Argopecten uselmae (Pilsbry & Johnson, 1917)
PILSBRY, H. A. & C. W. JOHNSON. 1917. New Mollusca of the Santo Domingan Oligocene. Proceedings of the Academy of Natural Sciences of Philadelphia, 69: 150-202. [p. 194].
1917 Plagioctenium uselmae Pilsbry & Johnson, 1917
1922 Pecten costaricensis Olsson, 1922
1925 Chlamys (Plagioctenium) mansfieldi Woodring, 1925
1925 Chlamys (Plagioctenium) gonioides Woodring, 1925
1925 Chlamys (Plagioctenium) concinnatus Woodring, 1925
1983 Argopecten uselmae baracoensis Franco, 1983
1922 Pecten costaricensis Olsson, 1922
1925 Chlamys (Plagioctenium) mansfieldi Woodring, 1925
1925 Chlamys (Plagioctenium) gonioides Woodring, 1925
1925 Chlamys (Plagioctenium) concinnatus Woodring, 1925
1983 Argopecten uselmae baracoensis Franco, 1983
Pecten inaequalis Sowerby; R. J. L. Guppy, 1866, On the Tertiary Mollusca of Jamaica, plate 18, figure 6. Not Pecten inaequalis Sowerby.
Pecten uselmae Pils. and Johns.; H. A. Pilsbry, 1922, Revision of W. M. Gabb's Tertiary Mollusca of Santo Domingo, plate 45, figure 9.
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«Pecten uselmae n. sp.
Pecten inaequalis Sowerby, Guppy, Q. J. Geol. Soc, XXII, 1866, p. 294, Pl. 18. fig. 6. Not of Sowerby. The shell is inequilateral, inequivalve, the left valve is larger, conspicuously surpassing the right throughout the lower margin and ends. It is strongly convex; the right valve weakly so. Sculpture of 17 rounded ribs. In the left valve these are wider than the intervals, which are deeply cut, with concave bottoms, which are closely sculptured with delicate transverse threads. In the right valve the ribs are lower, and not quite as wide as the intervals, in which the transverse striation is often weak. The submargins are smooth. Ears with several ribs. Ctenolium is very short, of three teeth, which are small in adult shells. Cardinal crura strong, vertically striate. Length of left valve 28, alt. 27.3, diam. 8.25 mm. Type No. 11124, A. N. S. P., from Bowden, Jamaica, collected by Mr. Uselma C. Smith. This species is noticed here because it has been identified as P. inaequalis Sowb. of Santo Domingo. It differs from that by having fewer, stronger ribs in the left valve and more strongly ribbed ears. The left valve is more convex, and the form more inequalateral. Having seen large series of both, we find these differences constant. It is not represented in our Santo Domingan collections». HENRY AUGUSTUS PILSBRY & CHARLES WILLISON JOHNSON, 1917
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«Remarks.— Pilsbry & Johnson (1917: 194) described Pecten uselmae in a paper dealing with fossils from the Dominican Republic only because this Bowden species had previously been identified as P. inaequalis, a species then known only from the Dominican Republic. Woodring (1925: 69) provided a more detailed description of the Bowden species, noting that this species "is the most abundant Bowden Pecten."
Woodring (1925) also named three new species of Argopecten [as "Chlamys (Plagioctenium)"] from the Bowden shell bed: A. concinnatus, A. mansfi eldi, and A. gonioides. Based on the examination of type and nontype material in the Smithsonian collections identified as these species by Woodring, all three are herein regarded as junior synonyms of A. uselmae. In fact, Woodring himself indicated the synonymy of the first two of these on the labels of the holotypes (USNM 352784 and 352786, respectively). On the label of the holotype of the third species (USNM 352787), Woodring used pencil to indicate with a question mark that he was unsure of its synonymy with A. uselmae. Indeed this specimen, a left valve (13.8 mm Ht), displays a difference from the majority of specimens of A. uselmae in having distinct keels on the crests of its plicae (Woodring, 1925: pl. 8, fi g. 11). Such keels, however, are within the range of variation of A. uselmae, as demonstrated by specimens in the nontype collection. Woodring (1925: 72, pl. 8, fig. 12) wrote that his paratype of Argopecten gonioides (USNM 353076) is a right valve. It is in fact another left valve but is not the same species as the holotype. The paratype has strongly costate disk flanks, a posterior auricular margin that apparently formed an obtuse angle with the dorsal margin, and a foliated-calcite re-entry that extends only to the level of the middle of the adductor scar, characters that in combination suggest that it is closely related to the extant species A. nucleus. The three right valves of Argopecten uselmae reported here are the first records of this species from the Dominican Republic. They agree well in shape, rib profile, configuration of commarginals, and pattern of foliated-calcite re-entry with A. uselmae from the Bowden Formation, the only detectable difference being that the rib crests of the Dominican Republic specimens are slightly narrower relative to the width of interspaces than in most of the Bowden specimens. There is little doubt that the living counterpart of Argopecten uselmae is A. noronhensis. It is a poorly known species that ranges today from Brazil through the Antilles to the Bahamas and Bermuda, but it is unknown from the western Caribbean and Gulf of Mexico (Waller, 1973: 46). The depths at which living specimens have been taken range from 66 to 110 m, commonly associated with coarse sand and coral rubble. Argopecten uselmae probably was also a fairly deep-water species living in a similar habitat. In the Dominican Republic, the specimens from NMB 16885 are from the topmost beds of the Mao Formation on the Río Cana. Saunders et al. (1986: 22) commented that the paleoenvironment in these beds is difficult to judge because of the mixture of different kinds of sediment and fossils, possibly representing transport of shallow-water material into deeper water. They noted, however, that planktic foraminiferans are common in these beds and suggest a water depth of > 100 m. Comparisons.