Nautilida

Pieroni, Vittorio, 2022, Middle Triassic Nautilida from the Besano Formation of Monte San Giorgio, Switzerland, Swiss Journal of Palaeontology (21) 141 (1), pp. 1-12 : 9-10

Nautilida gen. et sp. indet. Figure 6 a–d .

Material Specimen PIMUZ 38668, internal and external mold of a poorly preserved phragmocone (about half a whorl, fractured and deformed), with some recrystallized shell remains.

Measurements Deformed preserved portion with maximum dm of 105 mm, estimate dm (at this position) = 125, estimate wh = 29 mm aperturally, estimate wh = 23 mm apically, ww= 58 mm aperturally, ww = 51 mm apically.

Locality and age Point 902/ Mirigioli (Meride, Canton Ticino). Middle Besano Formation, Illyrian, Secedensis Zone (bed 98, laminated bituminous dolostone).

Description Te specimen consists of about half-whorl, lacking inner whorls and umbilical wall not visible (included in the matrix?). It has rather rapidly expanding whorls, with strongly depressed whorl sections (estimate ww/wh = 2). Some blunt ribs are visible on the flank. Te suture line shows a shallow rounded lobe on the flank, while it forms a shallow arched ventral saddle.

Discussion Tis specimen represents an additional taxon, although it is too poorly preserved for identification even on family level. Among the Middle Triassic Nautilida from the Tethys, strongly depressed whorl section with blunt lateral ribs and rapidly expanding whorls occur in some species of Mojsvaroceras Hyatt, 1883 (see M. gianii Pieroni & Prinoth, 2021 ). Te ventral saddle is a character known from the family Syringonautilidae . Teir conchs are usually without ribs (in this family, only Weitschatinautilus Engeser, 1995 bears ribs in the mature stage), and in the genus Holconautilus Mojsisovics, 1902 ( Tainoceratidae ), the whorls are usually expanding more slowly.

Figures

Fig. 6 Nautilida gen. et sp. indet., PIMUZ 38668,P. 902/ Mirigioli (Monte San Giorgio), Middle Besano Formation (bed 98), uppermost Anisian: a lateral view, left side,b lateral view,right side, c ventral view, d aboral portion of ventral view showing suture line (ventral saddle). In figs. a and b the umbilical wall is not visible because is embedded in the matrix (here not showed),so it is difficult to judge where exactly the dorsum ends.The black point beside a, b, c, marks where was observed the suture line showed in d