Propeamussium, DE GREGORIO, 1884

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 : 31-32

PROPEAMUSSIUM DE GREGORIO, 1884

Type species —by monotypy, Pecten (Propeamussium) ceciliae de Gregorio (1884) . Miocene , Sicily .

There are many available genus-group names for the fossil glass scallops. Although Propeamussium itself is based on a Miocene fossil, shell morphology is distinctive. The free-living P.jeffreysii (E.A. Smith, 1885) ( Fig. 20A, B ) illustrates the difference between the functionally dorsal right valve (A) with its uncalcified flexible prismatic apron, which is adpressed against the rigid interior margin of the fully calcified (functionally ventral) left valve (B) when the valves are closed. Although swimming has never been observed in live specimens, Morton and Thurston (1989) inferred rapid valve adduction and strong swimming capability by combining shell morphology with detailed study of adductor muscle components and insertions on the shell. Their reconstruction is presented in an elegant series of illustrations (Morton and Thurston 1989 figs. 3; 4a, b; and 5a, b) of the airfoil shape, opening and closure of the shell, and flexure of the apron during valve adduction.

An early functional reconstruction of the Keasey propeamussiid ( Hickman 1984, Fig. 8A, B ), recognized the design of jet propulsion system, swimming in an edge upward orientation but without evidence of the lateral gape that is more evident in Propeamussium s.s. Potential multiple functions of the gape during valve opening and adduction in swimming and carnivorous feeding (swimming jets, intake of meiobenthonic prey, and cleansing of the mantle cavity) have not been explored, and theory requires additional anatomical data and testing in live animals. There is strong evidence of sex-selective predation on meiobenthonic copepods ( Hicks and Marshall 1985), although hypothesized mechanisms are yet to be tested.

Propeamussiids are the only pectinoideans in which the right valve is functionally dorsal or uppermost. If they do not comprise a monophyletic group, it is even more remarkable that an uppermost right valve and discrepant shell size and surface sculpture have evolved multiple times.

The well-developed slat-like ribs of lathic calcite, which act as buttresses in both valves, are partially imbedded in the shell interior layer of crossed lamellar aragonite. Interior ribs are often visible externally through the thin translucent shells ( Fig. 20A, B ). All species are apparently capable of swimming even if normally attached by a weak byssus (Waller 2006), and all available evidence suggests that propeamussiids are living “relicts of the past” (Waller 1971), surviving extinctions at the close of the Paleozoic and Mesozoic to persist and radiate at bathyal and abyssal depths during the Cenozoic.

Stratigraphic range —Jurassic–Holocene.

Figures

Figure 8. Glycymeris andersoni Dickerson (1917), Gries Ranch Formation. A. Interior of right valve, hypotype UCMP 110730. B, C. Exterior and interior of right valve, hypotype UCMP 110729. D, E. Ventral and do2rsal of articulated valves, hypotype UCMP 110731. Scale bars=1 cm.

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.