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

Nucinella taylori sp. nov.

(Fig. 3)

Diagnosis. A small Nucinella having a nuculoid shape, smooth shell except for growth lines with three subumbonal and two anterior pointed blade-like teeth, with no triangular flat area below the dentition; opisthodetic ligament.

Holotype. Disarticulated left valve, NHMUK PI MB 1206, LD-04; length = 1.6 mm, height = 1.6 mm.

Paratypes. Disarticulated right valve, NHMUK PI MB 1209, LD-04; length = 1.6 mm, height = 1.6 mm; disarticulated right valve, NHMUK PI MB 1210, LD-05; length = 2.1 mm, height = 2.3 mm.

Other material. Two specimens from LD-04 (NHMUK PI MB 1207–1208) and three specimens from LD-05 (NHMUK PE PEI 5500; NHMUK PE PEI 5506; NHMUK PE PEI 5517). Fifty-three prodissoconch valves from LD-04 (NHMUK PI MB 1211–1217; NHMUK PE PEI 5481) and 36 prodissoconch valves from LD-05 (NHMUK PI MB 1218).

Derivation of name. Named after Dr John Taylor (Natural History Museum, London, UK) in recognition of his work on chemosymbiotic bivalve molluscs.

Description. Shell is small, thin, inequilateral, equivalve, with a nuculoid, suboval outline. Posterior dorsal margin is slightly incurved; anterior margin almost straight. Opisthogyrate, beaks close to posterior margin, umbo prominent. Smooth shell, except for concentric, irregularly spaced growth lines. Monomyarian: posterior adductor muscle scar absent; anterior adductor large, oval. Opisthodetic ligament, prominent, external does not invade the hinge plate. Hinge made by five pointed blade-like teeth as a single arched series, with the anterior subumbonal teeth being smaller and wider than the posterior ones. A single, long lateral tooth. Left valve with a secondary ridge creating a shallow socket.

Prodissoconch valves: outline nuculoid and suboval. Posterior margin is distinct, long and slightly incurved. Inequilateral, with beaks close to posterior margin, and sculpture consists of irregularly spaced growth laminae. Ventral valve margin has a narrow flat platform. Five anterior and 11 posterior hinge teeth. Amphidetic ligament, lying between the beak and anterior hinge plate.

Remarks. These specimens are most similar to the extant nucinellid Nucinella serrei in their small size, number of posterior and anterior hinge teeth and opisthodetic ligament. However, they lack a flat triangular area below the teeth dentition and a small circular pit at the end of the lateral tooth, which supports their separation.

The majority of extant Nucinella range from intertidal to 500 m deep (La Perna 2005), but some species have been described from water depths exceeding 3000 m (Oliver & Taylor 2012). A large fossil Nucinella species has been described from a Late Cretaceous cold-seep deposit (Amano et al. 2007), showing that this genus may inhabit a wide range of sulphide-rich environmental settings. Bacterial symbiosis with sulphur-oxidizing bacteria is confirmed for N. owenensis and has been inferred for all species of the Nucinellidae (Oliver & Taylor 2012). Nucinella taylori sp. nov. supplants N. birkelundi from the Late Jurassic (Clausen & Wignall 1990) and Nucinella ? sp. from the Late Triassic (Nutzel & Kaim 2014) as the oldest known species of Nucinella, and extends the range of the genus to the basal Triassic H. parvus Conodont Zone.

The prodissoconch valves are very similar to adult specimens of Nucinella taylori sp. nov. except that they have more hinge teeth, which appears to reflect their premetamorphosis stage of development. The position of the ligament in the prodissoconch valves also differs from adult specimens of N. taylori sp. nov. in being amphidetic rather than opisthodetic, but this character is known to change after metamorphosis (Bernard 1898). The prodissoconch valves most resemble N. taylori sp. nov. rather than N. nakremi sp. nov., but may represent larval stages for either or both species.

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