Halwaxiids

Definition and members

The name "halwaxiid" was formed by combining the names of two members of the proposed group, Halkieria and Wiwaxia. The group was defined as a set of Early to Mid Cambrian animals that had: a "chain mail" coat of three concentric bands of small armor plates that are called sclerites; in some cases, a small cap-like shell at the front end and in some cases both ends. Some scientists are unhappy with this loose definition, arguing that such traits may have arisen convergently rather than being inherited from a common ancestor. This objection implies that the group may not be monophyletic, and that their similarities are not biologically equivalent but evolved separately. In fact the originators of the term "halwaxiid" prefer an evolutionary "family tree" in which the halwaxiids are not monophyletic. The possibility of convergence is reinforced by the presence of a scleritome in a derived gastropod mollusc.