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.

Introduction

The genus Bipennatus Mawson, 1993 is distinguished by possessing a blade-like Pa element that has a more or less centrally situated basal cavity, and a narrow or broad platform in its middle part. It has a worldwide distribution and is generally recorded from the lower Eifelian to middle Givetian (Mawson, 1993, Kononova and Kim, 2005). Taking its possible palaecological significance into consideration (Narkiewicz and Narkiewicz, 2010; personal communication with Katarzyna Narkiewicz in 2019), this genus is of great importance for stratigraphical subdivision and correlation especially in the shallow-water facies where coral- and stromatoporoid-bearing carbonates predominate. Although Mawson (1993) and Narkiewicz (2015) have accomplished important studies on the taxonomy and phylogeny of Bipennatus, this genus is still poorly understood. For example, the taxonomic composition of Bipennatus varies greatly between different researchers. When first named by Mawson (1993), it includes three taxa, B. palethorpei (Telford, 1975), B. bipennatus bipennatus (Bischoff and Ziegler, 1957), and B. bipennatus scalaris (Mawson in Mawson and Talent, 1989). In contrast, Kononova and Kim (2005) suggested that Bipennatus taxonomically consists of B. palethorpei, B. bipennatus bipennatus, B. bipennatus montensis, and B. bipennatus auritus (Bai et al., 1979). Later, Narkiewicz (2015) assigned only two species, B. bipennatus and B. montensis, to this genus. Additionally, two remarkably different opinions on the phylogeny of Bipennatus were proposed. Mawson (1993) believed that Bipennatus originates from Pandorinellina Müller and Müller, 1957 via B. palethorpei, whereas Narkiewicz (2015) indicated that Bipennatus is a descendant of Ozarkodina Branson and Mehl, 1933.

The main goals of this study are to describe four species of Bipennatus, of which two are new, collected from two Eifelian limestone samples in Guangxi, South China, to propose a taxonomic revision of Bipennatus, and to discuss the phylogeny of Bipennatus on the basis of new conodont material.