Crenella, T. BROWN, 1827

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 : 4-6

CRENELLA T. BROWN, 1827

Type species — By monotypy, Mytilus decussatus Montagu (1808) . Holocene , northern Norway. Subsequently recognized in the Arctic Ocean and circum-boreal, extending to lower latitudes in both the Atlantic and Pacific .

The type species ( Figs. 1 , 2 ) is well represented in museum collections, although it is likely a species complex based on considerable variation observed by this author. This is consistent with a major difference in life habit between a North Atlantic population, described by Morton et al. (2016) as lacking a byssus and occupying crypts built of mucus-cemented sand grains, and the Northeastern Pacific forms that construct a network of fine byssal threads ( Fig. 2A ). These threads attach firmly to shell fragments and other particles within the sediment ( Fig. 2B, C ). Brooding and lack of a planktonic larval stage, also documented by Morton et al. (2016), are consistent with reduced dispersal and gene flow between isolated populations.

Recognition of the type species in the Northwestern Pacific is complicated by the introduction of additional names. Although this species has long been recognized in the far northern seas of Russia and Japan, there are unevaluated species names based on Japanese specimens, notably those proposed by A. Adams (1862) without illustration and no record of deposition in The Natural History Museum, London. These names along with discovery of the type species in a subtropical Korean fauna led Lutaenko (2014) to suggest a “regional complex” in need of much work .

In addition to the persistent problem of diagnosing the type species of Crenella and defining its distribution in the Northern Hemisphere, there are very similar small-shelled crenellids in the Subantarctic fauna. Melville and Standen (1912, p. 235) recorded the type species from the Burdwood Bank (an undersea plateau delimited by the 200 m isobath and situated approximately 200 km south of the Falkland Islands at 55 o S). Linse (2002) subsequently described this form as Crenella magellanica , with excellent scanning electron micrographs and list of diagnostic features. A similar subantarctic species is Crenella marionensis (E.A. Smith, 1885) from the Marion and Prince Edward Islands in the sub-Antarctic Indian Ocean. The genus also is reported from the southern Atlantic coast of Argentina (Zelaya 2016). The identity and even the generic assignments of southern hemisphere crenellids remain unclear. A very brief review of some of the difficulties assessing genus-group names based on Australasian species is in order because some of these names are now used for species in the North Pacific.

In the Australian fauna, Solamen (type, by original designation, Solamen rex Iredale, 1924 ), was proposed with inadequate figures, although Iredale clearly stated that the hingeline was narrow and edentulous and that the ligament groove was semi-internal. Larger shell size (15–26 mm), relatively thinner shell, and greater infla- tion of the valves also are typical of living North Pacific species assigned to Solamen . Megacrenella Habe and Ito (1965) and Exosiperna Iredale (1929) have been treated as synonyms of Solamen (e.g., Coan et al. 2000, Coan and Valentich-Scott 2012). To complicate matters further, the type species of Exosiperna was described originally under Arcoperna Conrad (1865) , a genus based on an Eocene fossil from Mississippi. Prashad (1932, p. 81) noted the strong similarity of Conrad’s type species ( Arcoperna filosa ) to Solamen . It is therefore possible that Solamen is a synonym of Arcoperna .

In New Zealand Crenella is represented in the living fauna by C. radians Suter (1908) , described from Hen Island off the east Coast of the North Island. Aside from a report of this species from the early late Pliocene of New Zealand ( Beu and Maxwell 1990). The genus is absent from Cenozoic faunas. However, three small specimens from the Maastrichtian of the Chatham Islands, described and figured as Crenella n. sp. (Stillwell 1998) clearly establish its presence in the New Zealand fossil record. Stillwell noted its similarity to coeval species from Maastrichtian and Paleocene of North and South Dakota. Marincovich (1993) noted the presence of the genus in cool-water faunas of the Late Cretaceous of the Western Interior Seaway of North America as well as the earliest Paleogene (Danian) in Arctic Alaska. The Arctic Ocean was presumably geographically isolated from the North Pacific at this time (Marincovich et al. 1985, Marincovich 1993). Detailed study of the high latitude fossil crenellids may prove useful in reconstructing seaway connections during the Late Cretaceous and Early Paleogene.

Greater attention to the Neogene fossil record and names proposed for taxa in the North Pacific, especially Japan and Alaska, is essential to unraveling Cenozoic history of crenellids. Although it is tempting to introduce a new genus-group name for the two Paleogene species in the Keasey Formation, they are retained in Crenella in the treatment that follows.

Stratigraphic range —Cretaceous–Holocene.

Figures

Figure 1. Type species of Crenella, C. decussata (Motagu,1808). A. Left valve exterior of double-valved specimen with byssal threads cemented to shell fragments. B. Right valve interior. SBMNH 214640. Scale bar=1 mm.

Figure 2. Alternative reconstructions of the life habit of Crenella decussata. A. Completely encased in crypt of cemented sedimentary grains, living below sediment-water interface in North Atlantic. B, C. Byssus attached to shell fragments, living at sedimentwater interface in North Pacific.