Delectopecten, STEWART, 1930

Hickman, Carole S., 2023, Paleogene marine bivalves of the deep-water Keasey Formation in Oregon, Part II: The pteriomorphs, PaleoBios 40 (5), pp. 1-51 : 24-25

DELECTOPECTEN STEWART, 1930

Type species —by original designation, Pecten (Pseudamussium) vancouverensis Whiteaves (1793) . Holocene, North Pacific. Pliocene of California .

A peculiar form of surface ornamentation, defined by Waller (1972b) as camptonectes microsculpture, covers the surface exterior of both valves in geometrically regular patterns of very fine foliated calcite striae (Waller 1972a, b). The striae are oblique or antimarginal, forming a variety of net-like patterns in which elements are neither radial (continuously produced) nor commarginal (periodically produced and conformable with the shell margin). Classifications of antimarginal sculpture are supported by computer simulations (e.g., Hayami and Okamoto 1986) and theoretical morphospaces including camptonectes patterns (Ubukata 2005). Other forms of divergent sculpture occur on the exterior surface of bivalve shells, although their taxonomic significance is less clear than their microstructural expression and potential functional roles. However, the microscopic, foliated calcite striae, also referred to by some authors as “diverging scratches,” “radial scratches,” or “antimargial striae” (e.g., Dijkstra 1991), are treated as a clade within Pectinidae (e.g., Habe 1977, Waller and Marincovich 1992, Dijkstra and Kastoro 1997, Dijkstra and Gofas 2004, Bouchet and Rocroi 2010, Carter et al. 2011, Coan and Valentich-Scott 2012).

The North Pacific living and fossil Delectopecten vancouverensis , type species of the genus, has microscopic camptonectes sculpture on both valves ( Fig. 16A, B ) that is comparable to that of the new species in the Keasey Formation described below. The type species also has a prominent anterior auricle ( Fig. 16B ), deep byssal notch with ctenolium ( Fig. 16C ), and poorly delineated posterior auricle that is continuous with the posterior margin of the disc ( Fig. 16A ). Delectopecten vancouverensis is especially pertinent to interpretation of Pacific coast fossil Delectopecten because of its tolerance to severe hypoxic conditions (0.0–0.2 ml/l O2) (Suarez-Mozo et al. 2019). It occurs in assemblages that include chemosymbiotic solemyid and lucinid bivalves as well as other oxygen minimum zone taxa capable of tolerating hypoxia (Levin 2003).

Delectopecten is cosmopolitan in distribution and has been considered a descendant of the Jurassic–Cretaceous Camptonectes Agassiz in Meek (1864) (Waller 1972a, b). Although the Cenozoic fossil record of Delectopecten is best documented taxonomically in strata of the west coast of North America (MacNeil 1967, E.J.Moore 1984b), it is well represented and often abundant in Japanese deep-water facies. However, Paleocene occurrences in Australia ( Darragh 1997) and Argentina (del Rio et al. 2008) as well as Late Cretaceous pectinids with camptonectes sculpture from New Zealand and the Chatham Islands ( Crampton 1988) are consistent with hypothesized long cosmopolitan history.

Stratigraphic range —Lower Paleocene (Danian)– Holocene.

Figures

Figure 16. Delectopecten vancouverensis (Whiteaves, 1893), North Pacific live-collected, articulated representative of the type spe- cies of Delectopecten. SBMNH 130085. A. Left valve. B. Right valve. C. Enlargement of byssal notch with ctenolium (yellow circle). Scale bars=1 mm.