Conorbis grotriani

Conorbis grotriani (Koenen, 1865)

Descrizione e caratteristiche:

Si differenzia dal Conorbis dormitor grazie all’ampia depressione presente sulla parte superiore delle spire e i solchi regolari presenti sull’ultimo giro.

L’altezza massima degli esemplare adulti è di 32 mm., per una larghezza di 15 mm.

L’originale assegnazione all’Oligocene del Conorbis grotriani è errata, quella corretta è l’Eocene Medio. Gli strati più antichi dell’area di Helmstedt sono del Luteziano: “On the basis of invertebrate fossils from the overlying marine succession the coal bearing succession of the Helmstedt district was originally regarded as Oligocene (Barth, 1892; Dorstewitz, 1902; Schmierer, 1913), but an Eocene age was also suggested by Linstow (1907) and Harbort (1909). The latter was confirmed by remains of Middle Eocene vertebrates at the base of the overlying marine succession (Schröder, 1913, 1918). Later, Quitzow (1948) was able to assign an Early Eocene age to the lower group of seams, now Schöningen Formation, based on lithologic arguments and scarce diatoms from glauconitic sediments of what is now called Emmerstedt Formation. Pflug (1952) distinguished different associations of spores and pollen (“Pollenbilder”) from the lower and the upper group of seams (“HU-Bild” respectively “HO-Bild”). Since the “HO-Bild” was closely comparable with microfloras from the Middle Eocene of the Geiseltal he regarded the upper group of seams (Helmstedt Formation) as Middle Eocene and the lower group of seams as Lower Eocene. This has generally been confirmed by later studies (Pflug, 1986; Lenz, 2005)” ⁽⁵⁾.