The successive evolution of hexangulaconulariids and the growth pattern of carinachitiids revealed by new materials from the lower Cambrian of South China

Abstract

Hexangulaconulariids and carinachitids are widely reported small shelly fossils from the early Cambrian in South China. They have relatively robust single-apertural periderms, longitudinal corner grooves that separate the faces of the periderm, and stable facial ornamentation. Although they are thought to be closely related to the widely reported conulariids of the early Palaeozoic and have been placed in the subclass Conulata, many features of this group are unclear. Here, we describe and analyse new material from the second bed of the Zhangjiagou section (Cambrian Fortunian Stage), in Xixiang, Shaanxi, South China. The facial ornamentation and six inner faces of similar widths support the establishment of Hexangulaconularia amylofacialis n. sp. Based on the new material, we proposes additional modifications to the hypothesis on the evolution of the hexangulacounulariids characterised by gradually increasing numbers of faces. Diagnosis of Carinachites tetrasulcatus is emended. An ontogenetic sequence for Carinachites tetrasulcatus is established based on new specimens exhibiting adaperturally arcuate ribs.