Lituyapecten lituyaensis (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, K [p. 231, pl. 39, figs. 1, 3; pl. 40, figs. 1-5; pl. 41, fig. 1; pl. 42, figs. 1, 2, 4; pl. 43, figs. 1-4]
1961 Patinopecten (Lituyapecten) lituyaensis MacNeil, 1961
«Shell large, asymmetrical; posterior extension greater than anterior extension, and greatest height posterior of median line; right valve more inflated than left valve; shell of medium thickness. Dorsal margins concave in region of ears, but become nearly straight distally; dorsal margins low; left valve may have a low steplike marginal slope, particularly along left side; left valve ears inflated and lie higher than adjacent disk; byssal fold may be greatly swollen. Dorsal margin of ears straight centrally but curves ventrally towards distal end; anterior ear of right valve curves strongly in large specimens. Ears of moderate size for a large shell; no denticles along margin; byssal sinus moderately deep; ears sculptured by scabrous or beaded radial riblets which tend to die out distally and which may be very weak or absent on some specimens; a weak dorsal fold and a corresponding terminal indentation on anterior ear of left valves. Both valves with about 23 or 24 broad radial ribs. Ribs of right valve higher centrally and low to nearly flush terminally; central ribs smooth and rounded on top; terminal ribs have weak lateral frills and concave tops; interspaces smooth or have frilled interstitial ribs in one or more of terminal interspaces. Left valve has frilled ribs, which may be single and narrow terminally but broad centrally with a raised row of strong but narrow beadlike frills on each side, or more rarely with a middle row of frills as well; interspaces wider than ribs in adults but narrower than ribs in juveniles, sometimes with beaded or frilled interstitial riblets in one or two interspaces. Juvenile sculpture of left valve consists of raised concentric growth lines which are slightly convex towards beak and which are not raised on bottoms of interspaces, but which are raised in median trough on ribs for some distance beyond first appearance of marginal frills. Microsculpture of adults consists of growth lines convex towards margin in interspaces and concave towwds margins on ribs but not raised as in juveniles. Right valve microsculpture consists of faint radial markings but otherwise smooth.
Holotype, a right valve (USNM 563598), memes: diameter 210 mm, height 220 mm. The left valve paratype is numbered USNM 563599. The holotype of this species is one of the largest Tertiary pectinids known —if a larger specimen has ever been reported— and fragments of other specimens indicate that an even larger size was attained; possibly as much as 250 mm. It may have the distinction also of being the first pectinid observed in the rocks of western North America, the "manteau royal" mentioned in the narrative of La Pérouse's expedition of 1786. This species is descended probably directly from P. (L.) yakatagensis. It is larger and asymmetrical, whereas yakatagensis is nearly round. Most of the known specimens appear to be deformed by rock flowage in one way or another; some are shortened, some lengthened, and others are flattened. It has fewer and broader ribs than yakatagensis. The ears are larger and higher, and the byssal notch and the byssal fasciole are not as broad. The terminal ribs on the right valve of yakatagensis usually have a row of weak frills on the outer side, but the terminal ribs of lituyaensis usually have frills on both sides, and the ribs tend to be broader with a concave top. However, there is considerable variation in this respect; compare figure 1 of plate 40 with the corresponding area on figure 3. The terminal ribs of the left valve of both species tend to be narrow and to have only a single set of beads or flanges; this feature differs from specimen to specimen, and opposite ends of the same specimen may be unlike. The left valve of lituyaensis sometimes has a strong third beaded riblet on some of the middle ribs (pl. 43, fig. 1), possibly a function of its wider ribs. Only one specimen of yakatagensis seen (pl. 42, fig. 3) has any indication of a third row of beads, and on it the third row is very weak. It is interesting to note that this specimen also has a fine beaded interstitial riblet, a variation which in lityuaensis accompanies the presence of the third beaded riblet on the ribs. Patinopecten (L.) lituyaensis has its flangelike beads in well-separated rows, the variation being from well to widely separated; in yakatagensis the rows range from narrowly to well separated, the maximum separation in yakatagensis being about the minimum separation in lituyaensis. The minimum separation in yakatagensis is about equal to the maximum separation in poulcreekensis, the latter species ranges to specimens having no division of the flanges. The frilled interstitial ribs seen on several right valves of this species (pl. 39, fig. 3; pl. 40, figs. 2, 5) seem to be a common occurrence, and they resemble to a lesser degree the interstitial ribs of P. (L.) poulcreekensis. Similar ribs have not been observed in yakatagensis. The microsculpture on juvenile left valves of lituyaensis (pl. 42, fig. 2) is similar to that of yakatagensis (pl. 37, figs. 3, 5). On both species the transition to the adult stage with side frills on the ribs is less abrupt than on poulcreekensis (pl. 36, fig. 6). All three species are in contrast to a specimen, identified as P. (L.) cf. yakategensis, collected by Miller (pl. 45, fig. 3; pl. 46, fig. 5) from above the P. (Y.) poulcreekensis zone but still below the base of the Yakataga formation (included in USGS 17733). This single and only known unfrilled specimen from the upper part of the Poul Creek formation poses a problem regarding the ancestry of both yakatagensis and lituyaensis. There seems to be little doubt that lituyaensis is descended directly from yakatagensis. However, in some respects the unfrilled Poul Creek form appears to be a more likely ancestor of typical yakatagensis than poulcreekensis does; poulcreekensis, although related, may belong to a different lineage that is more closely related to P. dilleri. The strongest resemblance between yakatagensis and poulcreekensis lies in the possession by both of divided flanges. The flanges of poulcreekensis are more exaggerated and have a greater range of variation than those of yakatagensis, its possible successor. On the other hand, the unfrilled Poul Creek form is what might be expected for the ancestor of yakatagensis. It has narrow ribs although they are fewer than in typical yakatagensis. The beaks and ears are similar, even to the fold along the dorsal margin of the anterior ear of the left valve, a condition also strongly developed in P. coosensis. (See Arnold, 1906, pl. 6, fig. 2.) The fact that the Poul Creek form does not have frills on its ribs may be outweighted by the fact that on some specimens of typical yakatagensis they are very weak and by the fact that the adult sculpture of the Poul Creek form is like the juvenile sculpture of yakatagensis. It may be unsafe to draw conclusions on the few specimens known at present. It is possible, however, that poulcreekensis and the unfrilled form, here indentified as P. (L.) cf. yakatayensis, represent already divergent stocks of common ancestry and that poulcreekensis may be the ancestral stock of dilleri, whereas the unfrilled form may be the ancestral stock of the typical yakatagensis and lituyaensis, as well as of coosensis. The next younger members of this group of pectinids are from the Pliocene of California; after lituyaensis none are known from Alaska. Distribution.— Late Miocene(?) and (or) early Pliocene, Lituya Bay district, Alaska: Occurs in both the lower sandstone-siltstone unit and the upper mudstone unit. Type locality.— Upper mudstone unit of unnamed upper Tertiary formation, about 300 to 340 feet above the base of the upper mudstone unit, southwest shore of Cenotaph Island, Lituya Bay, Alaska, USGS M270. Other occurrences.— Lower sandstone(?)-siltstone unit, USGS D223 (T), D264 (T); upper mudstone unit, USGS Dl74 (T), M270.» FRANCIS STEARNS MACNEIL, 1961
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