PROPEAMUSSIIDAE ABBOTT, 1954

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

PROPEAMUSSIIDAE ABBOTT, 1954

A second family group of small-shelled, fragile deep-water pectinoideans also appeared during the Triassic. The ancient propeamussiids (glass scallops) have a long fossil record independent of the pectinid mud pectens. Propeamussiids have been recognized, described, and treated under Propeamussium de Gregorio (1854) from numerous Paleogene formations on the Northeastern Pacific margin for more than 100 years, distinguished primarily by the presence of slat-like interior radial ribs of lathic calcite imbedded in the inner aragonitic layer. The ribs have been interpreted as an adaptation strengthening the fragile shells for efficient swimming and a free-living mode of life. Experimental study of valve design identifies an airfoil system of complex features that increase the lift required for level swimming (e.g., Hayami 1991). For more than 50 years some propeamussiids have been recognized as carnivorous, feeding on small epibenthic and planktic protists and invertebrates (e.g., Knudsen 1967, Tëmkin and Strong 2013). Fossil propeamussiids also have been recognized as microcarnivores, and sometimes abundant in bathyal mudstone faunas (e.g., Maxwell 1988).

The concept of the family has been refined substantially by detailed studies of both hard and soft parts, including details of shell microstructure (Waller 1971, 1972b, 1976, 2006). Although all propeamussiids are apparently capable of swimming (Waller 2006), byssal attachment is characteristic in some species, and interior radial ribs are not diagnostic of the entire family. An important feature of the most efficient swimming propeamussiids is the lateral gape in the margin of both valves, creating fluid jets during rapid valve adduction (Morton and Thurston 1989).

Examples of living Northeastern Pacific species in three genera are illustrated here ( Fig. 20A–F ). The most reliable features distinguishing Cenozoic fossil propeamussiids from the Northeastern Pacific are lack of a ctenolium in the byssal notch (if present) of the right valve and size difference of right and left valves in the taxa with prominent internal radial ribs. Most of the nominal species are easily distinguished from Delectopecten by discrepant sculpture of right and left valves and the lack of camptonectes microsculpture. Interior radial ribs are present in most of the Paleogene species examined, and species are commonly distinguished by rib number, point of origin, and extent.

Refined morphological data bearing on pectinoidean family relationships, paleontological data, and molecular analyses do not agree, and homoplasy in shell features exacerbates the problem by increasing the number of evolutionary hypotheses. The concept of Propeamussiidae is especially unstable in terms of molecular analyses suggesting non-monophyly (Smedley et al. 2019). If more comprehensive sampling supports non-monophyly, major revision of the family will be required.

Propeamussiids are treated here as monophyletic and recognized as an important element in a cosmopolitan post-Cretaceous radiation in deep water. Because shells are small, thin, and often poorly preserved they are difficult to distinguish from the similarly fragile pectinids. Their significance in the Keasey is the abundance and remarkable preservation of shell morphology in exterior and interior molds at the famous Mist crinoid locality where they are part of a diverse macrofauna at one of the three methane seeps in the formation.

Stratigraphic range —Middle Triassic–Holocene.

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.