Lituyapecten MacNeil, 1961
MACNEIL, F. S. 1961. Lituyapecten (new subgenus of Patinopecten) from Alaska and California and Stratigraphic occurrence of Lituyapecten in Alaska. United States Geological Survey Professional Paper, 354-J: 225-239, pls. 35-46. [p. 227]
«Genus PATINOPECTEN Dall, 1898
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Subgenus LITUYAPECTEN MacNeil, new subgenus
Type: Patinopecten (Lituyapecten) lituyaensis Mac-Neil,n. sp.
Diagnosis.— Shell medium large to very large; a right valve of the type species measures 8Ji inches in diameter; this size makes it one of the largest pectinids known. Outline ranges from subrounded to elliptical; long axis in some species is the height; in others, the length. Ears moderately large; byssal notch moderately strong to strong; anterior ear of right valve elongate, tends to become narrower, and curves ventrally in full grown adults; left valve ears more symmetrical, and anterior ear tends to develop a marginal fold (concave on the inside) opposite narrow ear of right valve. Ribs strong, and those of right valve moderately broad to broad; they tend to be undercut on sides, flat to rounded or irregular on top. Interspaces usually deep and round bottomed. Juvenile sculpture consisting of thin concentric raised lines.
Interstitial ribs may occur on either valve and they are usually beset by a single row of rounded frills or flanges. Right valve of Lituyapecten probably slightly more inflated than left valve. Crushing of known specimens makes it difficult to determine the relative inflation; certainly some known left valves more inflated now than right valves. Hinge plate relatively heavy and with a single nearly marginal groove in each of the left valve ears. Base of the hinge plate conspicuously swollen. Discussion.— The subgenus is founded primarily on the left-valve sculpture. The ribs are moderately narrow to wide and rounded to flat topped. The juvenile sculpture consists of raised concentric growth lines that are fine and evenly spaced; they are similar to those normally covering the entire valve of P. caurinus. The adult sculpture of Lituyapecten consists of sharply raised flanges or lamellae, which may be single or divided into two or more rows of segments.
I am indebted to Mrs. Ellen Trumbull, of the U.S. Geological Survey for a photograph of an unusually well-preserved specimen of Patinopecten propatulus (Conrad) from the Astoria formation (Miocene) of Oregon, a specimen which she will figure in a forthcoming paper. This specimen has several patches of imbricating microsculpture preserved. In addition, it has small frill-like flanges very close to the beak on three or four of the posterior ribs of the left valve, They are closer to the beak than I have seen them on other species having similar flanges, and they appear to have died out at the stage of growth at which they normally appear in other species. This specimen contains the only combination of imbricating microsculpture and frill-like flanges I have seen. On the basis of the frills alone, P. propatulus might appear to belong to Lituyapecten, even though the frills occur at a different growth stage than is common. I am inclined to believe that the juvenile frills on P. propatulus indicate that typical Patinopecten and Lituyapecten are closely related after all, something that I did not feel I could say positively from other species. Possibly P. (Patinopecten) and P. (Lituyapecten) diverged from each other in Oligocene time. P. propatulus certainly seems to be a typical Patinopecten, whereas I believe coosensis is an aberrent Lituyapecten. Clark (1932, p. 808) in his description of Pecten (Patinopecten) yakatagensis said: "Pecten yakatagensis of all the species referable to the subgenus Patinopecten is unique. * * * It is very possible that Pecten yakatagensis should be placed in a new section." The new subgeneric name, Lituyapecten, is here proposed to include the species of Patinopecten having one to several rows of frill-like flanges on the ribs of the left valve. Two groups of species are included under Lituyapecten, the Patinopecten dilleri group and the Patinopecten yakatagensis group. The P. dilleri group usually has subrounded ribs on the left valve and single flanges continuous across the ribs; rarely the flanges may be divided on one or two of the central ribs with the disconnected segments of the flanges alternating. In the P. yakatagensis group the flanges are broken up into two or more well-defined series located on ribs, which at least in full grown adults tend to be broad and flat topped. Two rows of stort spinelike flanges, one along each side of flat-topped ribs, is the most common condition. However, the distinction of the flanges as single or multiple rows is sharp only in the Pliocene species, the last-known members of the group. The earliest known members are irregular in the division or nondivision of the flanges and in the distance of separaseparation; the same specimen may have single or divided flanges on different ribs, or on different parts of the same rib. Thus, while the end members of these two groups may seem quite distinct, the earliest representatives indicate that they had a common ancestry and that the characters which distinguished their lastknown representatives were characters of no more than individual variation in their earliest known representatives. Lituyapecten commonly has large barnacles on its left valve, but none have been observed on right valves. (See pl. 37, fig. 7; pl. 42, fig. 3; pl. 43, figs. 1, 4.) Presumably it rested on its right valve. If they were active, specimens with large barnacles must have had their activity impeded; some barnacles on Alaskan specimens have bases measuring over 30 millimeters. The Geological Survey collection contains a Recent specimen of Patinopecten (Patinopecten) caurinus whose right valve is clean but whose left valve is covered by barnacle bases. Schools of this species are believed to move from location to location periodically. As far as is known, Lituyapecten is typically North American. The known species are all from Alaska and California; apparently it never invaded Asiatic waters. It ranges from early Miocene, possibly late Oligocene, to late Miocene, possibly early Pliocene in Alaska. The California occurrences are Pliocene.» FRANCIS STEARNS MACNEIL, 1961
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Patinopecten (Lituyapecten) lituyaensis MacNeil, n. sp; F. S. MacNeil, 1961, Lituyapecten (new subgenus of Patinopecten) from Alaska and California and Stratigraphic occurrence of Lituyapecten in Alaska, plate 40, figures 1-5.
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