Pleurotomaria indistincta, 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 : 21-24

Pleurotomaria indistincta n. sp.

( Figs 61–73 )

Type specimens – Holotype: GBA 2018/002/0005/01; paratypes: GBA 2018/ 002/0005/02–03.

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 – Indistincta (Latin) not clear, obscure.

Diagnosis – Conoidal shell with slightly convex whorls, suture impressed. Ramp narrow and indistinctly developed, outer face feebly convex or flat with shallow concave belt between angulation and selenizone, running at midwhorl. Transition to base rounded; suture running barely below periphery. Base slightly convex with flattish convex wall and moderately broad umbilicus. Umbilical lip obliquely bounding basal and parietal lip, developed as rim of parietal callosity. Early teleoconch bearing network ornament then ramp and peripheral area ornamented with collabral ribs and thin threads, crossed by sparse spiral threads. Ribs of peripheral region abaxially terminating in outermost belt of base, ornamented also with unequally spaced spiral threads, forming network with collabral threads.

Material – Three fragmentary specimens (GBA 2018/002/0005/01–03); one is a shell of a probably full grown animal, the others lack the latest whorls, they represent only the early teleoconch.

Measurements – Holotype (GBA 2018/002/0005/01): height 22.4 mm, early spiral angle 56°; paratype (GBA 2018/002/0005/02): height 10.2 mm, pleural angle 57°; paratype (GBA 2018/002/0005/03): diameter 10.5 mm, pleural angle 60°.

Description – By a reconstruction, the full grown shell has consisted of eight slightly convex or flattened whorls in undamaged state; a moderately impressed suture separates them. From the fourth whorl, a narrow and steep but indistinct ramp is observable. Actually it is so poorly developed also on the subsequent whorls that does not cause clearly gradate shell outline, which is a marked character of a typical Pleurotomaria shell. For the latest whorls of the holotype, only the ornament indicates the place of the ramp. The outer face of the whorls slopes just slightly more steeply than the ramp. Its surface is composed of a shallow concave adapical and a low convex abapical belt. The selenizone is placed to the midwhorl that coincides with the abapical half of the concave zone. The early selenizone is concave but alters into convex from the fourth whorl; its width is about 18% of the suture to suture distance. Position of the periphery coincides to one of the spiral threads near the suture in the convex band of the outer face. Transition of the outer face to the base is rounded angular; the suture is placed somewhat below the periphery of the shells. The base is slightly convex as a whole with flattish convex wall and a moderately broad umbilicus. The umbilical lip obliquely connects the basal and the parietal lip, which has developed as rim of a parietal callosity, smoothing the basal ornament of the former whorl. The outer lip is not preserved in the specimens in hand.

The early ornament of the shell is a network; few spiral threads are crossed by similarly dense and strong collabral threads above and below the selenizone, which is ornamented here by two limiting threads and lunulae, arranged in a density similar to those of the collabral threads. Together with the development of the angulation on the fifth whorl, the sculpture starts to change. On the ramp and in the abapical belt of the outer face, some collabral threads strengthen as ribs, the others in the interspaces of the formers become much thinner; all they are regularly repeated. The ribs and threads of the ramp are orthocline or faintly prosocline and straight. The ribs extend and gradually strengthen from the suture to the limiting angulation where they suddenly terminate. The ribs on the outer face are prosocyrt and feebly prosocline. Crossings of the ribs with the spiral threads result in nodule like undulations. Together with the development of the ramp and the ribs, a submedian thread appears on the selenizone that becomes a marked, smooth cord during the growth; at the same time, the lunulae become thinner, denser and subregularly repeated. The sculpture of the base consists of the terminal parts of the ribs of the lower part of the outer face and a network of threads. The spiral threads are distributed with unequal interspaces, sparser threads occur along the rim of the base and around the umbilicus. Subregularly repeated, thinner collabral threads cross the spiral ones. The growth lines are orthocline on the ramp and prosocline between the ramp edge and the selenizone; these two parts meet almost angularly at the thread on the ridge of the angulation, together they form a prosocyrt-prosocline belt. The growth lines are also prosocyrt-prosocline between the selenizone and the base, and parasigmoidal/ sickle shaped as it is usual in pleurotomariids on the base.

Remarks – The available specimens show considerable variability in the shape and also in the ornament. The ramp is less distinct in the early whorls of the holotype than in the two paratypes, which have not reached the full grown stage. However, the paratypes are also different in this respect since one shows quite clearly gradate shell outline but the other does not. There are differences also in the profile of the outer face; it is flattened with concave belt in the paratypes but feebly convex with concave belt in the corresponding shell parts of the holotype. All specimens differ from each other slightly in the strength, the number and the position of the ornamental elements (see Figs 61–73 ).

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

Figures

Figs 61–73. Pleurotomaria indistincta n. sp. – 61–65. Paratype (GBA 2018/002/0005/02), bearing poorly developed angulation and ramp on the whorl surface. – 61–62. Apertural view (×4 and ×1). – 63. Dorsal view (×4). – 64. Basal view (×4). – 65. Details of the earliest whorls (×7). – 66–68. Paratype (GBA 2018/002/0005/03) with rather clearly visible angulation, outer face and ramp. – 66–67. Lateral view (×4 and ×1). – 68. Apertural view (×4). – 69–73. Holotype (GBA 2018/002/0005/01), the ramp and angulation have vanished from this specimen for the latest whorls, only the row of weak nodules indicates their former position. – 69–71. Lateral views (×3, ×3 and ×1). – 72–73. Basal view (×2.7 and ×1)