Eutrephoceras aff. sublaevigatum (D’ ORBIGNY , 1850)

Frank, Jiří, Sklenář, Jan & Ekrt, Boris, 2014, Late Cretaceous Nautilid Juveniles Of Cymatoceras Reussi And Eutrephoceras Aff. Sublaevigatum - Scarce Fossils Under Risk Of Pyrite Degradation, Acta Musei Nationalis Pragae Series B 70 (3 - 4), pp. 143-152 : 145-146

Eutrephoceras aff. sublaevigatum (D’ ORBIGNY, 1850)

Pl. 1, fig. 2; Pl. 2, figs 1–8

1840 Nautilus laevigatus D’ ORBIGNY , 1, s. 84, tab. 17. non 1845 Nautilus inaequalis SOWERBY ; Reuss, p. 21, pl. 7,

fig. 12. 1850 Nautilus sublaevigatus D’ ORB. ; d’Orbigny p. 189. 1872 Nautilus reussi FR. IN FRITSCH ET SCHLÖNBACH ,

p. 25, pl. 12, fig. 5a-c. 1893 Nautilus reussi FR. ; Frič, p. 73, fig. 47. 1895 Nautilus reussi FR. ; Frič, p. 71, fig. 47. 1934 Nautilus reussi FRIČ ; Andert, p. 392. 1956 Nautilus reussi FRITSCH ET SCHLÖNBACH ;

Kummel, p. 344. 2007a Eutrephoceras sp. ; Frank, p. 112. 2007b Eutrephoceras sp. ; Frank, p. 35.

D e s c r i p t i o n. Specimens have nautiliconic, involute ovoidal shell with depressed broadly rounded whorl cross section on ventral and lateral sides. The suture line is very simple with a relatively shallow lateral lobe and shallow/ almost straight ventral lobe. The umbilicus is broad, not completely filled with shell material, which could result from the method of preservation. The siphuncle is in a central position. Where the shell is partly preserved early juvenile reticular patterns (Pl. 2, figs. 2b, 4a, 5b-c,) or prominent growth lines (Pl. 2, fig. 8c) are noticeable.

D i s c u s s i o n. Other determined specimens of Frič’s “ Nautilus Reussi ” are most probably juveniles of Eutrephoceras sublaevigatum (D’ ORBIGNY, 1840) . Those specimens are similar in simple suture line, shape of the shell, whorl cross section as well as siphuncle position. There are no specimens of later ontogenetic stages or complete juvenile specimens with preserved living chamber and thus features firmly indicating ranking within E. sublaevigatum are absent. Therefore these specimens are left here in open nomenclature as Eutrephoceras aff. sublaevigatum .

Frič (in Fritsch and Schlönbach, 1872) synonymized E. aff. sublaevigatum juvenile specimens with Nautilus inaequalis ( SOWERBY, 1812) which was described in detail by Reuss in 1845, but according to the description and figures, the latter species differs from E. aff. sublaevigatum in its almost flattened ventral side and centro-dorsal position of the siphuncle.

Prominent growth lines on the juveniles are very similar to Eutrephoceras expansum (SOWERBY, 1824) from the Cenomanian, but this species differs by its sinuous suture line with prominent lateral lobe.

There are fragments of shell preserved on the right lateral side of specimen NM-O7134 (Pl. 2, fig. 1b). This specimen was figured by Frič (1893, 1895) with a reconstructed living chamber. The shell is affected by diagenesis but still includes a considerable percentage of aragonite constituting the original shell. When examined by means of SEM, typical nacreous ultrastructure, consisting of flat hexagonal crystals arranged in columns, is still distinguishable (Pl. 3, fig. 2). This ultrastructure is comparable with the fabric of the nacreous layer of a modern nautiloid shell (Pl. 3, fig. 1).

O c c u r r e n c e: Czech Republic, Bohemian Cretaceous Basin – Březno (Louny district, Lower Coniacian), Lenešice (Louny district, Late Turonian), Třebívlice-Měrunice area (Lovosice district, redeposited in Pleistocene sediment).