree of inflation of the valves can be found within single samples and do not appear to display any stratigraphic trends. A few specimens have faintly keeled ribs, but there is intergradation between this state and evenly rounded, nonkeeled ribs. The same applies to trigonal rib profiles, which in some cases approach those of some specimens of A. thetidis but are not as high and narrow.
Comparisons.— Argopecten inaequalis differs from A. eccentricus s. l. in having an extensive foliated-calcite re-entry that extends around the ventral sides of the adductor scars on both valves (Pl. 6, Fig. 9). Unlike both A. eccentricus s. l. and A. thetidis, the rib crests of A. inaequalis lack medial troughs. Unlike any of the other species of Argopecten in the Neogene of the Dominican Republic, A. inaequalis has a glossy external surface on which growth lamellae become exceedingly thin and flat-lying, detectible mainly as white commarginal lines on the exterior surface. The glossy surface and rounded ribs of A. inaequalis resemble comparable features of the extant species A. lineolaris (Lamarck, 1819), but the latter has much lower, nearly obsolete ribs in late ontogeny, a posterior auricular margin that forms an obtuse angle with the dorsal margin, weak or obsolescent auricular costae, and a shallower byssal notch. Evolution.— The extensive foliated-calcite re-entry, separated striate and nonstriate adductor scars on the right valve, acute posterior auricles, and detailed confi guration of auricular costae suggest that Argopecten inaequalis is in the same clade as A. thetidis. This is further indicated by the presence in a few specimens of A. inaequalis of lateral rib profiles with just a hint of angularity (Pl. 6, Fig. 14), as well as a hint of incipient (or vestigial) crestal troughs as indicated by defl ections of growth lines on rib crests. Stratigraphic relationships suggest that A. inaequalis is the descendant species, although the absence of costellae on its disk flanks suggests that it evolved from A. thetidis before the latter developed prominently costate disk flanks, a feature that is more common in the stratigraphically higher A. thetidis. The morphological features that distinguish Argopecten inaequalis from other late Miocene-Pliocene Caribbean Argopecten are similar to those that distinguish modern A. lineolaris from other extant Caribbean Argopecten. Basically, the shell smoothness, rounded ribs, and lack of projecting commarginal lamellae are characters (carried to the extreme in the genus Amusium) that facilitate mobility on shifting sandy or silty sea bottoms. A facility for swimming is indicated in A. inaequalis by its relatively thin shell, low convexity, nearly obsolete ctenolium, shallow byssal notch, adductor scars that appear to be larger than normal for Argopecten, and a lack of a lobate pedal retractor scar on the left valve. These similarities, however, are analogous rather than homologous. The auricular shapes and early ontogenetic rib profiles of A. lineolaris suggest that it has a different ancestry, possibly lying within the A. gibbus (Linnaeus, 1758) stock (Waller, 1991: 34). Juvenile specimens of A. inaequalis (Pl. 6, Fig. 14), in which rib profiles of some specimens are angular, suggest descent from A. thetidis. If Argopecten inaequalis is indeed descended from A. thetidis, it is not clear how speciation occurred, because there is good evidence that the two species were sympatric during the time of deposition of the Gurabo Formation. All but two of the 19 localities that yielded A. inaequalis also yielded A. thetidis. The most likely explanation is that speciation was allopatric, with a population of A. thetidis becoming morphologically divergent and reproductively isolated, and then spreading and overlapping ancestral populations of A. thetidis. Occurrence.— Argopecten inaequalis is restricted to the Gurabo Formation (Table 9). In the Río Gurabo section, it was collected from 218 to 314-317.5 m above the base of the section. These beds are below the Miocene/Pliocene boundary, determined by Saunders et al. (1986: text-fig. 14) to be at ca. 400 m above the base of the section, and lie within nannoplankton zone NN-11. On the Río Mao, A. inaequalis occurs in the Gurabo Formation at Maury's Bluff 1 and downstream from Bluff 1 at least as far as Arroyo La Sibirma, 8 km (airline) south of Mao. The latter might be the youngest occurrences of the species, although the age of these beds relative to the Miocene/Pliocene boundary has not been determined. The species is unknown from the Mao Formation, the type section of which is further downstream (TU 1337, 3 km airline south of Mao) and which is in nannoplankton zone NN-16, of late middle Pliocene age (Saunders et al., 1986: 32). Other occurrences are listed in Table 9. Distribution.— Argopecten inaequalis is known with certainty only from the Gurabo Formation of the Dominican Republic.» 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. [p. 53-55]
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Argopecten inaequalis (G. B. Sowerby I, 1850); T. R. Waller, 2011, Neogene Paleontology of the Northern Dominican Republic, plate 6, figures 8-14.
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