Terzileria gregaria, Kiel, 2018

Kiel, Steffen, 2018, Three new bivalve genera from Triassic hydrocarbon seep deposits in southern Turkey, Acta Palaeontologica Polonica 63 (2), pp. 221-234 : 224-225

Terzileria gregaria sp. nov.

Figs. 2, 3.

2017 Caspiconcha -like bivalve; Kiel et al. 2017: fig. 4B, C.

Etymology: For its gregarious occurrence at the type locality.

Type material: Holotype: Mo184051, an internal mold showing features of the pallial line and muscle attachment scars. Paratypes: Mo184052–184060 from type locality.

Type locality: The Terziler III seep deposit, southern Turkey .

Type horizon: Late Carnian, Late Triassic seep deposits of the Kasımlar shales.

Material.—The type material and 75 specimens from Terziler III; five specimens from Terziler II; one specimen from Leo’s Terziler seep deposit; see Table 1 for measurements.

Diagnosis.—As for genus.

Description.—Shell elongate modioliform, largest specimen 33 mm long; length:height ratio about 1.7–2.0; moderately inflated, width:length ratio about 0.38–0.40; beak subterminal, prosogyrate; shell surface smooth except for fine growth increments; escutcheon elongate-lenticular and marked by a sharp ridge; ligament nymph short, slightly less than 1/3 of shell length, close to shell margin, indicating external ligament; short, heart-shaped lunule. Posterodorsal margin very gently convex, posterior end well rounded, ventral margin gently convex, anterior margin truncate and pointing anteroventrally. Anterior adductor muscle scars D-shaped, steeply deepening toward the anterior, terminating anteriorly at a triangular wall perpendicular to the anterior-posterior shell axis; concentric growth increments. Anterior pedal retractor scars elongate-oval, nearly parallel to anterior-posterior shell axis, positioned below and just anterior to the umbones. Pallial line close and subparallel to ventral and posterior shell margin with a distinct bulge just posterior to the anterior adductor muscle scar; reaches the posterior adductor scar at the center of its posterior margin; some specimens show strong secondary pallial attachment scars radiating toward the shell’s interior, especially along the ventral and posterior margins. Posterior adductor scar positioned in the middle of the posterior half of the shell, roundish, with a sinuous indentation underneath the onset of the posterior pedal retractor scar, which projects spine-like from the anterior side of the dorsal margin of the adductor scar. The hinge plate is edentulous, the posterior lateral tooth is elongate, and is well-developed in the left valve but inconspicuous in the right valve.

Remarks.—The “possible permophorid” reported from a Late Triassic seep deposit in Oregon, USA ( Peckmann et al. 2011), might also belongs to Terzilieria, and differs from T. gregaria by being more elongate and having a broader anterior margin. The Early Jurassic Myoconcha neuquena Leanza, 1940 was recently discussed as an early member or possible ancestor of Caspiconcha ( Jenkins et al. 2018) . It could therefore, at least stratigraphically speaking, be a link between Terzileria and Caspiconcha . Myoconcha neuquena differs from T.gregaria mainly by having the anterior pedal retractor and adductor muscle scars fused, by its oblique posterior adductor muscle scar with a much broader ventral extension, and by its thicker hinge plate (as shown in Griffin and Pastorino 2006: figs. 4-1, 4-2).