Lyropecten estrellanus (Conrad, 1856)
CONRAD, T. A. 1857. Descriptions of three new genera; twenty-three new species middle Tertiary fossils from California and one from Texas. Proceedings of the Academy of Natural Sciences of Philadelphia, 8: 312-336. [Reprinted in Dall 1909: 173-175] [p. 313]
Pecten (Lyropecten) estrellanus Conrad; R. Arnold, 1906, The Tertiary and Quaternary pectens of California, plate 19.
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«Suborbicular; lower valve ventricose, slightly undulated; ribs 17, broad, little prominent, convex, with an intermediate linear rib, from which the larger ribs are separated by an impressed line; upper valve convex, somewhat undulated, ribs flattened and the intermediate small ribs with a longitudinal impressed line on the lower part of the valve. Height 2½
inches.
Locality. Estrella valley, Cal. Dr. Newberry.» TIMOTHY ABBOT CONRAD, 1857
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«Holotype.— Missing. Conrad (1856) refers to a doublevalved specimen and to a single-valved individual (Conrad, 1857). Lectotype, USNM 13317, designated by Woodring (1938), is an incomplete right valve about 9 cm high, with 16 ribs.
Type locality.— San Luis Obispo County, Calif. The type area was originally given as "Estrella Valley." Although Angel (1883, p. 241) included the species in a list of Conrad's fossils from "the Estrella River at Panza," subsequent work suggests it came from the San Juan River Valley from the Santa Margarita Formation. The poor preservation and hard calcareous matrix of the L. estrellanus lectotype is matched in specimens washing out of Carnaza Creek (LSJU 1080), eastern tributary to the San Juan River. Late Miocene. Description.— Juvenile valves flat, adults convex. Left valves usually more inflated than right valves, commonly with rounded ledges. Beaks project equally beyond hinge line. Auricles equal to subequal, the anterior one with 4-5 radial riblets and a deep byssal notch. Hinge length greater than half of shell length. Right-valve ribs 16-17, interspaces with a single radial riblet crossed by fine, distantly spaced lirae. Growth lines more prominent and interspace furrows more numerous in gerontic specimens. Left-valve ribs 16, with 0-5 irregularly spaced round ledges produced by periodic growth. Growth series well represented in collections from Kimballs Wells (CAS 503). Adults commonly 10 cm in height; the largest individual is 14.8 high and 14.5 cm long; it has five ledges (CAS loco 503, near Coalinga, Calif.). Variability.— Valve profiles vary in convexity from juveniles (flatter) to adults and in number of constrictions or ledges. Rib counts vary from 15-18, but only a few individuals have as few as 15. Comparative notes.— Lyropecten estrellanus is more restricted in geologic and geographic range than L. crassicardo, but they commonly occur together in the upper Miocene sandstones of the Santa Margarita Formation of central California. Lyropecten estrellanus is distinguished by more convex valves, rounded rather than angular ledges, and 16-17 ribs compared to 14-15 in L. crassicardo. Lyropecten estrellanus has one midriblet in each interspace, no raised or "key" ribs on the left valve. At the few localities where L. estrellanus is found with L. terminus, the former is probably reworked. At these localities L. terminus is distinguished by low rib counts (12-14) and multiple striae in left-valve interspaces. Single valves or fragments of L. cerrosensis and L. catalinae have been confused with L. estrellanus but both have higher rib counts, incipient key ribs on left valves (R 3r Rc 3r R), and no tendency to form ledges. Transitional morphologic forms between L. miguelensis and L. estrellanus from the Cuyama area (USGS M2418) are difficult to separate, although older forms of L. miguelensis, s.s. have a more convex right valve and narrower umbonal angle, the beak projecting higher above the hinge line. Both taxa have a radial riblet in each interspace, but the riblet almost fills the space in L. miguelensis. After an inflated juvenile stage, L. miguelensis valves are flatter in comparison to the profile of L. estrellanus, which may have as many as 3-4 rounded ledges. Rib counts differ, 14-15 in L. miguelensis, 16-17 in L. estrellanus. Arnold's "flat form grading into catalinae" (pI. 22, figs. 5, 6) is a flat, short-hinged scallop from the Vineyard Canyon-Indian Valley area of the Stockdale Mountain 7½-minute quadrangle (localities USGS M3995, USGS 12922 = 16833, CAS 28473). It is a convergent Argopecten sp. related to but different from undescribed forms from eastern Baja California Sur (CAS localities 56493, 56499); in the Cholame Hills it is restricted to a hard sandy facies of the Monterey Formation (T.W. Dibblee, Jr., oral commun. 1973, and field map of the San Miguel 15-minute quadrangle). This form is an Argopecten that has been identified as both L. estrellanus and L. catalinae. It differs from both of them in hinge crura and a proportionally shorter hinge line that is not perfectly straight. Beaks meet at the hinge line, auricles are not rectangular, and valves are flat and unledged. Looped lamellar growth lines are distantly spaced. Despite all of these Argopecten features, the species is easily confused with L. estrellanus; it also has 17-18 ribs and a single midriblet in each interspace. Phylogenetic affinities.— Lyropecten estrellanus belongs in the bioseries L. miguelensis-L. estrellanus-L. terminus; this classification is based on overall morphologic similarity, fine macrosculpture, and growth forms that have rounded ledges and beaks standing above the hinge line. The bioseries constitutes a biostratigraphic succession in middle and upper Miocene shallow-marine units and becomes progressively more restricted in geographic distribution toward the end of the Miocene.» 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. 56]
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Lyropecten estrellanus
(Conrad, 1856); J. T. Smith, 1991, Cenozoic Giant Pectinids from California and the Tertiary Caribbean Province, plate 1, figures 4, 5; plate 22, figures 1, 4, 7. |