Syringonautilus sp.

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 : 8-9

Syringonautilus sp. Figure 5 a–c .

Material Specimen PIMUZ 37903, partially preserved as internal mold (about the last half-whorl, fractured and slightly deformed), partially as external mold from which a silicon cast was produced.

Measurements At maximum dm (deformed) of 44.5 mm other dimensions are: wh = 23.5 mm, uw = 10 mm, ww= 25 mm.

Locality and age Point 902/ Mirigioli (Meride, Canton Ticino). Middle Besano Formation, Illyrian, Secedensis Zone (bed 104, light grey dolomite).

Description Te specimen consists of about the last whorl, lacking its inner whorls. It shows a rapidly expanding smooth conch, with whorls deeply embracing. Te whorl section is more or less elliptical. Te anterior whorl section is higher than wide, well rounded and with the maximum width at the umbilical shoulders. Te umbilical shoulder passes to the inclined umbilical wall forming a continuous curve. Te flanks are convex, but slightly flattened, converging externally. In the last half-whorl, the ventrolateral shoulder becomes angularly rounded and corresponds to the maximum width of whorl. Te venter is convex and rounded, slightly flattened. Te other characters are not observable.

Discussion Tis poorly preserved specimen shows similarities with species of Syringonautilus (e.g., Mojsisovics, 1882; Pieroni, 2020) from the Middle Triassic of the Alps. Following Kummel (1953, p. 60), Syringonautilus can be distinguished from Syringoceras only by the position of its siphuncle: while in Syringonautilus it is subcentral, in Syringoceras it is nearly marginal. However, this character is still unknown in many species, and it is not visible in the present specimen. Te whorl section of Syringoceras is usually slightly compressed ( Kummel, 1953, p. 61) and more expanded in Syringonautilus as in the present specimen. Among the species belonging to Syringonautilus documented in literature, Syringonautilus bosnensis ( Hauer, 1892, p. 6 [254], pl. 1, Fig. 3a–c ; Trinodosus Zone of Han Bulog, Bosnia) shares the shape of whorl section and the umbilical width with the specimen here described. Te whorls of Syringonautilus lilianus ( Mojsisovics, 1882) are less rapidly expanding and the conch has a wider umbilicus and circular to subquadrate whorl sections. Syringonautilus cf. lilianus ( Mojsisovics, 1882) from Rasa di Varese (Varese, Lombardy, Rasa Dolomite, San Salvatore Dolomite, Reitzi Zone sup.) was described by Pieroni (2011, p. 57, pl. 2 cefalopodi, Fig. 3 ). It shows a more subquadrate whorl section and is more evolute.

Figures

Fig. 5 Syringonautilus sp., PIMUZ 37903, P. 902/Mirigioli (Monte San Giorgio), Middle Besano Formation (bed 104), uppermost Anisian: a lateral view, left side, b oral view, c ventral view

Fig. 3 Germanonautilus aff. ellipticus Parnes, 1986, PIMUZ 29942,Valle Stelle/Tre Fontane (Monte San Giorgio), Middle Besano Formation (bed 73), Secedensis Zone, Illyrian: a original specimen partially included in the matrix, lateral view, b ventral view,associated with an external mold of Stoppaniceras cf. artinii, c plaster cast replica of the same specimen, lateral view, d plaster cast replica of the same specimen,ventral view, e plaster cast replica of the same specimen, ventrolateral view, f drawing of the ventral view showing the hypothetical whorl section (in grey the flank areas)