Unionites aff. subrectus ( Bittner, 1901 )

Foster, William J., Danise, Silvia & Twitchett, Richard J., 2017, A silicified Early Triassic marine assemblage from Svalbard, Journal of Systematic Palaeontology 15 (10), pp. 851-877 : 862-863

Unionites aff. subrectus ( Bittner, 1901)

( Fig. 8 )

aff. 1901 Anodontophora subrecta Bittner : 100, pl. 7, figs 28–30.

aff. 2003 Unionites subrectus (Bittner) ; Szente & Vor̈os: 131, pl. Biv-II, figs 25, 26.

2009 Triaphorus aff. multiformis Kiparisova ; Kumagae & Nakazawa: 171, fig. 145, 18–25.

Material. Seven specimens from LD-04 ( NHMUK PI MB 1245–1247 ; NHMUK PE PEI 5480; NHMUK PE PEI 5485; NHMUK PE PEI 5507; NHMUK PE PEI 5519), and nine specimens from LD-05 ( NHMUK PI MB 1195 ; NHMUK PI MB 1197 ; NHMUK PI MB 1201 ; NHMUK PE PEI 5495; NHMUK PE PEI 5506; NHMUK PE PEI 5508; NHMUK PE PEI 5516) .

Description. Outline elongate-elliptical. Shell equivalved, inflated below the umbo, and inequilateral with beak positioned approximately 85% along the length of the dorsal margin from the posterior. Lower part of anterior dorsal margin projects slightly beyond plane of commissure. Posterodorsal margin is almost straight to slightly round and gently sloping. Lunule is long and narrow, with deeply impressed escutcheon. Umbo is prosogyrate and rises above the hinge margin. Ornamented with fine concentric growth lines. The entire inner margin is smooth. Prodissoconch is smooth and orbicular.

A small subumbonal groove limits the shell projection posteriorly. Short lateral tooth, left valve possesses a weak secondary ridge creating a shallow socket. Hinge of left valve has an anterior platform that bears a depression for the corresponding anterior hinge margin of the right valve; posterior to this is a small, tuberculiform subumbonal tooth fitting above posterior lateral tooth of right valve. Ligament is fixed to a nymph, which extends about half the length of the posterior dorsal margin. Isomyarian muscle scars with a deeply impressed anterior adductor muscle scar.

Remarks. Insufficient knowledge of the internal morphology of Early Triassic bivalves in general, and Unionites in particular, has created uncertainty regarding their systematic position. Based on their external morphology alone these specimens would be assigned to Triaphorus aff. multiformis ( Kumagae & Nakazawa 2009) , but they possess the following characters that are diagnostic of Unionites : the anterior hinge margin of the right valve overlaps that of the left; a nymph that extends nearly half the length of the posterior hinge margin; an impressed anterior adductor muscle scar; a deeply impressed lunule; and a long and narrow escutcheon. Thus, these specimens are assigned to Unionites .

These specimens differ from U. brevis , U. fassaensis , U. letticus , U. griesbachi , U. borealis and U. rhomboidalis because their length greatly exceeds their height. They are most similar to U. subrectus described from the Middle Triassic of Hungary by Szente & Voros (2003), except for their more elliptical outline.

Mode of life. Shallow infaunal, facultatively motile, unattached, suspension feeder ( Hautmann et al. 2013).

Figures

Figure 8. Unionites aff. subrectus (Bittner, 1901). A, NHMUK PI MB 1246, external view, right valve; B, NHMUK PI MB 1245, external view, left valve; C, NHMUK PI MB 1246, internal view, right valve; D, NHMUK PI MB 1245, internal view, left valve; E, NHMUK PI MB 1195, external view, right valve; F, hinge margin, NHMUK PI MB 1245, left valve; G, hinge margin, NHMUK PI MB 1246, right valve. Scale bars = 1 mm.