Parvamussium, SACCO, 1897

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 : 32-33

PARVAMUSSIUM SACCO, 1897

Type species — By original designation, Pecten duodecimlamellatus Bronn (1832) . Upper Miocene, northern Italy .

Parvamussium is treated here as a subgenus of Propeamussium , accommodating small, rounded shells, with interior shell slats developing later in ontogeny and coinciding with the inception of distinctive scabrous exterior sculpture on the initially smooth disc of the left valve. Discrepant sculpture on the right and left valves is illustrated here in the living Parvamussium alaskense (Dall, 1871) , a typically sedentary form with a well-developed byssal notch in the anterior auricle of the right valve ( Fig. 20E ). Both auricles are clearly demarcated from the disc, and both are ornamented with radial and commarginal striae. The scabrous pattern of intersecting radial and commarginal striae on the left valve ( Fig. 20F ) is in marked contrast to the subdued ornamentation of the right valve with commarginal striae only ( Fig. 20E ). Small size, scabrous sculpture of the left valve, and presence of a byssal notch do not constitute a strong argument for monophyly. However, the lack of a ctenolium in the byssal notch, discrepant valve size and sculpture, and interior slat-like ribs do exclude it from the Pectinidae and clearly distinguish it from the two new Keasey Delectopecten species.

Mesozoic species that can be assigned to either Parvamussium or Propeamussium s.s. first diversified during the Cretaceous, surviving the end Cretaceous extinction to begin a second and continuing radiation in the deep sea (Waller2006). Parvamussium achieved cosmopolitan distribution by the Eocene, with records as early as Danian in Patagonia (del Rio et al. 2008). Paleocene and Eocene records in the southern Hemisphere from both Australia (e.g., Darragh 1994, 1997, Stilwell 2003, 2005) and New Zealand (e.g., Beu and Maxwell 1990, Maxwell 1992). Del Rio et al. (2008) provide additional references to Paleogene records from Europe and the Gulf Coast of North America but suggest that the Paleogene taxa inhabited shallow-water facies at this time. In the Northwestern Pacific, however, Parvamussium is well documented in Japan from Upper Cretaceous forearc slope mudstones along the subduction zone on Kyushu (e.g., Komatsu et al. 2008) and Hokkaido (e.g., Tsujino and Maeda 2007). These Late Cretaceous forearc sequences also include methane seeps (e.g., Takahashi et al. 2007). The Parvamussium bivalve associations are remarkably like those in the Keasey and indicative of an even earlier shift of the minute squamose glass scallops to low energy, oxygen depleted conditions associated with methane seeps.

The major difficulty assessing monophyly and re- lationships of the squamose glass scallops is the large number of genus group names that have been placed in synomymy under Parvamussium . For example, Dijkstra (2013) lists five names based on extant species, includ- ing Squamamussium Oyama (1944) . Harold Vokes (pers. comm. 1981) intended to assign the new Keasey species to Squammamussium and obtained USNM type numbers for the specimens described and figured here under Parvamussium .

Stratigraphic range —Upper Jurassic?–Holocene. Cosmopolitan, bathyal and abyssal depths (modern), shallower in Mesozoic and Early Paleogene.

Figures

Figure 20. Variation in shell morphology in living North Pacific Propeamussiidae. A, B. Right and left valves of Propeamussium jeffreysi (E.A. Smith, 1885), SBMNH 126833. C, D. Right and left valves of Cyclopecten davidsoni (Dall, 1898), SBMNH 123955. E, F. Right and left valves of Parvamussium alaskense (Dall, 1871), SBMNH 103661. Scale bars=1 cm.