Austrotindaria antiqua Foster & Danise & Twitchett 2017, sp. nov.

Austrotindaria antiqua sp. nov.

(Fig. 5)

1899 Anodontophora (Myacites) fassaensis (Wissmann); Bittner: 22, pl. 3, figs 28–33.

1908 Anoplophoria (Myacites) fassaensis (Wissmann); von Wittenburg: 33, fig. 15.

1926 Anodontophora fassaensis (Wissmann); Matsushita: pl. 8, fig. 11.

1963 Unionites fassaensis (Wissmann); Ciriacks: 81, pl. 16, fig. 13.

1988 Unionites fassaensis var. brevis Neri & Posenato: 94, pl. 2, fig. 8.

2009 Unionites fassaensis (Wissmann); Kumagae & Nakazawa: 166, fig. 145, 5–9.

2014 Unionites ? fassaensis (Wissmann); Pan et al.: 151, fig. 5G, H.

2015 Unionites fassaensis (Wissmann); Hofmann et al.: 8, fig. 4K.

2015 cf. Unionites fassaensis (Wissmann); Foster et al.: 381, fig. 4L.

Diagnosis. Shell small, smooth except for concentric growth lines, subtrigonal, inequilateral, prosogyrate; taxodont hinge dentition with more than 50% fewer anterior than posterior ones, 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 1240, LD-04; length = 4.0 mm, height = 3.0 mm.

Paratype. Disarticulated left valve, NHMUK PI MB 1241, LD-04; length = 4.6 mm, height = 3.4 mm.

Other material. Eight specimens from LD-04 (NHMUK PI MB 1194; NHMUK PE PEI 5479; NHMUK PE PEI 5486–5488; NHMUK PE PEI 5513; NHMUK PE PEI 5520) and 18 specimens from LD-05 (NHMUK PI MB 1191; NHMUK PI MB 1196–1197; NHMUK PI MB 1199; NHMUK PI MB 1242; NHMUK PE PEI 5475– 5476; NHMUK PE PEI 5483; NHMUK PE PEI 5494; NHMUK PE PEI 5497; NHMUK PE PEI 5499–5501; NHMUK PE PEI 5504; NHMUK PE PEI 5506; NHMUK PE PEI 5515). Two prodissoconch valves from LD-04 (NHMUK PI MB 1243–1244).

Derivation of name. Latin, antiqua (ancient) referring to this species being the oldest known of the genus.

Description. Equivalve, inequilateral shell with a subtrigonal outline and a beak positioned approximately 30% along the length of the dorsal margin from the posterior; H/L ratio 0.6–1. Conspicuously tumid. Umbo prosogyrate, prominent, moderately broad, rounded and projects above the hinge margin. Slightly rounded and gently sloping anterodorsal margin. Posterodorsal margin almost straight to slightly rounded, gently sloping, with a slight angled junction with the posterior margin. Ventral margin deeply rounded. Escutcheon short, relatively broad, elliptical; lunule narrow. Shell smooth with fine concentric growth lines; entire inner margin smooth. Small ligament, external, opisthodetic, with a well-defined margin and well-rounded triangular pit seated beneath the edentulous gap. Hinge plate with taxodont teeth in two series, sometimes separated by a narrow, plain area, without resilifer, that is narrow below the beak, broadening towards the anterior and posterior ends. Robust teeth, moderately long and blunt, with more than 50% fewer anterior than posterior ones. Smooth ventral margin. No pallial line, sinus or muscle scars are present. As the size of the shell increases the edentulous gap becomes proportionally smaller and more central, and moves externally.

Remarks. The external morphology of these specimens is identical to most Early Triassic specimens previously described as ‘ Unionites’ fassaensis, which is one of the most widespread bivalve species from the Lower Triassic and is also a problematic dustbin taxon that includes a range of different morphologies. These specimens differ from the original description and figures of ‘ Myacites’ fassaensis Wissmann, 1841 in having a less elongated posterior margin and a more prosogyrate beak. Due to poor preservation, however, little is known about the internal morphology of Early Triassic specimens assigned to U. fassaensis, which has created some uncertainty (Hautmann et al. 2013). Internally, these new specimens lack the following characters that Geyer et al. (2005) determined were diagnostic of Unionites: a nymph extending nearly half of the posterior margin; an impressed adductor muscle scar; an overlap of the anterior hinge; and a deeply impressed lunule and posterior keel. Thus, these specimens cannot be assigned to Unionites. The presence of taxodont hinge dentition, an opisthodetic ligament and lack of ornamentation are, however, characteristic of the genus Austrotindaria.

The more prosogyrate beak means that these specimens have a similar external morphology to Middle Triassic Unionites specimens (e.g. Geyer et al. 2005), but apart from possessing a faint posterior keel they lack the diagnostic criteria of Unionites. Furthermore, the Unionites specimens described by Geyer et al. (2005) and from this study (see below) all have a strong posterior keel. Thus, these specimens are not assigned to Unionites. These specimens, and all previous specimens assigned to Unionites fassaensis that possess a prosogyrate beak and lack a well-defined posterior keel, are therefore assigned to Austrotindaria antiqua sp. nov.

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