Interchlamys 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. 30]
«Genus INTERCHLAMYS n. gen.
Type species.— Pecten interlineatus Gabb, 1873, Pliocene, Dominican Republic.
Diagnosis.— Mimachlamydini of Aequipecten-like shape, height and length approximately equal, length of anterior auricle approximately equal to or greater than length of posterior auricle. Primary radial ribs of disks strong and persistent in central sector of disk but decreasing in amplitude and width adjacent to disk flanks; primary ribs originating after early smooth zone on which microsculpture is reduced or absent; commarginal lamellae tending to be prominent and thickened, forming distally concave loops over rib crests and distally convex loops in interspaces, reflecting dentate valve margin. Hinge dentition of RV dominated by single dorsal tooth on each side of resilifer; intermediate teeth absent; resilial teeth poorly developed.
Etymology.— The prefix inter refers to the medial costa that nearly fills rib interspaces of the type species; the suffix chlamys refers to the pectinid genus Chlamys Röding, 1798.
Remarks.-- Interchlamys n. gen. includes two groups of species that differ greatly in maximum size. The group with species of smaller size (< 50 mm Ht) is represented by Pecten interlineatus Gabb, 1873, from limestones of probable mid-Pliocene age in the Mao Formation of the Dominican Republic and Pecten (Aequipecten) jacobianus Cooke, 1919, from the Pliocene La Cruz Formation of the Santiago area, southeastern Cuba. The group containing species of larger size (commonly exceeding 70 mm Ht) is represented by four nominal taxa: (a) Pecten interlineatus aidei Harris in Hodson et al., 1927, from the Pliocene of Venezuela; (b) Pecten (Lyropecten) tamiamiensis Mansfield, 1932, from the Pliocene of southern Florida; (c) Pecten mediacostatus mediacostatus Hanna, 1926, from the Miocene? and Pliocene of southern California and Baja California, Mexico (Moore, 1984: B29); and (d) Chlamys mediacostata grewingki Hertlein, 1966, from the Pliocene of Baja California. All of these species share a similar early ontogeny of external sculpture, namely an umbonal smooth zone that in the unweathered state is glossy and nearly devoid of microsculpture, with radial ribs beginning to form as very low undulations within this zone. All of the species in the second group have in common the presence of a single medial riblet in rib interspaces that appears early in ontogeny, only slightly later than the radial ribs. A medial riblet is also present in all of the species that comprise the genus Spathochlamys, but these species lack the early umbonal smooth zone. Instead the early ontogeny of Spathochlamys is like that in Mimachlamys Iredale, 1929, with a clearly pitted microsculpture on the left beak and an earlier appearance of primary radial ribs.
In the Dominican Republic, only the two species of the first group of Interchlamys n. gen. are present, I. jacobiana and I. interlineata, both associated with coral-bearing limestones. Interchlamys jacobiana is the stratigraphically lower species, occurring at NMB 17277 on the Río Yaque del Norte near the base of the limestones that form Angostura Gorge. These basal limestones have not been precisely dated, but they are apparently no older than Late Miocene (based on ostracodes lower in the section that indicate a Late Miocene age), and they are no younger than the Miocene-Pliocene boundary, which occurs near the top of the limestone section (Saunders et al., 1986: 30, table 3). Interchlamys interlineata occurs in the stratigraphically higher limestones of probable mid-Pliocene age in the Guayubín area (Saunders et al., 1986: table 3). The same pair of species of Interchlamys n. gen. occurs in southeastern Cuba in the Santiago de Cuba area, described by Cooke (1919) as Pecten (Aequipecten) jacobianus and Pecten (Plagioctenium) crucianus, the latter regarded herein as a junior synonym of I. interlineata. The relative stratigraphic positions of the Cuban specimens, however, cannot be determined from the original locality data given by Cooke (1919). Both of Cooke's species share the same type locality (USGS 3440), the description for which is "northeast portion of Santiago; fossils in marl on hillside; Oligocene." All occurrences of both species in Cuba are within the broad area of exposure of the La Cruz Formation. Bold (1975: 131) regarded this formation to be Pliocene or younger based on the presence of the ostracode, Radimella confragosa (Edwards, 1944). Later studies, however, extended the first appearance of this taxon into the Upper Miocene (Bold, 1988: 7; Saunders et al., 1986: 23). The species of the second group on the eastern side of the Americas are concentrated in Middle and Upper Pliocene formations, Interchlamys aidei mainly on the Paraguaná and Araya Peninsulas of Venezuela, and I. tamiamiensis in the Tamiami and Pinecrest formations of Florida. Interchlamys n. gen. probably evolved from Mimachlamys by becoming more aequipectinoid in shape, i.e., becoming more equilateral, with a broader umbonal angle and auricles of more equal length, and developing stronger carinae on the edges of plicae on the inner shell surface. Clues to an origin within the Mimachlamydini are provided by traces of antimarginal striae at the margins of the umbonal smooth zone as well as in early rib interspaces in I. jacobiana and I. interlineata. Based on exceptionally well-preserved specimens, no trace of this microsculpture remains on the early smooth zones of I. tamiamiensis. It also seems to be absent on I. aidei, although the specimens of this species that are available are not exceptionally well preserved, and abrasion might have removed the striae. Additionally, the low, trigonal to rounded ribs and narrow interspaces of species of Interchlamys n. gen. are approached by some species of Mimachlamys, an example being M. vokesorum n. sp., which co-occurs with I. interlineata in the Pliocene "Mao Adentro Limestone" in the Guayubín area of the Dominican Republic. Although more material is needed to assess evolutionary change in Interchlamys n. gen. within the Dominican Republic, there is a suggestion of such change in the material on hand. The succession begins with I. jacobiana of the late Miocene of the Dominican Republic, which lacks medial costae in rib interspaces (Pl. 2, Fig. 16). In I. jacobiana from the somewhat younger Pliocene La Cruz Formation of Cuba, the overall morphology is similar but medial costae are variably developed (Pl. 2, Figs 14-16). Interchlamys interlineata is the most derived in the sense that it develops scales and has consistent medial costae in interspaces as well as a more complex pattern of secondary radial costae in late ontogeny (Pl. 2, Figs 10, 13). Evolution in the second group of Interchlamys n. gen. probably occurred independently of the first group after branching off from a common ancestor, because there seems to be broad temporal overlap between the two groups. Geographic range.— Tropical and subtropical western Atlantic and eastern Pacific, southern Florida, Cuba, Dominican Republic, Venezuela, southern California, and Baja California, Mexico
Stratigraphic range.— Upper Miocene?, Pliocene.»
THOMAS R. WALLER, 2011
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Interchlamys interlineata (Gabb, 1873); T. R. Waller, 2011, Neogene Paleontology of the Northern Dominican Republic. 24. Propeamussiidae and Pectinidae (Mollusca: Bivalvia: Pectinoidea) of the Cibao Valley, plate 2, figures 10-13.
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