Pleurotomaria nodulocincta, 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 : 16-18

Pleurotomaria nodulocincta n. sp.

( Figs 29–46 )

Type specimens – Holotype: GBA 2018/002/0003/01 ; paratypes: GBA 2018/ 002/0003/02–03 .

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 – Nodule (Latin) = nodule; cinctus (Latin) = belt; referring to the marked, nodulous ribbons of the latest whorl in the peripheral region.

Material – Eight fragmentary specimens (GBA 2018/002/0003/01–08); the holotype probably shows the latest whorls, the others represent the early growth stages.

Measurements – Holotype (GBA 2018/002/0003/01): maximum diameter of inner mould: 14.5 mm; paratype (GBA 2018/002/0003/02): height: 6.5 mm; paratype (GBA 2018/002/0003/03): height: 6.8 mm.

Diagnosis – Rather low spired, slightly cyrtoconidal shell of about six, convex whorls, having narrow subsutural ramp in all growth phases. Ramp appearing from third whorl and abaxially limited by distinct angulation from fourth whorl. Outer face flat initially then becoming convex for last whorl; slightly concave belt just below angulation on all whorls. Suture impressed, selenizone flush with midline cord above middle of outer face. Periphery widely rounded, base as whole somewhat convex and having convex wall and rather narrow umbilicus. Ornament composed of marked, sparse ribbons, nodose on whorl surface in latest growth phases but even on base.

Description – The available material indicates a shell form with rather low spire and somewhat cyrtoconidal outline. A shell of about six whorls with impressed suture can be reconstructed from the specimens in hand. It starts with a blunt apex, a depressed, almost planispiral protoconch of pleurotomariid type. On the third whorl, the initially evenly arched whorl surface changes into angular; a narrow subsutural ramp develops with sharp abaxial rim. A wide, concave early selenizone is observable above the midline of the outer face from the second whorl. The outer face is flat on the early teleoconch whorls then becomes convex for the last whorl; however, a narrow, slightly concave belt goes along with the angulation on its abapical side in its full length. The selenizone is built just abapically from the concave belt. It changes into convex form from the fifth whorl and has a marked submedian cord from the fourth whorl. The peripheral area is widely arched and turns into the base through a rounded angulation; the periphery itself is well above the rim of the base, it corresponds to the nodulous ribbon abapically just along the selenizone. The base as a whole is convex with a convex wall and a rather narrow umbilicus within a funnel like depression.

The earliest trace of the ornament is observable from the second whorl; it is a network initially on both sides of the selenizone but changes into simply ribbed for the third whorl on the ramp. Between the selenizone and the lower suture, the network ornament persists also on the third whorl. From the fourth whorl, the former collabral elements of the network fully vanish but denser, delicate growth-threads appear instead both on the ramp and the outer face. They also vanish from the penultimate whorl while the spiral threads become strong, wide and nodular ribbons, crossed only by very fine growth lines. The nods of the neighbouring ribbons are not aligned collabrally.

On the ramp, the earliest riblets are dense, regularly repeated, thin and collabral. Small nodules develop on each second riblets on the third whorl then the nodeless riblets soon disappear; only collabrally elongated sparse nodules with their top on the angulation ornament subsequently this region of the shell. These nodes strengthen during the growth. Initially, a spiral thread runs just on the angulation then it also changes into spiral ribbon in the latest growth phases.

The selenizone is formed between two thin spiral threads along full length of the teleoconch; initially thin lunulae ornament it then a new spiral thread appears at about the midline from the fourth whorl and gradually strengthens into a marked cord for the last whorl.

The growth lines are almost orthocline and feebly prosocyrt adapically from the selenizone, slightly prosocline-prosocyrt below the selenizone and parasigmoidal on the base.

Remarks – Mainly owing to the nodose ornament, P. nodulocincta n. sp. somewhat resembles P. wiesberghausensis n. sp. Their distinction is discussed amongst the remarks of the latter species above.

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

Figures

Figs 29–46. Pleurotomaria nodulocincta n. sp. – 29–38. Holotype (GBA 2018/002/0003/01). – 29–30. Apertural view (×3 and ×1). – 31. Details of the ornament from the penultimate whorl (×6). – 32–33. Apical view (×3 and ×1). – 34–35. Dorsal view (×3 and ×1). – 36. Details of the ornament from the last whorl (×6). – 37–38. Basal view (×3 and ×1). – 39–40. Paratype (GBA 2018/002/0003/02), an early shell of the species, having sparser nodules along the ramp than P. wiesberghausensis in the same growth stage; nodules below the selenizone of its latest whorl are just appearing, lateral view (×5 and ×1). – 41–46. Paratype (GBA 2018/002/0003/03), young specimen, with well-preserved growth threads; the sparse nodes on the ramp indicate its belonging also to P.nodulocincta though the abapical belt of the outer face is similar to that of P.wiesberghausensis. – 41–42. Lateral view (×5 and ×1). – 43–44. Basal view (×5 and ×1). – 45–46. Slightly oblique lateral view (×5 and ×1)