Debate About Cambrian Lophotrochozoans - Organisms Featured in The Debate

Organisms Featured in The Debate

Further information: Halkieriid, Wiwaxia, Orthrozanclus, and Odontogriphus

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 being inherited from a common ancestor. This objection suggests that the group is not monophyletic, in other words that the group does not contain all and only the descendants of a single common ancestor. In fact the originators of the term "halwaxiid" prefer an evolutionary "family tree" in which the halwaxiids are not monophyletic.

Other fossil organisms became involved in the debate as there were plausible arguments for considering them closely related to the halkieriids or Wiwaxia or both. One was the more recently discovered Orthrozanclus, which looked an intermediate between the halkieriids and Wiwaxia as it had a shell like that of halkieriids, and unmineralized sclerites and long spines like those of Wiwaxia – in fact the article which first described Orthrozanclus introduced the term "halwaxiid". New finds of Odontogriphus put this animal into play as well since, despite its lack of sclerites or shells, its feeding apparatus looks very like Wiwaxia’s. The siphogonotuchids, a very Early Cambrian group known only from isolated sclerites among the small shelly fossils, also appear in analyses as their sclerites suggest this group may have been close to the ancestors of halkieriids.