Neorotomaria subgradata, 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 : 38-41

Neorotomaria subgradata n. sp.

( Figs 120–129 )

Type specimens – Holotype: GBA 2018/002/0014/01 ; paratype: GBA 2018/ 002/0014/02.

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) interval .

Derivation of name – Referring to the obscurely gradate latest whorls.

Diagnosis – Widely conoidal shell with arched convex early teleoconch and obscurely gradate latest whorls; suture slightly impressed. Convex, wide selenizone bearing midline cord, placed somewhat below midwhorl. Narrow, very steep and feebly concave ramp developing for last three whorls. Outer face wide, just little steeper than ramp; two concave belts along selenizone. Periphery running at edge of base, being rounded angular. Base and its wall flattened, widely phaneromphalous. Network ornament on early whorls with sparse and coarse lunulae on midline cord of selenizone. Subsutural riblets becoming nodules in early growth phase, suprasutural nodules develop from penultimate whorl.

Material – Two specimens; a less incomplete one (GBA 2018/002/0014/01) that has remains of the early teleoconch and also the latest whorls (holotype); the other one (GBA 2018/002/0014/02) represents only early shell parts. Further specimens may be in the GBA 2008/69/43/2/1 inventory unit.

Measurements – Holotype (GBA 2018/002/0014/01) height of remnants: 22.7 mm, maximum diameter of shelly parts 25.5 mm; paratype (GBA 2018/ 002/0014/02) width: 9.4 mm.

Description – The specimen in hand is a fragmentary, wide conoidal shell that has originally consisted of about 10 ± 1–2 whorls, which are slightly convex in the early and obscurely gradate in the succeeding growth stages. The latter have narrow, gently sloping, feebly concave ramp and a much steeper outer face, composed of two concave belts that are separated by the convex selenizone. The selenizone is placed below the midwhorl in a position that roughly coincides with the midline of the outer face on the gradate whorls. The shell remnants are indicative of a low, flat or feebly concave and broadly phaneromphalous base, meeting the last whorl surface through a rounded angulation that corresponds also to the periphery of the shell.

The first preserved whorl, which is about the fourth one from the protoconch, has only a small fragment of shell, indicating network ornament just above the suture from equally strong spiral and collabral threads. This network is present also on the next whorl between the suture and the selenizone, where three spiral threads give the frame, one just above the suture, another abapically limits the selenizone and one between the formers; regularly repeated, equally strong, short threads/riblets cross them. A spiral thread edges the selenizone also on the adapical side. There is also a spiral thread in the midline of the selenizone that is stronger than the limiting ones; marked lunulae cross it. A distinct spiral thread is about halfway between the selenizone and the adapical suture, and a thin line above the former one. Short, dense riblets of two, alternating thickness start from the suture and strengthen toward the marked spiral thread, where the stronger riblets form nodules at the intersections then all riblets weaken but all reach the edge of the selenizone. On the next whorls, the strong spiral thread adapically from the selenizone and the two threads at and above the lower suture become gradually stronger. Nodules appear also on the latter two threads; all nodose riblets become gradually marked ribs and nodes. The adapical and abapical rows of nodes are not aligned but those along the lower suture and in the outermost belt of the base are arranged along low, collabral elevations/ribs. For the last whorl, the spiral threads in the neighbourhood of the periphery become ribbons that widen at all nodes.

Remarks – See distinction from N. obertraunensis above.

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

Figures

Figs 116–130. Neorotomaria obertraunensis n. sp. and Neorotomaria subgradata n. sp. – 116–119. Neorotomaria obertraunensis n. sp., holotype (GBA 2008/69/43/2/2). – 116–117. Lateral view (×1 and ×3). – 118. Apertural view (×1). – 119. Basal view (×1). – 120–128. Neorotomaria subgradata n. sp. – 120–124. Holotype (GBA 2018/002/0014/01). – 120–121. Lateral view to show the outline of the shell (×2 and ×1). – 122. Apical view (×2). – 123. Lateral view with the largest surface of preserved ornament (×2). – 124.Basal view to document the presenceof a broad umbilicus (×1). – 125–128. Paratype (GBA 2018/002/0014/02) representing early shell parts of the species. – 125–126. Lateral view (×4 and ×1). – 127–128. Apical view (×4 and ×1). 129–130. Comparative images of two Neorotomaria species to follow the similarities and divergences in the development of the shells. – 129. Neorotomaria subgradata n. sp. (×3). – 130. Neorotomaria obertraunensis (×3)