--Argopecten uselmae differs from A. inaequalis of the Gurabo Formation in the following: (a) the rib profiles on the right valve of A. uselmae are nearly rectangular, with the edges of the crests slightly overhanging the interspaces, whereas those of A. inaequalis are rounded; (b) both valves of A. uselmae are more convex relative to height especially in early ontogeny; (c) raised commarginal growth lamellae are present in interspaces but not on rib crests throughout most of ontogeny in A. uselmae, whereas in A. inaequalis growth lines are barely raised above the surface even in the rib interspaces, appearing mainly as light-colored lines crossing both interspaces and ribs; (d) the auricular costae of A. inaequalis are more delicate and tend to fade out completely in midontogeny; and (e) the left posterior auricle of A. inaequalis is relatively larger and forms more of an acute angle with the dorsal margin. Argopecten uselmae differs from A. thetidis of the Gurabo Formation in having broader ribs and lacking elaborate grooved rib crests with raised edges, and from A. eccentricus s. l. of the Cercado and Gurabo formations in having smooth rib crests that overhang rib flanks and a foliated-calcite re-entry that passes along the ventral side of the adductor muscles. Argopecten uselmae is very close to the extant species A. noronhensis. The latter species differs in having right rib crests that are considerably broader than the interspaces as well as sharper, knife-like horizontally directed edges along the rib crests, more pointed posterior auricles, a relative larger right anterior auricle with deeper, more acutely angular byssal notch, and left ribs with medial keels throughout ontogeny flanked by commarginal lamellae that nearly cross the rib crests except for the keels. Argopecten borinquenensis (Hubbard, 1920) from the Neogene of Puerto Rico is morphologically intermediate between A. uselmae and the extant species A. noronhensis. The left valve of the Puerto Rican species consistently has keeled ribs, but the ribs of its right valve lack the knife-edged rib crests of A. noronhensis and develop a secondary riblet at the base of each right-valve rib late in ontogeny. Specimens of A. borinquenensis in the Smithsonian's collections are from USGS 17952, a mile east of the Arecibo lighthouse. This locality was mentioned by Monroe (1980: 63), who assigned it to the upper member of the Camuy Formation, of probable Pliocene age. Argopecten uselmae also resembles the gibbous variant of the extant Caribbean species A. lineolaris, regarded by some workers as a distinct species, A. mayaguezensis (Dall & Simpson, 1901). Unlike A. uselmae, these taxa lack the overhanging right rib crests except in very early ontogeny, have smaller auricles with anterodorsal and posterodorsal margins intersecting dorsal margins at more obtuse angles, and auricular costae and disk ribs that tend to decrease in amplitude in late ontogeny, leaving a smooth and glossy shell surface. In effect, A. lineolaris displays features that become increasingly Amusium-like during ontogeny. Woodring (1925: 70) noted the strong similarity of Argopecten uselmae to A. costaricensis Olsson, 1922, from the Moin Formation of Costa Rica, which was said to have lower ribs. Specimens of Olsson's species in the nontype collections of the Smithsonian demonstrate that it is synonymous with A. uselmae. Argopecten uselmae baracoensis from the Guardarraya Formation, Upper Miocene to Pliocene, of Cuba was described on the basis of a single left valve which was said to have only 14 ribs. On the basis of the published figures, it appears that fairly prominent marginal ribs were not counted, meaning that this specimen could well be within the range of variation of typical A. uselmae. Evolution.— Argopecten uselmae is likely ancestral to the extant species A. noronhensis and is possibly a sister species of A. thetidis based on shared high ribs, steep disk flanks, acute posterior auricles, and extensive foliated-calcite re-entry, but no transitional species between A. thetidis and A. uselmae have been found. Another extant species that is likely in the same clade as the A. uselmae-noronhensis lineage is A. lineolaris, another species of the tropical western Atlantic and Caribbean but also occurring in the Gulf of Mexico. Its early shell ontogeny is similar to that of A. noronhensis, and the two species share common aspects of color patterns, particularly the presence of discontinuous commarginal brown or reddish pigment lines on the tops of ribs. Argopecten lineolaris has diverged by becoming amusiiform, with a polished surface, reduced or absent commarginal lamellae, lower ribbing, small streamlined auricles, and a reduced byssal notch with obsolete ctenolium. Occurrence.— In the Dominican Republic, Argopecten uselmae is known only from the Mao Formation, of Early to Middle Pliocene age. Distribution.— Jamaica: shell beds of the lower Bowden Formation (late Middle Pliocene in age). Haiti: unnamed "Miocene" formation (but probably Pliocene), USGS 9464, northern edge of Port-au-Prince (Woodring et al., 1924: 220- 221); unnamed Pliocene formation, USGS 9530, arrondissement of Jacmel. Dominican Republic: Mao Formation (Pliocene). Costa Rica: Moin Formation (upper Pliocene or lower Pleistocene). Cuba: Guardarraya Formation (Pliocene)». 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. 65, 66]
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Argopecten uselmae (Pilsbry & Johnson, 1917); T. R. Waller, 2011, Neogene Paleontology of the Northern Dominican Republic, plate 8, figures 1-5.
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