Nucinella nakremi Foster & Danise & Twitchett 2017, sp. nov.

Nucinella nakremi sp. nov.

(Fig. 4)

Diagnosis. A small Nucinella having a nuculoid shape, smooth shell except for growth lines. Prosogyrate beak, one to three subumbonal teeth. Ligament amphidetic and does not invade the hinge plate.

Holotype. Disarticulated left valve, NHMUK PI MB 1219, LD-04; length = 1.1 mm, height = 1.1 mm.

Paratype. Disarticulated left valve, NHMUK PI MB 1220, LD-04; length = 0.9 mm, height = 0.8 mm (transposed hinge).

Other material. Two specimens from LD-04 (NHMUK PI MB 1221–1222).

Derivation of name. Named after Dr Hans Arne Nakrem in recognition of his work on Permian and Triassic fossils from Svalbard.

Description. Shell small, nuculoid and ovate. Posterior dorsal margin distinct, slightly incurved; posterior margin rounded. Inequilateral, prosogyrate, with beaks close to anterior margin. Umbo prominent. Smooth, thin shell with very weak growth lines. Monomyarian: posterior adductor muscle scar absent; anterior adductor large, oval. One to two subumbonal, pointed blade-like teeth plus one anterior tooth. Ligament amphidetic, prominent, external does not invade the hinge plate.

Remarks. These specimens differ from other described nucinellid species in having fewer hinge teeth, a more elliptical shape and a prosogyrate beak. Such differences may occur during the ontogeny of Nucinella (e.g. Bernard 1898) and so are not sufficient for assignment to a separate genus. These specimens are, however, considered to represent a separate species rather than an intermediate ontogenetic stage between the protoconch and adult stage of Nucinella taylori. During ontogeny, the shape, size and position of nucinellid subombonal teeth also vary: in earlier stages of development they are more rounded and later they develop a chevron-blade shape with the older teeth making space below the beak for thinner, newer ones (Bernard 1898; La Perna 2004). In contrast, the subumbonal teeth of specimens assigned to N. nakremi and N. taylori have comparable shapes, and so indicate a similar stage of development. Furthermore, because the position of the ligament in nucinellids is fixed after metamorphosis (Bernard 1898; La Perna 2004), and the only subsequent ontogenetic change is an increase in ligament size with age, the differences in ligament position between N. nakremi and N. taylori cannot be ontogenetic. If the specimens assigned to N. nakremi were included as an intermediate ontogenetic stage of N. taylori, the ontogenetic pattern would not match any known nucinellid (cf. Bernard 1898; La Perna 2004), and, therefore, the differences in the subumbonal teeth and the position of the ligament support their separation.

A specimen of Nucinella nakremi sp. nov. includes an example of a transposed hinge on a left valve (Fig. 4D). Instead of the normal left valve arrangement of three hinge teeth and a lateral secondary ridge creating a secondary socket, this specimen has two hinge teeth and a lateral tooth (i.e. the normal right valve arrangement). An alternative interpretation is that this specimen represents an earlier ontogenetic stage, but this is rejected because even though the specimen is slightly smaller, its lateral tooth is more prominent than in a typical left valve arrangement and both the subumbonal and lateral teeth appear to fit with the expected corresponding valve arrangement. In addition, it has been reported that in the early stages of nucinellid development the second lateral tooth is very small and closer to the subumbonal teeth than later in ontogeny (Bernard 1898), which is not the case with this specimen. Transposed hinges have been reported in a number of bivalve families, but this is the first reported occurrence in a species of Nucinellidae.

Mode of life. Shallow infaunal, fully motile, slow, chemosymbiotic (Oliver & Taylor 2012).