Neithea alta Hayami in Hayami & Noda, 1977
HAYAMI, I. & M. NODA. 1977. Notes on the morphology of Neithea (Cretaceous pectinids) with taxonomic revision of japanese species. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 105: 27-54, pls. 5, 6 [p. 39, pl. 5, figs. 1-3]
1977 Neithea (Neithea) alta Hayami in Hayami and Noda, 1977
I. Hayami & M. Noda, 1977, plate 5.
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Types.— Holotype (UMUT MM 5698), right internal mould, and four paratypes (UMUT MM5699-5702), left external moulds, all collected by Takizawa from the Kobitawatashi Member of the Ayukawa Formation at Yokone, south coast of Ajishima island, Ojika-cho, Ojika-gun, Miyagi Prefecture.
Diagnosis.— Small Weyla-like species of Neithea, characterized by the moderately convex right valve with 11 radial ribs of almost one order of prominence and remarkably concave left valve provided with 12 angular and highly elevated radial ribs forming six contiguous pairs. Description.—Shell small for the genus, scarcely exceeding 20 mm in height, highly inequivalve, nearly equilateral, a little higher than long. Right valve moderately convex, subtriangular with slightly concave antero- and posterodorsal margins of disc, ornamented with 11 highly raised and simple radial ribs which are almost equal in prominence; intercalary ribs seemingly absent. Left valve remarkably concave even at the very early and later stages, provided with 12 highly elevated and angular radial ribs which are at a glance similar to those of Weyla but seem to form six contiguous pairs; numerous concentric scales crossing the ribs and their interspaces. Posterior auricle of left valve obtusely truncated and comparatively small, while other auricles are not actually observable owing to the incomplete material. Apical angle of two valves apparently small for Neithea, though it may be due to the secondary deformation. Table 2. Measurements in mm [Neithea (Neithea?) alta sp. nov.]
Specimen Length Height Thickness
UMUT MM5698 Right in. mould (holotype) 17.5 18.5 ca. 4.5 UMUT MM5699 Left ex. mould (paratype) 14.0+ 15.0+ ? Remarks.— Several years ago one of us (I. H.) had an opportunity to observe various Jurassic and Early Cretaceous molluscan fossils from Ojika Peninsula of north Honshu collected by Takizawa of the Geological Survey of Japan. Although many of them are secondarily deformed and poorly preserved, several specimens of a pectinid attracted our attention, because it appeared to be one of the earliest representatives of Neithea.
This species is now proposed on the basis of five specimens from one and the same locality: one is a right internal mould with partly preserved shell and four are incomplete left external moulds. The detailed surface characters of right valve and auricles are actually unknown. Nevertheless, the present species is clearly distinguishable from hitherto described species of Neithea by the fewer and scarcely differentiated radial ribs in right valve and remarkably concave left valve with well developed concentric scales and only six pairs of angular ribs. Neithea (Neithea) aequicostata (LAMARCK, 1819) and a few other species from the European Cretaceous have also radial ribs of only a single order, but the number of ribs are much more numerous. In the mode of radial ribbing the present species is more similar to Neithea biangulata IMLAY, 1940, from the Hauterivian of Mexico, particularly to its young specimen. Although the left valve of that Mexican species has not come to science, the principal ribs of the right valve are almost uniform in prominence, numbering 11 to 13. In N. biangulata, however, the shell becomes much larger and one or two weaker riblets occur on each interspace after the middle stage. Such riblets are hardly expected in the present species, judging from the impression of ornaments on the right internal mould and the radial ribs impressed on the left external moulds. Although the radial ribs in the holotype (right internal mould) are apparently equal in strength, the six contiguous pairs of ribs on the left valve seem to foretell the characters of six-ribbed species of Neithea (s. s.). Moreover, the orientation of concentric scales on the left valve strongly suggests a more or less digitated ventral margin in accordance with the six pairs. The number of radial ribs on the right valve, which is 11, may be also meaningful, if one regards the ribs as homologous with the primary ribs plus median secondary ribs in ordinary six-ribbed species. As noted before, in almost all the species of Neithea (s. s.) radial ribs are basically composed of six primaries plus a multiple of five for the secondaries. Anyhow, the surface ornamentation of the present species is interpreted as representing an early stage of differentiation of radial ribs into two orders. Taxonomically, the present species is tentatively referred to Neithea (s. s.) with a query, since it possibly constitutes an infrageneric group together with N. biangulata. Occurrence.—This species is known only from the above mentioned type locality. All the specimens were obtained from the middle part of the Ayukawa Formation, where Berriasella sp. was found in association (SATO and TAKIZAWA in TAKIZAWA, 1970). Most of the associated species of Bivalvia, such as Limatula akiyamae, Astarte cf. spitiensis, Parallelodon (Cosmetodon) kesennumensis and Parallelodon (Torinosucatella) kobayashii are the elements of the Late Jurassic Kogoshio fauna, which actually survived until the earliest Cretaceous (HAYAMI; 1961). As suggested by TAKIZAWA, the age of this fossil bed is almost certainly Berriasian. According to DHONDT (1973) the first appearance of Neithea in Europe is also from the Berriasian but it becomes common after the ValanginIan. In North America, the above cited Hauterivian species, N. biangulata, is probably the earliest. In Japan, a small pectinid from the Upper Jurassic Torinosu Group was once referred to Neithea (KIMURA, 1951), but, as restudied by TAMURA (1959), it probably belongs to "Aequipecten". The present specimens, though poorly preserved, seem to be an interesting material for the consideration about the origin and phylogeny of the genus Neithea as well as the process of differentiation of radial ribs into two orders.» ITARU HAYAMI & MASAYUKI NODA, 1977
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