Pleurotomaria? homoruspira, Szabo, 2017

Szabó, János, 2017, Gastropods of the Lower Jurassic Hierlatz Limestone Formation, part 3. New pleurotomarioideans from the fauna of Hierlatz Alpe (Hallstatt, Austria), Fragmenta Palaeontologica Hungarica 34, pp. 9-48 : 30-32

Pleurotomaria? homoruspira n. sp.

( Figs 97–109 )

Type specimens – Holotype: (GBA 2018/002/0009/01); paratype: (GBA 2018/002/0009/02).

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 – Homorú (Hungarian) = concave; spira (Latin) = spiral.

Diagnosis – Conoidal early and coeloconoidal latest teleoconch outline. Early teleoconch whorls flat with almost flush suture, latest whorls becoming convex with clearly impressed suture. Selenizone from early near midwhorl position shifting close to edge of base/periphery. Latest whorl with hardly distinguishable ramp and feebly concave outer face. Periphery sharply angular; base low with convex wall and broad umbilicus. Ornament of riblets on both sides of suture and dense spiral threads, crossing them. For last whorl, large, low nod-like swellings developed on ramp and peripheral band. Base with dense spiral threads.

Material – Two strongly worn, badly fragmentary specimens; however, one informs about the early teleoconch whorls, the other shows also characters of the subsequent whorls of a probably full grown specimen. The protoconch and the earliest teleoconch parts are not preserved, the peristomal parts are also lacking.

Measurements – Holotype (GBA 2018/002/0009/01): height 16.5 mm; paratype (GBA 2018/002/0009/02): height: 11.7 mm.

Description – The early spire whorls have conoidal outline but the latest whorls are coeloconoidally added. On the early whorls, the surface is flat or feebly convex with almost flush suture then, from the penultimate whorl, the whorl surface changes into moderately convex with impressed suture. For the last whorl, a rather wide, slightly convex ramp and a feebly concave outer face also develop; an indistinct angulation divides them. Parallel to the ramp, a narrow swollen belt develops adapically along the suture. The moderately wide selenizone runs at about the middle of the early whorls then gradually shifts abapically; on the latest preserved whorl/last whorl, its position is below the midline of the outer face, quite near to the periphery. The periphery is angular in the earlier whorls and sharp on the last whorl. The base is flattened with concave central area and having a convex wall around a broad umbilicus. No peristome part is preserved.

Sparse spiral threads and riblets form a network ornament on the earliest available teleoconch whorls. The riblets are arranged into two rows along the sutures; the adapical ones are prosocline-prosocyrt while the abapical ones incline opisthocline-prosocyrt way. Both rows extend from the suture to the selenizone. The riblets and the spiral threads are nearly of the same strength, excepting the two spiral threads, limiting the selenizone, which are initially much weaker than the others; the selenizone bears weak lunulae and a midline, flattish spiral thread with low, spirally elongated nodules on the early whorls. On the later whorls, the midline becomes even. Nodules appear also at intersections of the riblets and the spiral lines. The riblets and the spiral threads, with increasing strength and increasing number, are present also on all subsequent whorls. During the growth, low nodes, rather undulations, appear then strengthen on the ramp and on the swollen belt along the lower suture. The base is ornamented by spiral threads that are strongest and sparsest near the periphery; their density increases towards the umbilicus whereas the threads become thinner.

Remarks – This species shows a unique combination of some characters which recall different genera. The coeloconoidal outline and the selenizone that shifts near to the abapical edge of the latest whorls refer to Pyrgotrochus ; the flat early teleoconch whorl surface with swollen sub- and suprasutural belts also support this genus arrangement. However, development of an angulation and ramp on the penultimate whorl together with the low, subsutural nodes refer to Pleurotomaria . Actually, this morphology indicates a new genus level taxonomic unit but the poor preservation does not permit a satisfactory characterisation; no similar species has been found in the literature.

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

Figures

Figs 97–109. Pleurotomaria? homoruspira n. sp. – 97–102. Paratype (GBA 2018/002/0009/02), representing early shell parts of the species. – 97. Lateral view (×3). – 98–99. Lateral view (×3 and ×1). – 100.Basal view (×3). – 101. The early selenizone (×2.8).– 102. Ornament of the base (×7). – 103–109.Holotype (GBA 2018/002/0009/01), a full grown specimen.– 103–104. Lateral view (×3 and ×1). – 105–106. Apical view (×3 and ×1). – 107. Apertural view (×3). – 108. Basal view (×3). – 109. A relatively well preserved part of the selenizone on the last whorl, oblique lateral view (×5)