Chagrepecten 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. 66]
Genus CHAGREPECTEN n. gen.
«Type species.— Pecten (Chlamys) cactaceus Dall, 1898, from the Coatzacoalcos Formation of Spencer (1897: 23), Veracruz, Mexico (Dall, 1898), and the Chagres Sandsone of Panama (Woodring, 1982: 595), Upper Miocene (Collins et al., 1996). Woodring (1982: 594) designated as lectotype "the immature right? valve illustrated by Dall, USNM 135058," from Tehuantepec, Mexico. It is definitely a right valve on the basis of its ctenolium and adductor scar and is figured herein (Pl. 8, Figs 6-7).
Diagnosis.— Aequipectinini close to Cryptopecten Dall, Bartsch, & Rehder, 1938, in having prominent vesiculate sculpture and two distinct orders of radial costae on posterior auricles; shell equiconvex and of low to moderate convexity, L exceeding Ht, disk gapes absent or very narrow; byssal notch becoming shallow and ctenolium becoming weak or obsolete in late ontogeny; radial costae in rib interspaces beginning in early ontogeny.
Etymology.— The prefix chagre- refers to the Chagres Formation of Panama, in which the type species, Pecten cactaceus, is abundant.
Remarks.— Woodring (1982: 594) placed Pecten cactaceus from the Upper Miocene of Mexico and Panama in the genus Cryptopecten, noting similarities in sculpture with extant C. alli Dall, Bartsch, & Rehder, 1938 [= Pecten (Chlamys) bullatus Dautzenberg & Bavay, 1912] from the Hawaiian Islands and C. phrygium from the western Atlantic. In a thorough review of Cryptopecten in the world's oceans, however, Hayami (1984: 90) regarded the placement of Pecten cactaceus in Cryptopecten as debatable, noting that this species differs considerably from other Cryptopecten in having an "undeveloped ctenolium, well-developed and persistent intercostal threads, relatively shallow byssal notch, and unusually thin shells."
In the present study, Pecten cactaceus, Chagrepecten paracactaceus n. gen., n. sp. from the Pliocene upper Gurabo and Mao formations of the Dominican Republic, and Pecten (Chlamys) dallasi Jordan & Hertlein, 1926, from the middle Pliocene of Baja California Sur, Mexico, are brought together into the new genus Chagrepecten. Although these species have the vesiculate sculpture present in Cryptopecten, they differ from species in that genus in the characters pointed out by Hayami (1984: 90). Their active ctenolium, although similar to that of Cryptopecten in early ontogeny, becomes nearly obsolete or has only weakly developed, small teeth in late ontogeny. In maturity, the byssal notch of Chagrepecten n. gen. is shallow, its depth, commonly less than one third the length of the right anterior auricle, whereas in Cryptopecten the byssal notch remains deep throughout ontogeny and its depth is usually at least half the length of the auricle. Although I would not characterize the shell of Chagrepecten n. gen. as being "unusually thin," it is nonetheless thinner, less convex, and more flaring than in Cryptopecten. The foliated-calcite reentries on the shell interior are also thinner in Chagrepecten n. gen. In Cryptopecten, the deposition of foliated-calcite ventral to the adductor scars raises these scars on a thickened platform; in Chagrepecten n. gen., the same depositional areas of inner foliated-calcite are present, but they scarcely elevate the adductor insertions above the inner shell surface. In well-preserved specimens of Chagrepecten n. gen., as in Cryptopecten, the commarginal lamellae that merge and form enclosed vesicles also produce a secondary surface that is of lower relief than the original surface with sharp commarginal lamellae. In essence, the more flaring shape, reduced byssal notch and ctenolium, and secondary smoothened surface are indications that Chagrepecten n. gen. evolved from an aequipectinoid form to more of an amussioid form, concomitant with living unattached on a soft sediment surface in deep water. Although Chagrepecten paracactaceus n. gen., n. sp. in the middle Pliocene part of the Mao Formation is the geologically youngest defi nite member of the new genus, there is an extant species that might have branched off early from Chagrepecten n. gen. It is Pecten glyptus Verrill, 1882, which lives in the western North Atlantic from southeast of Cape Cod to the Florida Strait and in the northern Gulf of Mexico at depths from ca. 150-250 m. It resembles species of Chagrepecten n. gen. in shell outline, convexity, byssal notch, and ctenolium, as well as shell thickness and the distribution of foliated-calcite on inner shell layers. Although it also has broad, narrow-crested ribs comparable in profi le and number to the fossil species of Panama and the Dominican Republic, it lacks radial costellae (except in early ontogeny), a vesiculate secondary surface, and prominent commarginal lirae. There is only a vestigial trace of the latter in the form of rows of tiny cuspate lamellae in late ontogeny. Unlike the fossil species, P. glyptus has prominent disk gapes. Pecten (Aequipecten) preglyptus Olsson, 1922, from Olsson's "Gatun Formation" in Limon Province, Costa Rica, is possibly ancestral to the extant species. Olsson's species, however, is known only from the holotype (PRI 21140), a damaged and badly worn left valve (Olsson, 1922: pl. 17, figs 2, 7). Dall also noticed the resemblance between P. cactaceus and the extant P. glyptus. In his faunal list published by Spencer (1897: 24) preceding his formal description of P. cactaceus, Dall listed the fossil species as "Pecten (like glyptus, Verr.)." Chagrepecten n. gen. is represented on the Pacific side of the Americas by Pecten (Chlamys) dallasi, from the middle Pliocene Carmen Formation and Piedras Rodadas Sandstone on the eastern side of Baja California Sur, Mexico (Durham, 1950: 65; Piazza & Robba, 1994; see the latter for a discussion of lithostratigraphic units and ages in this area). Geographic and stratigraphic ranges.— Dominican Republic: upper Gurabo and Mao formations, Pliocene; Panama: Chagres Sandstone, Upper Miocene; Costa Rica, Limon Province, "Gatun Formation" of Olsson (1922), Pliocene?; Mexico, Veracruz: Coatzacoalcos Formation of Spencer (1897: 23); Baja California Sur, Mexico: Carmen Formation and Piedras Rodadas Sandstone, Pliocene. Inferred to have lived in water > 100 m.»
THOMAS RICHARD WALLER, 2011
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Chagrepecten cactaceus (Dall, 1898); T. R. Waller, 2011, Neogene Paleontology of the Northern Dominican Republic. 24. Propeamussiidae and Pectinidae (Mollusca: Bivalvia: Pectinoidea) of the Cibao Valley, plate 8, figures 6, 7.
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