Towaipecten Beu, 1995
BEU, A. G. 1995. Pliocene Limestones and their scallops. Lithostratigraphy, pectinid biostratigraphy, and paleogeography of eastern North Island late Neogene limestone. Institute of Geological & Nuclear Sciences Monograph, 10: 1-243, figs. 1-95. (New Zealand Geological Survey Paleontological Bulletin, 68). Institute of Geological and Nuclear Sciences, Ltd., Lower Hutt, New Zealand. [p. 46]
«Genus Towaipecten n.gen.
TYPE SPECIES: Pallium mariae Finlay, 1927 (new name for Pecten semiplicatus Hutton, 1873, preoccupied), latest Pliocene and earliest Pleistocene (Nukumaruan), New Zealand.
DESCRIPTION: Shell moderate-sized to moderately large known species 90 to 130 mm in maximum height), weakly to moderately inflated, left valve more inflated than right. Shell thin and fragile. Disc slightly higher than long in early species (T. ongleyi) to subcircular or, in large specimens of younger species, slightly longer than high. Auricles relatively small (particularly in Mangapanian and Nukumaruan species, compared with those of coeval Phialopecten species); posterior margins of both posterior auricles inclined forwards, slightly sinuous; anterior margin of both anterior auricles markedly sinuous, on right valve inclined backwards over dorsal two-thirds of is height and then forwards over ventral third to form a wide, shallow byssal notch, on left valve more gently and symmetrically sinuous; dorsal margins weakly serrate and inclined dorsally above umbo on right valve, smooth and inclined slightly below umbo on left valve; each auricle demarcated from disc by a shallow groove and an abrupt depression of valve surface. Ctenolium not functional in adult, but represented up much of byssal groove by a row of prominent, narrow, elongate teeth (in T. mariae: not seen in older species, probably because most material has been removed from hard limestone). Sculpture of many low, wide radial costae, on earliest known species (T. ongleyi) simple and strap-like with one or two weak subdividing grooves and interspaces largely filled by intercalated secondary costae, but in T. katieae simple, strap-like and becoming very weak and broad over outer half of disc on right valve, and on left valve consisting of alternating strap-like, raised, smooth primary costae and intercalated, much lower, secondary costae that become equal in prominence to primary costae over outer half of disc; on left valve of T. mariae narrow, well raised, widely spaced radial costae cover the whole surface. Six or seven low, narrow plicae develop over umbonal third to (in a few specimens) two-thirds of disc in T. mariae, but are absent from earlier species. Auricles bear low radial costae over proximal half, but radial sculpture fades out over distal half. Radial costae plicate interior surface to form low, narrow, widely spaced internal costal carinae in T. mariae and T. katieae (interiors of older species not seen), but valve margins are smooth and straight (in plain of commissure), not crenulated by either intemal or external sculpture. Radial interspaces of both disc and ears closely sculptured with thin, well raised, moderately widely spaced commarginal lamellae, but commarginal sculpure is much lower and less prominent than in Kaparachlamys. Muscle scars not seen. Hinge with wide, triangular resilifer, hinge teeth very weat wide, smooth, shallowly triangular contact surface present below dorsal margin on both sides of resilifer.
REMARKS: The new genus Towaipecten differs from Phialopecten Marwick, 1928 in its much weaker sculpture, its thinner shell and its smaller auricles. Towaipecten appears to represent a group (apparently a single lineage of phylogenetically related species) that evolved from a similar ancestor to that of Phialopecten (perhaps from P. tolagaensis), but which evolved towards progressively weaker sculpture as Phialopecten was evolving towards progressively coarser sculpture.
The new genus Towaipecten much more closely resembles Kaparachlamys Boreham, 1965 than it does Phialopecten It is concluded above (under K. hectori) that the distinctive low, narrow, very numerous, relatively widely spaced radial costellae of K. hectori and the unusually prominent commarginal lamellae of K. hectori and an unnamed North Canterbury early Nukumaruan species of Kaparachlamys demonstrate that the similarity of Kaparachlamys to Towaipecten results from convergence rather than from close phylogenetic relationship.»
ALAN GLENN BEU, 1995
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Towaipecten mariae (Finlay); A. G. Beu, 1995, Pliocene Limestones and their scallops. Lithostratigraphy, pectinid biostratigraphy, and paleogeography of eastern North Island late Neogene limestone, figures 23a, 23d.
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