Pleurotomaria seminodosa, 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 : 19-21

Pleurotomaria seminodosa n. sp.

( Figs 47–60 )

Type specimens – Holotype: GBA 2018/002/0004/01 ; paratypes: GBA 2018/ 002/0004/02–03 .

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 – Semi (Latin) = half, nodosa (Latin) = having nodes; referring to the single row of nodes on the whorls, instead of the two rows, common in Pleurotomaria .

Diagnosis – Conoidal shell of about seven whorls. Whorls low, latest four with increasingly distinct but narrow ramp, slightly sloping abaxially. Much wider outer face, sloping also abaxially but steeply. Wide, convex selenizone in middle of outer face between two concave belts, abapical one narrower; lowermost swollen belt providing periphery. Base flattened as whole with slightly convex wall, and moderately broad umbilicus with wide, funnel like opening. Early whorls bearing nodules along both sutures, latest whorls nodosed only on ramp. Selenizone flat and having sparse lunulae on early teleoconch whorls but becoming markedly convex without lunulae prior to penultimate whorl. Few spiral threads on whorls, more but thinner ones on base. Growth lines delicate on whorls but thin thread-like on base.

Material – Three fragmentary specimens are available (GBA 2018/002/ 0004/01–03); one (paratype) is represented by remnants of the penultimate and the last whorl from the “background collection”; another one (holotype), which is a more complete shell, and an inner mould with small shell pieces have been found during the revisit of the type locality area in 1999.

Measurements – Holotype (GBA 2018/002/0004/01): diameter 13.2 mm; paratype (GBA 2018/002/0004/02): height of the fragment 10.5 mm; paratype (GBA 2018/002/0004/03): diameter of inner mould 14.8 mm.

Description – The shell has a conoidal, slightly gradate outline and consists of about seven whorls, five of which have remained more or less preserved. These whorls are low; the last one is more than four times wider than high. Four of the preserved whorls have narrow, spirally undulating ramp, which is indistinct initially but becomes marked during the growth. Its abaxial edge is a rather sharp angulation with the outer face that is about three times wider than the ramp. The outer face steeply but the ramp just slightly slope abaxially. At the middle of the outer face, wide selenizone runs between two concave belts; the abapical one is narrower than the other one. The selenizone itself is flat initially but markedly convex subsequently; its width is about 20% of the suture to suture distance. The lowermost belt of the outer face is swollen; this belt provides the periphery of the shell and also on the whorls just above the slightly impressed suture. The base as a whole is flattened and has a slightly convex wall in about the abaxial two thirds of the diameter, and a moderately broad umbilicus with wide, funnel like opening peri-axially.

The poorly preserved early whorls bear periodically repeated nodules along both sutures but the abapical row of nodules vanishes for the latest whorls. The nodes of the ramp gradually strengthen along the full length of the whorls; these nodules extend from the suture to the angulation, there usually suddenly terminate. They are gradually thicker toward the angulation so they cause a characteristic undulation of the ramp and sometimes also the adapical concave zone. The nodules are connected by a spiral thread/cord that is also undulating. Within the concave outer face zones, no ornament appears on the latest whorls. However, short collabral threads are in this belt that are connected to the nodules, which seat on the suprasutural elevation of the early teleoconch whorls that correspond to the subsequently swollen belt. These short threads terminate at the selenizone, which is flat on the early shell with two weak, limiting threads, and bear sparse lunulae. The selenizone gradually changes into convex and the lunulae fade away prior to the penultimate whorl; the limiting threads occasionally also disappear. On the last whorl, two spiral threads are grown also on the periphery; the adapical one is exposed also on the spire whorls but the lower one is just overlapped by the suture there. Differently strong, sparsely spaced spiral threads are on all visible parts of the base that are usually much thinner than the threads of the whorls. The growth lines are delicate on the whorls but thin, thread-like on the base.

Remarks – Pleurotomaria seminodosa n. sp. is well distinguishable from the other species, discussed in this paper on its single row of characteristic nodes, lowest whorls and its very simple and poor accessorial ornament. The ratio of the width to the suture to suture distance is 4.3 in P. seminodosa ; only P.nongradata n. sp. approaches this high ratio (~4) but it has two rows of differently shaped nodes.

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

Figures

Figs 47–60. Pleurotomaria seminodosa n. sp. – 47–53. Holotype (GBA 2018/002/0004/01). – 47– 48. Lateral view (×3 and ×1). – 49. Development of the ornament in the best preserved section (×9.5). – 50–51. Lateral view (×3 and ×1). – 52–53. Apical view (×3 and ×1). – 54–59. Paratype (GBA 2018/002/0004/02), the ornament of the base is preserved only on this specimen. – 54. Lateral view (×1). – 55. Lateral view (×1). – 56. Basal view (×1). – 57. Lateral view (×6). – 58. Basal view (×8). – 59. Lateral view (×3). – 60. Paratype (GBA 2018/002/0004/03), an inner mould with shell particles, the undulation of the periphery does not appear on the outside of the shell (×3)