Csigabaga nillae, Szabó, 2018

Csigabaga nillae n. sp.

( Figs 37–50 View Figs 20–50 )

Type specimens – Holotype: GBA 2019/009/0023 , paratypes: GBA 2019/009/0024–27 . 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 – See together with the genus name.

Diagnosis – Same as for the genus.

Material – Eight specimens (GBA 2019/009/0023–30).

Measurements – Paratype (GBA2019/009/0024),H= 9.7mm, holotype (GBA 2019/009/0023), W = 6.5 mm, H = 8.8 mm, paratype (GBA 2019/009/0025), W = 5.5 mm, H = 7.2 mm, paratype (GBA 2019/009/0026), W = 6.6 mm, H = 10.5 mm, paratype (GBA 2019/009/0027), W = 7.5 mm, H = 9.8 mm.

Description – Shell is slightly cyrtoconoidal with a convex, anomphalous base that is convex but obliquely flattened in the latest growth stage. The last whorl is abaxially extending along the flattened base part near the last peristome. The apex is blunt, the earliest shell part is almost planispiral. All whorls are low; their number is about eight in the full-grown stage. Surface of the whorls is convex and the suture is impressed in the earliest shell parts then gradually changes into flush. The last half whorl becomes convex again and strongly convex at the latest periphery. The latest peristome has strongly thickened parietal and columellar lips, joined into broadly lunuliform inner lip with wide outer face. Single fold is formed around the middle of the low columellar lip. It seems to be missing from the spire whorls. Latest outer lip has not been found in undamaged state; by its remnants, it seems to be thin. All full-grown specimens show traces of a moderate outward extension of the last whorl near the last peristome and development of a concomitant subsutural concave belt.

Ornament of few spiral lines is found only on one early whorl, which belongs to the protoconch or immediately follows it. The growth lines are prosocline, straight, and delicate on the whorls, sickle shaped on the base before the last growth phase shell modifications then change into parasigmoidal.

Remarks – A damaged specimen ( Figs 49–50 View Figs 20–50 ) exposes an inner basal fold that is probably present also in the other specimens. Following a marked early phase, it gradually weakens then terminates on the basal lip near the foot of columella but here it is already very low. Sometimes a shallow outer deepening seems to indicate its presence inside ( Figs 47–48 View Figs 20–50 ). Neither the columellar nor the basal fold have been found in the spire whorls of an axially cut subadult specimen. However, that specimen went through intensive, multi-phased recrystallization that could mask also the folds.

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