Gatunek typowy
Hemilistrona depkei Chauff et Dombrowski, 1977 from the basal shale member of the Sulphur Springs Formation near Pevely, Missouri, lower costatus Zone.

Opis

Polygnathids with elaborate platform developed also in P2 elements; platform of P1 elements with ridges parallel to blade in rostral ventral part and with tuberculated convex and strongly asymmetric dorsal part; remaining elements of the apparatus similar to Polynodosus.
The ancestry of Hemilistrona remains unsolved. The morphology of P2 elements in its oldest species, H. perplexa, suggests relationship to the early Famennian Polygnathus. Prominent ridges backing the blade in the P1 element are also similar to those in P. webbi group but, even if they are in asymmetric pairs, this is obscured by the tremendous population variability. Moreover, the ridges extend much farther dorsally than in Polygnathus, in smallest juveniles being rather like that of Polynodosus. Also the shape of M elements with transversely oriented processes speaks in favor of a relationship to the Mehlina-Polynodosus lineage. A relationship to the Frasnian polygnathids of similar platform ornamentation remains to be tested by apparatus reconstructions.

Synonimy

Zasięg czasowy

Literatura

Dzik, J. 2006. The Famennian "Golden Age" of conodonts and ammonoids in the Polish part of the Variscan sea. Palaeontologia Polonica 63, 1-359.