Phylogeny and taxonomy of Halkieria: progress and prospects

Halkieria is a problematic Cambrian metazoan with mineralized sclerites covering its body. It possesses a large shell plate on both anterior and posterior parts of the body. The arrangement of its calcareous sclerites is bilaterally symmetric, forming several regular longitudinal bands on the body. For over half a century, the biological affinities of Halkieria or halkieriids have been contentious. This paper summarizes the research progress of Halkieria fossils in aspects of taxonomy, palaeogeographical distribution, and stratigraphic range, and reviews the following viewpoints regarding its biological affinities: 1) Halkieria is a stem group brachiopod, and has a close relationship with tommotiids; 2) Halkieria and Wiwaxia are monogenic, forming a monophyletic lineage, Halwaxiidae, which belongs to a stem group mollusk or brachiopod and annelid; 3) Halkieria is a stem group of the Aculifera that includes the Polyplacophora and the Aplacophora; and 4) Halkieria is the ancestor of annelids. Of these hypotheses, it is currently widely accepted that Halkieria is a stem group aculiferan because of the close comparability of scleritome construction between Halkieria and chitons. However, this hypothesis has been challenged by new fossil materials discovered around the globe and the ontogenetic studies of these animals. Through review of its taxonomy and summary of research progress on its biological affinities, this paper discusses the significance of Halkieria in the taxonomy of small shell fossils (SSFs), reconstruction of scleritome, and origin and evolution of the lophotrochozoans. And we further point out the limitations of current studies of Halkieria and highlight the research emphasis in the future.

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