Austrotindaria svalbardensis, Foster & Danise & Twitchett, 2017

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 : 860-861

Austrotindaria svalbardensis sp. nov.

( Fig. 6 )

1864 Anoplophora fassaensis (Wissmann) ; Alberti: 137, pl. 3, fig. 8.

1930 Nucula sp. juv. ind; Spath: 53, pl. 7, fig. 12.

2013a Unionites fassaensis (Wissmann) ; Hofmann et al.: 887, fig. 8, 17–18.

2015 Unionites cf. fassaensis ; Hautmann et al.: 890, fig. 10H.

Diagnosis. Shell small, smooth except for concentric growth lines, subtrigonal, inequilateral, orthogyrate; taxodont hinge dentition with more posterior than anterior teeth, obtusely chevron-shaped teeth, hinge plate interrupted below the beak by an edentulous gap, a small rounded triangular pit below the edentulous gap; ligament opisthodetic.

Holotype. Disarticulated right valve, NHMUK PI MB 1223 , LD-04; length = 1.8 mm, height = 1.3 mm.

Paratype. Disarticulated left valve, NHMUK PI MB 1231 , LD-04; length = 1.5 mm, height = 1.0 mm .

Other material. Twenty-one specimens from LD-04 ( NHMUK PI MB 1190 ; NHMUK PI MB 1192–1193 ; NHMUK PI MB 1224–1230 ; NHMUK PI MB 1232– 1233 ; NHMUK PI MB 1235–1236 ; NHMUK PE PEI 5478; NHMUK PE PEI 5482; NHMUK PE PEI 5489; NHMUK PE PEI 5505; NHMUK PE PEI 5509–5510; NHMUK PE PEI 5512; NHMUK PE PEI 5520) and 12 specimens from LD-05 ( NHMUK PI MB 1189 ; NHMUK PI MB 1197 ; NHMUK PI MB 1200 ; NHMUK PE PEI 5476; NHMUK PE PEI 5491–5492; NHMUK PE PEI 5496; NHMUK PE PEI 5498; NHMUK PE PEI 5503– 5504; NHMUK PE PEI 5517). Twenty-four prodissoconch valves from LD-04 ( NHMUK PI MB 1234 ; NHMUK PI MB 1237 ; NHMUK PI MB 1238 ; NHMUK PE PEI 5520) and five prodissoconch valves from LD-05 ( NHMUK PI MB 1239 ) .

Derivation of name. Named after the Svalbard archipelago.

Description. Shell outline subtrigonal, equivalve, inequilateral with beak positioned approximately 60% of the distance along the dorsal margin from the posterior; Height/Length (H/L) ratio 0.6–1; conspicuously tumid. Orthogyrate umbo, prominent, moderately broad, rounded and projected above the hinge margin. Anterodorsal margin slightly rounded and gently sloping. The posterodorsal margin is almost straight to slightly rounded and gently sloping. Ventral margin is smooth and deeply rounded. Escutcheon is short, relatively broad, elliptical; lunule narrow. No sculpture except for fine concentric growth lines; entire inner margin is smooth. Prodissoconch is smooth, broadly subovate, with H/L ratio of 0.6–0.9. Ligament small, external with well-defined margin, opisthodetic, with a well-rounded triangular pit seated beneath the edentulous gap. Hinge plate with taxodont teeth in two series; separated by narrow, plain area, without resilifer, narrow below the beak, broadening towards the anterior and posterior ends. Teeth are robust, moderately long and blunt, with more posterior than anterior teeth, separated by an edentulous gap. As the size of the shell increases the edentulous gap becomes proportionally smaller, more central and moves externally. No pallial line, sinus or muscle scars present.

Remarks. These specimens differ from Austrotindaria antiqua sp. nov. in having a more equal number of anterior to posterior teeth and an orthogyrate beak. The direction of the beak also separates these specimens from unequivocal species of Unionites . In the specimens described here and other Austrotindaria species the beak is orthogyrate to posteriorly opisthogyrate, whereas in Unionites it is prosogyrate. Based on this key character, most specimens previously assigned to Unionites fassaensis , and similar Early Triassic specimens with an orthogyrate beak and no internal morphological detail preserved, are herein assigned to Austrotindaria svalbardensis sp. nov.

These specimens have a very similar shape, size and ornamentation to the type species of Austrotindaria ( A. wrighti Fleming ); however, they differ in possessing a small, rounded, triangular pit below the edentulous gap, and differ from other Austrotindaria species, such as A. mawheraensi s, in lacking a weak posterior rostrum. For these reasons, they are therefore considered a separate species.

Mode of life. Shallow infaunal, fully motile, slow, miner (Stasek 1961; Stanley 1968).

Figures

Figure 6. Austrotindaria svalbardensis sp. nov. A, paratype, NHMUK PI MB 1231, external view, left valve; B, holotype, NHMUK PI MB 1223, external view, right valve; C, NHMUK PI MB 1190, left valve, external view; D, larval shell, NHMUK PI MB 1234, external view, right valve; E, NHMUK PI MB 1225, internal view, left valve; F, NHMUK PI MB 1227, internal view, left valve; G, larval shell, NHMUK PI MB 1237, internal view, right valve; H, paratype, posterior and anterior hinge plate junction, NHMUK PI MB 1231. Scale bars = 500 µm.