Pleurotomaria nongradata, Szabó, 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 : 25-27

Pleurotomaria nongradata n. sp.

( Figs 74–84 )

Type specimens – Holotype: GBA 2018/002/0006/01 ; paratype: GBA 2018/ 002/0006/02 .

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 – Non- (Latin) = not, gradatus (Latin) = gradate; referring to the obscure ramp and lack of the gradate shell outline from the latest teleoconch whorls.

Diagnosis – Conoidal shell, composed of very low, concave whorls. Angulation and ramp hardly developed on whorls; shell not gradate. Selenizone wide and flush little below midwhorl. Suture feebly impressed, undulating. Periphery coinciding with swollen, nodose abapical rim of last whorl. Base slightly concave as whole with flattened wall, and moderately broad umbilicus. Few spiral threads and rows of nodes along both sides of suture ornamenting whorls. Selenizone running between two thin spiral threads and having strong, submedian cord and dense, weak lunulae. Rather sparsely spaced spiral threads on base. Growth lines delicate on whole surface.

Material – Two specimens; the holotype is a multi-damaged specimen from a variety of fissure-filling Hierlatz Limestone, predominated by coarse grained crinoid debris and cemented by diagenetic calcite (from field trip in 1999). The apical shell and parts of the latest whorls are lacking; the rest has broken into two parts that are slightly slipped apart. The paratype is in even worse state of preservation. Its shell is at least one whorl longer so shows a slightly different latest whorl.

Measurements – Holotype (GBA 2018/002/0006/01): diameter 15.3 mm; paratype (GBA 2018/002/0006/02): diameter 20.8 mm, preserved height: 18.7 mm.

Description – The available specimens indicate a conoidal shell of numerous (~10 ± 2) very low whorls; the right number depends on the early shell shape, which may be different from the simple cone of the available latest whorls. The width of the latest teleoconch whorls is ~4 times larger than their height. They have an indistinct angulation close to the adapical suture and, as consequence, an extremely narrow, rudimentary ramp has developed that is not enough to cause gradate shell outline. The suture is a narrow but rather deep groove that becomes undulating for the latest whorls owing to the marked nodular ornaments of the neighbouring swollen belts. Between the swollen belts, the “outer face” is concave with a flattish deepest part, occupied by the selenizone somewhat below the midwhorl. The selenizone is rather wide, about 20% of the suture to suture distance on the latest whorls. The periphery coincides with the abapical swollen belt that turns rounded-angular way to the base, which is slightly concave as a whole with mostly flat wall, and has a moderately wide umbilicus in a funnel like depression.

Two rows of nodules ornament the swollen belts of the whorls, one below, the other above the suture. The adapical row is crossed on the earliest preserved whorls by a single spiral thread, marking the abapical rim of the ramp, then another, weaker one is also added; equally strong, double spiral threads cross the abapical series of nodules on all visible whorls. Two thin threads limit the selenizone that bears also a strong submedian cord, crossed by dense, thin lunulae. Rather sparsely and unequally spaced, thin spiral threads ornament the base. The growth lines are delicate on the entire shell surface; some of them appear as thin, subregularly repeated riblets on the latest whorl between the selenizone and the lower swollen belt.

Remarks – Since a narrow but doubtless ramp is present on the earliest whorls, identification of this species as Pleurotomaria seems correct in spite of some similarity to the type species of Pyrgotrochus P. Fischer, 1885 [ P. strobilus (J. A. Eudes-Deslongchamps,1849) ] and some closely related species as P. precatoria (J. A. Eudes-Deslongchamps,1849) and P. princeps (Koch & Dunker 1837) . However, the extremity of this shell form within Pleurotomaria is obvious.

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

Figures

Figs 74–84. Pleurotomaria nongradata n. sp. – 74–81. Holotype (GBA 2018/002/0006/01). – 74. Details of the ornament in a slightly oblique lateral view (×6.5). – 75. Apical view (×3). – 76–77. Lateral view (×3 and ×1). – 78–79. Basal view (×3 and ×1). – 80–81. “Best outline” view (×3 and ×1). – 82–84. Paratype (GBA 2018/002/0006/02), a larger specimen, which probably represents fully or near fully grown stage (see more in remarks). – 82–83. Lateral view (×3 and ×1). – 84. Basal view (×1)