Pleurotomaria nodulocostulata, 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 : 10-13

Pleurotomaria nodulocostulata n. sp.

( Figs 1–13 )

Type specimens – Holotype: GBA 2018/002/0001/01 ; paratypes: GBA 2018/ 002/0001/02–03 .

Type locality – Hierlatz Alpe (Hallstatt, Austria) .

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

Derivation of name – nodulus (Latin) = small node, costula (Latin) = small rib; name refers to the ornament of the ramp where no true nodes, characteristic of the genus, can be found.

Diagnosis – Shell of few (6–7) gradate whorls with blunt apex. Angulation and ramp present from second whorl. Angulation sharp on early teleoconch but rounded on penultimate and last whorls. Ramp slightly sloping abapically. Outer face flat or somewhat convex, steeply sloping abapically. Rather wide selenizone at midwhorl. Outer face joined to base along even, wide arch, continued to rim of moderately wide umbilicus. Aperture suborbicular, columellar lip slightly prosocline, strong and smooth. Parietal lip appearing as edge of shell inductura, smoothing coarse ornament of base. Whorls and base spirally ornamented, ramp bearing collabral riblets with granules at crossings with spiral elements. Delicate, dense collabral threads on whole surface of latest whorls and base.

Material – Three fragmentary specimens (GBA 2018/002/0001/01–03); two of them are in about full grown stage, one is an early teleoconch.

Measurements – Holotype (GBA 2018/002/0001/01):width 17.7 mm, height 18.2 mm, apical angle 83°, pleural angle 77°; paratype (GBA 2018/002/0001/02): height 17.5 mm; paratype (GBA 2018/002/0001/03): height 7.5 mm.

Description – This species has a shell of rather few whorls, forming a low spire that is about as high as the last whorl. The protoconch and the initial whorls form a blunt apex. Narrow, obscure ramp has developed already on the second whorl then its limiting angulation is also present from the third whorl. The angulation is sharp initially but becoming rounded from the penultimate whorl. The ramp is narrow relative to the outer face; the former belt just slightly the latter area steeply slopes abaxially and abapically. Both belts are flat initially but becoming feebly convex from the penultimate whorl. Position of the selenizone is nearly at the midwhorl, quite near to the outer rim of the ramp. Width of the selenizone reaches 16–17% of the distance between the nearest sutures. On the last whorl, the outer face is connected to the base along a wide and even arch that turns into a moderately broad umbilicus finally. The aperture is subovate with a broad angle at the meeting of the parietal and umbilical lip. Partially, the peristome is preserved in the holotype: its umbilical lip is straight in prosocline orientation and the parietal lip is represented by the slightly prosocyrt edge of a smooth callosity that evens the rough basal ornament.

The teleoconch is predominantly ornamented by spiral threads and cords; marked transverse ornament appears only on the ramp in form of short, collabral riblets, extending from the suture to the angulation. They bear also granules or nodules at the intersections with the spiral threads. The granules are initially restricted to the thread running on the abaxial rim of the ramp, but from the penultimate whorl, a new spiral thread appears also adaxially from the former one and it becomes also granulate. Weak collabral riblets are present also on the outer face of the earliest teleoconch whorls and form subregular networks with the spiral threads without granules. On this shell part, two of the spiral threads limit the selenizone that is ornamented by similarly strong lunulae. Subsequently the two limiting threads do not strengthen during the growth but a submedian thread also develops and it becomes a cord for the latest whorls as the earlier threads farther along the selenizone also do it. On the base between the indistinct periphery and the umbilicus, several variably wide spiral ribbons also developed. Very fine, nearly periodically repeated, dense growth threads/riblets connect the spiral ornamental elements on the latest whorls and the base; they are present also in the interspaces of the stronger riblets of the ramp. The lunulae of the early selenizone fade away during the growth and only fine growth-lines cross the median thread.

Remarks – The most similar species to P. nodulocostulata n. sp. is P. wiesberghausensis n. sp. The latter species has higher shell (more whorls), steeper ramp, the angulation of the whorls does not change into rounded form on the latest whorls and its nodes reach considerable size. On the outer face, fewer spiral threads/ cords develop that become weakly nodular on the latest whorls; the outer face has a concave belt between the angulation and the abapical edge of the selenizone on the latest teleoconch whorls (for more details see below). Appearance of a spiral thread on the ramp adaxially from the abaxial angulation is also a specific feature of P. nodulocostulata .

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

Figures

Figs 1–13. Pleurotomaria nodulocostulata n. sp. – 1–8. Holotype (GBA 2018/002/0001/01).– 1–2. Apertural view (×2.3 and ×1). – 3–4. Apical view (×2.3 and ×1). – 5–6. Basal view (×2.3 and ×1). – 7. The best preserved shelly side (×2.3). – 8. Details of the ornament (×5.5). – 9–12. Paratype (GBA 2018/002/0001/02), having a thin coating of granulose calcite. – 9–10. Apical view (×2.3 and ×1). – 11–12. Lateral view (×2.3 and ×1). – 13. Paratype (GBA 2018/002/0001/03), an early shell in a slightly oblique view, with the onset of the specific spiral thread, developing on the ramp along the angulation of the whorl surface (×5.5)