CRENELLIDAE J.E. GRAY, 1840

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 : 4

CRENELLIDAE J.E. GRAY, 1840

This family-group of small to minute mussels (<10 mm) is represented in the Paleogene fossil record of the Northeastern Pacific by a single genus, Crenella T. Brown (1827) . Following the proposal of Morton et al. (2016), the group is elevated here to family status. Five additional genera have been recognized in the living fauna of the Northeastern Pacific (Coan et al. 2000). They are represented by species that nestle or live infaunally in byssal nests, mucus-bound sedimentary crypts, or attached by byssal threads to shell fragments and larger particles in sandy mud. The shallow infaunal life habit at the sediment-water interface is associated with miniaturization and a departure from the more familiar epifaunal, hard-substrate mytiloid mode of life. Convergent reduction in shell size may be more common than currently suspected, confounding efforts to define natural groups.

In the North Pacific crenellids occur primarily in cold, deep-water settings at high latitudes. In the Southern Hemisphere crenellids are less diverse but also a predominantly high latitude faunal element.

Post-Paleogene diversification of crenellids in the tropical Eastern Pacific fauna ( Coan and Valentich-Scott 2012) is recorded in the living fauna by additional genera, including species that bore in calcareous substrata and are not part of earlier Cenozoic infaunal lineages.

Fossil crenellid specimens are a minor element in faunas ranging from late Eocene to early Miocene in the Pacific Northwest and Alaska. Although never abundant, fragments of the thin shell have been recovered from numerous localities. These fragments are easily recognized by their peculiar opalescent luster as well as fine radial and concentric ornamentation and very finely crenulate shell margin.

Crenella is not present in the tropical Eocene fauna of the Northeastern Pacific. It appears after the onset of high-latitude global cooling in the late Eocene and early Oligocene in the deep-water (bathyal) transition faunas. In California the first records of the genus are Pliocene and Quaternary ( Hertlein and Grant 1972, E.J. Moore 1983). In the Western Pacific it appears as a high latitude element in Neogene and Quaternary faunas of eastern Russia (predominantly Kamchatka and Sakhalin) (Kavanov et al. 2000, 2001) .

Although the genus is absent from the tropical Paleogene faunas of the North Pacific, it apparently was present at much higher latitudes in cold water during the Late Cretaceous and earliest Paleogene. Marincovich (1993) described Crenella kannoi from Danian rocks of the Prince Creek Formation of northern Alaska, deposited in a detached and isolated Arctic Ocean. He suggested that the Danian Arctic Ocean acted as a refugium, subsequently giving rise to elements in the high latitude faunas of both the Pacific and Atlantic. Mesozoic history is even more obscure, with apparent Late Triassic origin of the family and a nestling mode of life that subsequently gave rise to epibyssate, hard-substrate forms as well as a variety of endobyssate and free-living forms in which life habit is not clearly reflected in shell morphology .

Crenellid evolutionary history is further complicated in the Northeastern Pacific by the presence in the living fauna of two larger-shelled species of similar ovate-elongate form that have been reassigned ( Coan et al. 2012) to Solamen Iredale (1924) , a genus based on a living Australian species. However, the earliest records of Solamen are in the Eocene Blanche Point Formation in South Australia and a silicified Eocene fauna in Western Australia ( Darragh and Kendrick 1980). The tendency for crenellids to become more elongate as they increase in size was noted by Fleming (1959) and is noted below in discussion of the two Keasey species. Distinctions among crenellid genera and species are tenuous and remain subject to change with additional study of both fossil and living taxa.

The unsettled state of mytiloid systematics has been characterized more colorfully as a “taxonomic farrago” and as “nomenclatural chaos” (Morton et al. 2016). Convergent miniaturization of the shell associated with infaunal nestling ( Harper and Skelton 1993) masks potentially significant anatomical differences. Although much-needed biological study is not likely to assist with systematics of fossil taxa, the shell ultrastructure underlying the distinctive luster of the shells of the two Keasey species is addressed in the treatment below.

Stratigraphic range —Upper Triassic–Holocene.