Eucyclomphalus? subgradatus, 1892

Eucyclomphalus? subgradatus n. sp.

( Figs 1–4 View Figs 1–19 )

Type specimen – Holotype: GBA 2019/009/0004.

Type locality – Hierlatz Alpe (Hallstatt, Austria).

Type strata – Lower Jurassic Hierlatz Limestone from the Semicostatum Zone (Lower Sinemurian) to Jamesoni Zone (Lower Pliensbachian) interval.

Derivation of name – The name is formed as reminder to one of the specific characters of this new species, the weakly developed gradate outline of the spire.

Diagnosis – High conoidal shell with low whorls. Earliest and latest teleoconch whorls evenly convex; intermediate whorls having weak angulation at midwhorl and at periphery; latter one bearing strong, single cord. Second preserved whorl having dense, collabral threads and four spiral threads. Lowermost spiral one overlapped by suture; adapical neighbour becoming peripheral cord, other two becoming ribbons afterwards. Collabral threads lacking from latest whorls. Base bearing dense spiral ornament, mostly thin threads and few cords.

Material – Single specimen with damages and matrix pieces on the shell, risky to remove.

Measurements – Holotype (GBA 2019/009/0004), D = 10.4 mm.

Description – The species is represented by a moderately high conoidal shell that consists of about nine low, convex whorls, which are more than three times wider than the maximum distance of the limiting sutures in the latest growth phases. Already on the earliest observable whorl, a weak angulation is visible slightly above the suture; it persists also on all following whorls though it becomes rounded for the latest growth stages. This angulation provides the periphery for all whorls. The suture is deeply impressed below the peripheral angulation and the outermost belt of the base remains exposed also on the spire. Another but obscure angulation also develops somewhat below the midwhorl for the latest 3–4 whorls. Initially, the subsutural ramp is fully convex then its abapical, wider belt becomes slightly concave; the outer face between the two angulations is feebly convex on all whorls. The base below the rounded part of the peripheral angulation is subglobularly convex as a whole, and it has convex wall and a broad umbilicus. No peristome part is preserved.

The earliest small whorl fragment has two spiral threads in peripheral position; no more ornament remained observable around. The next four whorls have dense, sharp, suture to suture threads that are crossed by four, similarly strong spiral threads initially, so about the abapical half of two earlier whorls shows a network ornament. On the third available early whorl, the obscure angulation of the whorl surface develops in line with the most adapical spiral thread. At the same time, the thread itself and its abapical neighbour become flat elevation but the thread, third from the apex, strengthens as a cord on the peripheral angulation. The fourth line is overlapped by the suture initially then exposed on a narrow but widening belt of the base between the periphery and the suture. In the latest growth stages, few additional, thin threads are also visible in this belt. For the penultimate whorl, the collabral threads change into irregularly spaced, flat ridges, and the uppermost two spiral treads transformed into slightly convex, wide ribbons. The shell remnants of the base in the peripheral region and at the rim of the umbilicus are covered with thin spiral threads. However, the elevations on the inner mould of the base indicate also presence of few sparsely distributed spiral cords; their subregular undulation may be caused by nodes.

The growth lines seem slightly prosocline and opisthocyrt between the sutures, actually they consist of a prosocline and gently prosocyrt part on the ramp then turn into a just feebly prosocline part on the narrow outer face then into feebly opisthocline on the peripheral part of the base.

Remarks –The shape of the early shell parts is similar to that of Eucyclomphalus hierlatzensis von Ammon, 1892 but the opisthocyrt form of the collabral threads, and the formation of an angulation on the whorl surface between the upper suture and the periphery in Eucyclomphalus ? subgradatus n. sp. distinguish the early teleoconchs of these species. On the latest whorls, the periphery becomes rounded in both species; however, the size of the full-grown E. hierlatzensis specimens much exceeds the size of the full-grown E.? subgradatus n. sp. Moreover, spiral ribbons on the latest whorls develop only in E.? subgradatus n. sp.

Occurrence – Within Sinemurian to lowermost Pliensbachian (Lower Jurassic) part of the Hierlatz Limestone Formation in Hierlatz Alpe, Hallstatt, Austria.