The endemic and morphologically remarkable nautilid genus Deltocymatoceras Kummel, 1956 from the Late Cretaceous of Central Europe
Abstrakt
The endemic Late Cretaceous nautilid genus Deltocymatoceras Kummel, 1956 is represented by Deltocymatoceras leiotropis (Schlüter, 1876) and D. rugatum (Fritsch&Schlönbach, 1872). Characteristic for this genus is a bulbous involute shell with strong radial ribbing and a ventral keel, which appears in post-juvenile ontogenetic stages.
The combination of these morphological features is unique among post-Palaeozoic nautiloids. Both species are restricted to the Late Turonian–Middle Coniacian of Central Europe and England.
Until now, the type species D. leiotropis was known based only on the holotype specimen. This specimen is re-described with revised occurrence and for the first time photographically illustrated.
The only preserved syntype from the type specimens of D. rugatum was re-discovered during this revision and more than thirty other specimens were studied. Representatives of Deltocymatoceras show clear morphological changes during their ontogeny and acquisition of maturity, most notably the appearance of a ventral keel in premature stages and its disappearance at maturity.
The hypothesis of a close phylogenetic affinity of Deltocymatoceras to the genus Cymatoceras Hyatt, 1884 is based on similar morphological features such as the suture, ribbing, shell shape and early ontogenetic development. The ventral keel is quite an exceptional feature of morphology in post-Palaeozoic nautiloids.
In particular the combination with an involute depressed shell is only shared with representatives of the genus Angulithes Montfort, 1808 (Cretaceous–Palaeogene) and Gryponautilus Mojsisovic, 1902 (Triassic). The combination with a compressed and evolute planispiral shell is more common in Palaeozoic and some post-Palaeozoic nautiloids.
The ventral keel is discussed here in relation to its functional aspects (hydrodynamic stability) in combination with the strong radial ribbing (protection of the shell against mechanical damage).