Lyropecten tiburonensis Smith, 1991
SMITH, J. T. 1991. Cenozoic Giant Pectinids from California and the Tertiary Caribbean Province: Lyropecten, "Macrochlamis", Vertipecten, and Nodipecten species. United States Geological Survey Professional Paper, 1391: v, 155 p., figs. 1-18, pls.
1-38. [p. 67, pl. 16, fig. 2; pl. 18, fig. 5; text fig. 17A]
1991 Lyropecten tiburonensis Smith, 1991
J. T. Smith, 1991, plates 16, 18.
J. T. Smith, 1991, figure 17.
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«Holotype.— CAS 61215.01, fragment of an adult right valve, height 4.5 cm, length 8.7 cm (incomplete), hinge length 4.6 cm (figure 17A, ii, iii).
Type locality.— Southwestern Isla Tiburon, Sonora, Mexico (M9117 = CAS 61215 = Gordon Gastil field nos. S2G-8 and S2G-18), about 4 km northwest of Punta Willard. Unnamed marine conglomerate in "Arroyo 4," about 1f2 km from the beach where the arroyo forms a prominent delta near the center of Bahfa Vaporeta (fig. 17B). Late middle Miocene, based on associated volcanic rocks having a radiometric age of 12.9 ±0.4 Ma (Smith and others, 1985; Smith, in press). Paratype.— CAS 61974.01, an incomplete left valve measuring 7.1 cm high, 3.3 cm long (locality M9117 = CAS 61215). Significant hypotypes include UCR 7267/3 and UCMP 32291 (pI. 18, fig. 5) from the basal part of the Imperial Formation, Coyote Mountains, Calif. and UCR 5042/4 from the Whitewater River area, Riverside County, CaIif. Taxonomic comment.— Although the new species shares the rib scheme of the L. crassicardo-L. magnificus stock, it differs from those species in anterior and posterior disc sculpture and fine macrosculpture. Right valves are very similar to those of Nodipecten subnodosus, with which it was confused for many years until diagnostic left valves were collected. Description.— Valves moderately convex. Beaks project slightly above the hinge line; auricles unequal, the anterior one rectangular, twice as long as the posterior, and radially ribbed with 5 or 6 costae. Byssal notch moderately deep; umbonal angle 100º. Right valves with striae on anterior and posterior dorsal parts of the disc, 11-12 rounded rectangular striate ribs grouped in threes about a narrow central space bearing a single medial thread. Left valves with 9-10 ribs having a scheme of (r) r R 2r Rc 2r R r; interspaces bear one medial thread in juveniles, up to three in later growth stages; worn adult specimens may have no striae preserved in interspaces. Adult specimens from the southeastern Coyote Mountains could not be collected whole; specimens measured 12 or more cm in height. Comparative morphology.— Specimens are similar to L. colinensis, s.s. in having striate dorsal margins of the shell; they share the left-valve rib and node scheme common to L. colinensis, s.l., L. modulatus, Lyropecten sp. cf. L. magnificus, and L. crassicardo, but differ from these in fine macrosculpture, anterior and posterior striae, and slightly differentiated rather than nodose key ribs. Some individuals have ledges (fig. 17Aiv), most do not. Phylogenetic affinities.— Specimens of this taxon have been known from the Imperial Formation for many years, but available material is still insufficient or too poorly preserved for determining close phylogenetic relations. In rib scheme the taxon belongs with the L. crassicardo-L. magnificus stock. Geographic distribution and stratigraphic occurrence.— Isla Tiburon, Sonora, Gulf of California and the Salton Trough of California. Southwestern Isla Tiburon. Fragments from unnamed boulder and cobble conglomerate exposed in "Arroyo 4" (M9117 in fig. 17B and Smith, in press). The unit is interbedded with a volcanic debris flow of 12.9 ±0.4 Ma and overlain with an angular unconformity by an ash flow cap of 11.2±1.3 Ma (Gastil and Krummenacher, 1977). Salton Trough of California. Southern and southeastern part of the Coyote Mountains (UCR 7267, UCMP 738, UCMP A-1273, CAS 683), in the Latrania Sand Member of the Imperial Formation. Present in Alverson (= Fossil) Canyon and abundant as stacks of shells in the Painted Gorge 71/2-minute quadrangle, SW¼ . sec. 5 and S½ sec. 6, T. 16 S., R. 10 E. Also from the northernmost Salton Trough, in the Whitewater River area, Riverside County (UCR 5042), between the Banning and Mission Creek strands of the San Andreas fault; rocks mapped as Imperial Formation are overlain by the nonmarine Painted Hill Formation, which contains a basalt flow of 5.94±0.186.04± 0.18 Ma (Matti and others, 1985). Geologic age.— Late middle Miocene to late(?) Miocene. Although the Imperial Formation was deposited over an irregular surface, possibly over a span of 8 to 10 million years, faunal and radiometric evidence suggest that the sections containing L. tiburonensis, n. sp. are late middle to late Miocene in age.» JUDITH TERRY SMITH, 1991
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