Laevitomaria problematica ( Szabó, 1980 ), HGM J

Gatto, Roberto, Monari, Stefano, Szabó, János & Conti, Maria Alessandra, 2015, The Jurassic pleurotomarioidean gastropod Laevitomaria and its palaeobiogeographical history, Acta Palaeontologica Polonica 60 (1), pp. 217-233 : 219-222

Laevitomaria problematica ( Szabó, 1980)

Figs. 2, 3.

1980 Pyrgotrochus ? problematicus sp. n.; Szabó 1980: 63, pl. 4: 1–3. 1986 Trochotomaria ? cf. problematica (Szabó) ; Conti and Monari

1986: 182, pl. 2: 1, 2, 5.

Material.— Holotype ( HGM J 10120 ), 15 paratypes ( HNHM INV.2012.15.1–2012.15.7, HNHM INV.2012.16.1–2012.16.6, HNHM INV.2012.18.1, HNHM INV.2012.19.1), and 6 specimens ( HNHM INV.2012.17.1–2012.17.6). Additional 5 unnumbered specimens are stored in the HGM. Bajocian

( Stephanoceras humphriesianum Zone to Parkinsonia parkinsoni Zone ), Somhegy, Bakony Mountains ( Hungary). Four specimens ( MPUR MAC127 A–D) from Bivio Macerino ( Martani Mountains , central Apennine, Italy), Early Bajocian ( Stephanoceras humphriesianum Zone ) .

Description.—The shell is conoidal and composed of about ten whorls. The apical spire is slightly cyrtoconoid. The protoconch is dome-like, with globose nucleus followed by a well-rounded volution. The early teleoconch whorls are slightly and evenly convex gradually changing in moderately convex during the growth. The last three whorls are obtusely angular at the selenizone. Their outer face is narrow and flat, becoming a slightly concave band during the latest growth stage. The suture is moderately impressed. The selenizone is concave at the beginning. On the earliest teleoconch whorls it is wide, flat and edged by sharp marginal spiral threads. On the subsequent whorls, the selenizone becomes rather convex. Its width is about 20% of the whorl surface. It runs clearly below the mid-whorl and becomes markedly convex concomitantly to the development of the angulation of last whorls. The periphery is markedly angulated on the early shell and becomes rounded-angular and slightly swollen on the fully adult shell. The base is phaneromphalous, somewhat flat and with a slightly convex surface.The umbilicus is rather wide with a rounded but somewhat sharp periumbilical edge.

The early shell is ornamented by sharp, equally sized and regularly spaced spiral threads and collabral riblets which give rise to an even network with small granules at crossing points. On the first preserved whorls, there are three to four spiral threads above the selenizone and two spiral threads below it. They increase in number during the growth. The reticulate ornament persists, though gradually attenuating, on the first five teleoconch whorls. On the last whorls the col-

→ Fig. 2. Pleurotomarioidean gastropod Laevitomaria problematica ( Szabó, 1980) from the Bajocian ( Stephanoceras humphriesianum Zone – Parkinsonia parkinsoni Zone ) of Somhegy (Bakony Mountains, Hungary). A. Holotype HGM J 10120 , apertural (A 1), basal (A 2), and dorsal (A 3) views, detail of the apical shell (A 4), and detail of the surface of the last whorl (A 5). B . Paratype HNHM INV.2012.15.3, dorsal view. C . Paratype HNHM INV.2012.15.1, detail of the apical shell (C 1), dorsal view (C 2), and detail of the ornament (C 3). D . Paratype HNHM INV. 2012.15. 2, dorsal (D 1) and basal (D 2) views, and detail of the surface of the last whorl (D 3) .

labral riblets become sharp growth lines. These cross weak and quite closely spaced spiral threads, making them slightly granulated. About ten spiral threads are present above the selenizone and about five below it. The spiral threads attenuate on the last whorl. They also flatten and widen so that their interspaces become striae. The base is covered by thin, sharp and dense spiral threads. The selenizone of the earliest whorl is ornamented only by sharp and regularly spaced lunulae. From the second to third whorl onward, a median spiral thread appears. Other spiral threads appear during adult growth while the lunulae gradually change in growth lines. The selenizone of the last whorls bears about five shallow and slightly variably sized spiral threads which tend to vanish during the last growth stage. The growth lines are prosocline and widely prosocyrt above the selenizone, almost orthocline and prosocyrt below the selenizone, opisthocline-falciform on the base i.e., markedly opisthocyrt on its abaxial region and slightly and widely prosocyrt on its adaxial region.

Remarks.— Laevitomaria problematica ( Szabó, 1980) is a rather variable species. The spiral angle ranges from about 50° to 60°. The fully adult whorls are moderately to decidedly convex. The ramp of the last whorls is slightly convex to flat or also weakly concave on the final part of the last whorl. In some specimens (e.g., in the holotype), the surface of the last whorls above the selenizone shows weak and regularly spaced spiral lines. In other specimens, the spiral ornament is sharper and denser, in some cases more marked along the subsutural band. The prominence of the selenizone is also variable. The base is always very low, but the convexity of its surface may vary.

Conti and Monari (1986) tentatively ascribed some specimens from the Early Bajocian of central Italy to L. problematica . Subsequently, Conti and Szabó (1987) suggested that they probably belong to a different species, although the poor preservation prevents a safe assignment. However, the re-examination of that material and a better documentation of the variability of L. problematica as presented here enable to identify these specimens as L. problematica . They differ from the type material of L. problematica only in having a less prominent selenizone and slightly lower whorls.

Stratigraphic and geographic range.—Bajocian ( Stephanoceras humphriesianum Zone to Parkinsonia parkinsoni Zone ), Somhegy, Bakony Mountains ( Hungary). Early Bajocian ( Stephanoceras humphriesianum Zone ), Martani Mountains (central Apennine, Italy).