Aksumya krystyni, 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 : 229-231

Aksumya krystyni gen. et sp. nov.

Figs. 7–9.

2017 new genus of bivalve clade Anomalodesmata ; Kiel et al. 2017: fig. 4A.

Etymology: For Leopold Krystyn (Vienna, Austria).

Type material: Holotype: Mo184067, a specimen with preserved shell surface. Paratypes: Mo184066, Mo184068–184074 from the type locality.

Type locality: Leo’s Terziler seep deposit, southern Turkey .

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

Almost identical regarding shell shape, external surface and hinge characters is the Viséan, early Carboniferous genus Tellinomorpha de Koninck, 1885 (type T. cuneiformis de Koninck, 1885 ). However, there are several reasons why I hesitate placing Aksumya krystyni in this genus: (i) Tellinomorpha is based on a single right valve with unknown internal features; (ii) the posterior siphonal area is broken in that single specimen, hence its length is unknown, and furthermore, it is demarcated by the upturn of the growth lines (cf. Morris and Dickins 1991) and this upturn is angular in T. cuneiformis but gently rounded in Aksumya krystyni ; (iii) the only known specimen of T. cuneiformis is about 100 million years older than Aksumya krystyni and I find it unlikely that such decent-sized bivalves (exceeding 70 mm in length) should have remained unnoticed during such a long geologic time interval, spanning most of the Carboniferous, the well-sampled Permian, and half of the Triassic.

Among the Pholadomyidae , species of Arcomya Roemer, 1839 are less inflated, have much narrower gapes anterior and posterior of the umbones, the pallial line crosses the anteroventral margin obliquely, whereas it largely follows the ventral shell margin in Aksumya , and pallial line and posterior adductor muscle scar are more distant from the posterior margin than in Aksumya . In Pachymya Sowerby, 1826 the shell is more inflated posteriorly and also has pallial line and posterior adductor muscle scar more distant Material.—Ten specimens from the type locality, two from Terziler I, six specimens and many fragments from Terziler II, 14 specimens from Terziler III, 18 specimens and many fragments from Dumanlı I, 12 specimens and many fragments from Dumanlı II; southern Turkey, late Carnian, Late Triassic of the Kasımlar shales. See Table 3 for measurements.

Diagnosis.—As for genus.

Description.—Elongate nuculaniform shell, umbo slightly prosogyrate, positioned in the anterior half of the shell; surface sculptured by commarginal growth lines and tiny, irregularly distributed spines; ligament at least partially external and calcified; anterodorsal margin running more-or-less straight down to about 2/5 of the shell height, anterior margin below rounded or truncate; ventral margin straight to slightly concave; anterior variable, ranging from broad and blunt, to tapering into a blunt, pointed, posteroventral end, to acute-rounded and pointing slightly upward. Shell moderately inflated, with broad and shallow sulcus running from the umbo to the ventral margin; escutcheon lanceolate, deep, bordered by a sharp ridge; largest specimen ca. 74 mm long. Entire inner shell surface covered by fine radial grooves; ventral margin of inner shell surface crenulated; broad groove running from umbo to anterodorsal margin, bordering the anterior adductor muscle scar, which is indistinct, situated mostly below the midline of the shell, roundish to quadrate, and fused dorsally with the anterior pedal retractor scar; pallial line indistinct, onset apparently at the posteroventral margin of the anterior adductor muscle scar, running close to the ventral shell margin, no pallial sinus; posterior adductor muscle scar indistinct, roundish, situated in the posterodorsal corner of the shell, above a broad and shallow groove that radiates from about the middle part of the shell (anterior half of escutcheon) toward the posterior margin. Hinge plate narrow and short, with two blunt teeth in each valve; ligament nymph short and strong holding an external and calcified ligament

Remarks.—The “Nuculanid” from the early Norian, Late Triassic Greylock Butte seep deposits in Oregon, USA ( Peckmann et al. 2011: fig. 3G) almost certainly also belongs to Aksumya . That species reaches 50 mm in length and is the most common mollusk (n = 15) at this Halorella - dominated seep deposit. It has a shorter and taller shell with a steeper and more angular anterior margin than Aksumya krystyni sp. nov.