Phylogeny and taxonomy of Halkieria: progress and prospects

Halkieria is a kind of Cambrian problematic metazoan with mineralized sclerites covering its body. It possesses a large shell plate on both anterior and posterior parts of the body, and its calcareous sclerites are arranged bilaterially-symmetrically, which form 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, paleogeographical distribution, stratum spanning, and reviews the following viewpoints regarding its biological affinities: 1) Halkieria is a stem group brachiopod, and has a close relationship with tommotiids; 2) The sclerites fossils such as 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 class Polyplacophora and Aplacophora; 4) Halkieria is the ancestor of annelids. Of these, it is currently widely accepted that Halkieria is a stem group aculiferan because of the close comparability of scleritome construction between Halkieria and chitons, which, however has been challenged by new fossil materials discovered across the globe and ontogenetic studies of the animal. Through systematical determination of taxonomy and summary of research progress in biological affinities. The paper further discusses the significance of Halkieria in the taxonomy of SSFs, reconstruction of scleritome, origin and evolution of lophotrochozoans. And we further point out the deficiencies of current studies of Halkieria and highlight the research emphasis in future.

Project Supported:

The National Natural Science Foundation of China (General Program, Key Program, Major Research Plan)

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