Germanonautilus aff. ellipticus Parnes, 1986

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 : 6-8

Germanonautilus aff. ellipticus Parnes, 1986 Fig. 3

a–f.

Material Specimen PIMUZ 29942, a poorly preserved natural mold of the living chamber, filled by carbonate sediment; with slightly deformed coiling, partially included in the matrix. Te original shell is dissolved, with its morphology preserved on the ventral area.

Measurements At maximum dm (deformed) of 95 mm other dimensions are: wh = 40 mm, uw = 33 mm, ww= 75 mm; at dm of 70 mm other dimensions are: wh = 34 mm, uw = 24 mm, ww = 69 mm.

Locality and age Valle Stelle (Meride, Canton Ticino). Middle Besano Formation, Illyrian, Secedensis Zone (recovered from a laminated bituminous dolostone in a gallery of the former mine; correlating to bed 73). In association with an imprint of an ammonoid ( Fig. 4 ) classified here as Stoppaniceras cf. artinii ( Airaghi, 1912a, 1912b).

Description Evolute shell (uw/dm = 0.34), consisting of about the last whorl. Te umbilicus is broad, without any preserved structure. Siphuncle and suture line are not visible. Te whorl section, wider than high, is subelliptical, with the maximum width at the umbilical shoulders. Te ventral area is convex, widely arched, with more or less rounded distinct ventrolateral shoulders. Te flanks are rounded and convex. Te umbilical shoulder passes to the inclined umbilical wall forming a continuous curve. Te growth lines are observable only on the ventral area and form a wide and deep hyponomic sinus.

Discussion Te genus Germanonautilus is usually characterized by a narrow umbilicus and trapezoidal whorl sections, with more or less flattened or depressed venter. Te specimen described here shows similarities in particular with Germanonautilus ellipticus Parnes, 1986 . Tis species was described by Parnes (1986) from the Middle Triassic of Israel (Negev, Saharonim Formation, Lower Ladinian). It has a relatively wide umbilicus, a well-rounded elliptical whorl section, which is wider than high, and a regularly arched convex venter. Te present specimen is more evolute (uw/dm= 0.34 at dm = 70 mm) than Germanonautilus ellipticus (uw/dm = 0.24 at dm = 80 mm, Parnes 1986 showing a wider umbilicus and a last whorl only slightly embracing the former volution. Up to now, forms similar to G. ellipticus were not documented in Southern Alps. Germanonautilus brooksi Smith, 1927 from Alaska (Carnian) described by Kummel (1953), shows a well-rounded elliptical whorl section and a wide umbilicus. Following Kummel (1953, p. 29). the specimens described originally by Smith are small conchs probably corresponding to immature growth stages, with deeply embracing whorls and more distinct umbilical shoulders.

Family Syringonautilidae Mojsisovics, 1902

Genus Syringonautilus Mojsisovics, 1902

Type species. Nautilus lilianus Mojsisovics, 1882

Figures

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)

Fig. 4 Stoppaniceras cf. artinii (Airaghi, 1912a, 1912b) associated with PIMUZ 29942. Image obtained producing positive silicon cast from negative external mold