Eifelian (Middle Devonian) species of Bipennatus from the Beiliu Formation at Nalai, South China

Abstract

A conodont fauna from the uppermost part of the Beiliu Formation at the Nalai section (Guangxi, South China) is studied. Four species of Bipennatus Mawson are described, of which B. hemilevigatus n. sp. is differentiated by a short sulcus flanked by a smooth left margin and a nodose right margin, and B. planus n. sp. is characterized by a distinctly flat, smooth, and narrow platform in the middle part of the blade above the basal cavity. Another species in open nomenclature, Bipennatus? sp., is distinguished by an extremely primitive sulcus or a totally fused and adenticulate ridge above the basal cavity. This paper also reports occurrences of ‘Ozarkodina’? sp. B, Polygnathus nalaiensis n. sp., and P. costatus costatus Klapper. The conodont fauna situates the investigated samples in the lower Eifelian costatus Zone. Taking specimens of Bipennatus from the Hillesheimer Mulde and Bergischen Land (Germany) into consideration, the conodont material furnishes new data on the diversification and phylogeny of this genus. A taxonomic revision of Bipennatus is conducted, and this genus includes B. bipennatus (Bischoff and Ziegler), B. hemilevigatus, B. montensis (Weddige), B. planus, B. scalaris (Mawson), and B. mayri (Uyeno), the latter two are raised herein to species level. More importantly, the prominent diversification of Bipennatus in the costatus Zone cannot be interpreted by previously proposed hypotheses on the phylogeny of Bipennatus, which specifically focus on the development of a characteristic sulcus on the upper margin above the basal cavity. It is apparent that the ornamentation of the narrow or wide platform above the basal cavity is more complicated and variable than previously estimated, and that this genus needs further investigation, especially on the lowest occurrences of its assigned species.