Xenoglyphioceras, Korn, 2025

Genus Xenoglyphioceras gen. nov.

urn:lsid:zoobank.org:act:134ED820-9F3E-4524-83DC-E6D6154E7845

Type species

Xenoglyphioceras eidos gen. et sp. nov.

Diagnosis

Genus of the family Ferganoceratidae with a discoidal conch. Shell ornament with dense spiral lines. Adult stage with constrictions that bend forward in the ventrolateral area to continue in a single longitudinal groove.

Etymology

Combination of the Greek ‘ ξεΝΟΣ ’ = ‘strange’ and Glyphioceras Hyatt, 1884 (neuter).

Remarks

The new genus Xenoglyphioceras gen. nov. occupies a morphological position between the genera Beyrichoceras Foord, 1903 and Ferganoceras Librovitch, 1957 , meaning that it combines a thickly discoidal conch with a shell ornamentation composed of delicate spiral lines and a shape of the venter that is bordered by deep longitudinal grooves. At first glance, this may seem surprising, particularly as these two genera have distinctly different stratigraphic positions. Beyrichoceras is known from an interval ranging from the Entogonites nasutus Zone to the Goniatites crenistria Zone in the Rhenish Mountains, and from the B1 Zone to the P1a Zone in northern England. In Lancashire and Derbyshire, Beyrichoceras is particularly species-rich in the B2 Zone, where it is found alongside Goniatites hudsoni ( Bisat 1934; Korn & Tilsley 2006).

In contrast, Ferganoceras is known from considerably younger strata ( Ruzhencev & Bogoslovskaya 1971), specifically from the Viséan-Serpukhovian boundary interval ( Hypergoniatites – Ferganoceras genus zone). The genus, first described based on material from Fergana ( Uzbekistan) by Librovitch (1957), is one of the most important components of the ammonoid assemblages in the southern Urals ( Ruzhencev & Bogoslovskaya 1971). The genus Ruddelites Malinky & Mapes, 1982 , described based on material from Arkansas ( Malinky & Mapes 1982), is likely a junior synonym of Ferganoceras . To date, Ferganoceras is known from the Anti-Atlas by only a single species, F. torridum Korn & Ebbighausen, 2006 , represented by just one specimen ( Klug et al. 2006).

The new genus is characterised by the peculiarity that in its middle ontogenetic stage, it resembles typical representatives of Beyrichoceras , but in its adult stage, it develops a morphology more similar to Ferganoceras . This can be seen as a case of hypermorphosis in Xenoglyphioceras gen. nov. The ventrolateral grooves typical of Ferganoceras , which appear at a relatively small conch diameter of about 8 mm ( Ruzhencev & Bogoslovskaya 1971), do not develop in Xenoglyphioceras until the end of ontogeny, when the conch diameter reaches about 50–60 mm.

This raises the question of whether some of the species described from the British Isles or Central Europe and attributed to Beyrichoceras might actually belong to Xenoglyphioceras gen. nov. However, most of these species have been described from relatively small specimens, usually with diameters of less than 50 mm ( Bisat 1934; Korn & Tilsley 2006). It is worth noting that larger specimens of Beyrichoceras from these regions have been rarely reported. Beyrichoceras mempeli Schmidt, 1941 , with a conch diameter of 60 mm, is an exception ( Schmidt 1941; Korn 2006). Unlike Xenoglyphioceras , the type specimen at this size shows no evidence of ventrolateral longitudinal grooves.

The hypothetical evolutionary scenario suggested by this new discovery can be explained through alternating processes of heterochrony (e.g., McNamara 1986; McKinney 1990). The ancestral form, Beyrichoceras , does not yet exhibit a specific adult morphology that fundamentally differs from earlier ontogenetic stages. Through hypermorphosis, Xenoglyphioceras gen. nov. adds an additional stage at the end of ontogeny, characterised by sharply forward-directed constrictions in the ventrolateral area, which develop into ventrolateral longitudinal grooves. In a subsequent step, these features are shifted to an earlier ontogenetic stage in Ferganoceras through acceleration.

This new discovery also prompts a reconsideration of the systematic classification of the family Ferganoceratidae . This family was placed in the superfamily ‘Neoglyphiocerataceae’ by Ruzhencev & Bogoslovskaya (1971). However, this classification may now be questioned, or it might only be upheld if Neoglyphioceras and similar genera could also be derived from Beyrichoceras and Xenoglyphioceras gen. nov.

The origin of the Neoglyphioceratoidea remains unresolved. The superfamily appeared suddenly with its oldest known species, Neoglyphioceras spirale (Phillips, 1841) , in the Neoglyphioceras spirale Zone (P1d of the British zonation). No species that could be considered a direct ancestor has yet been identified. With its unusually slender conch geometry and prominent spiral lines, N. spirale is an anomaly in the Neoglyphioceras spirale Zone and cannot be phylogenetically linked to any stratigraphically older species.

It is possible that Xenoglyphioceras gen. nov. is the ancestor of Neoglyphioceras . However, this would require significant modifications to the conch (whorls becoming more slender) and ornamentation (reduction in the number of spiral lines). The suture line of Xenoglyphioceras differs less markedly from that of Neoglyphioceras . Both genera share similarities in the shape of the external lobe and the ventrolateral saddle.