Halkieria系统进化及其化石分类学研究进展

Halkieria是寒武纪一类躯体上披有骨质鳞片的疑难后生动物,前后各有一个大壳板,两侧对称,其钙质骨片在软躯体上排列形成几条规律的纵向带。半个世纪以来,学术界对Halkieria生物系统确切位置的讨论以及Halkieriidae不同属种亲缘关系的探索从未停止。本文总结了Halkieria化石在骨片属种分类、古地理和地层分布、生物系统分类学的研究进展,对当前的生物系统分类归纳出以下几种观点:1) Halkieria为腕足动物的祖先,并与托莫特壳类有一定的亲缘关系;2)Halkieria与Wiwaxia等骨片化石具有单源性,组成一种新的分类单元Halwaxiidae,属于软体动物干群或腕足动物和环节动物的干群;3)Halkieria为软体动物有刺亚门干群,与无板纲、多板纲更为亲近; 4) Halkieria为环节动物的祖先。目前,虽然多数人根据Halkieria与现生多板纲的骨片排列一致将Halkieria归属软体动物有刺亚门的干群,但近年来世界各地新化石材料的陆续发现及壳体发育研究使得Halkieria的系统分类位置出现新的争议。通过对化石分类学的系统厘定和生物分类研究进展的总结梳理,...

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.