Halwaxiid
| Halwaxiida Temporal range: Early Cambrian–Middle Cambrian |
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| Halkieria sp. | |
| Scientific classification | |
| Kingdom: | Animalia |
| Superphylum: | Lophotrochozoa |
| (unranked): | Halwaxiida |
| Families | |
Halwaxiida or halwaxiids is a proposed clade equivalent to the older orders Sachitida He 1980 and Thambetolepidea Jell 1981, loosely uniting scale-bearing Cambrian animals, which may lie in the stem group to molluscs or lophotrochozoa. Some palaeontologists question the validity of the Halwaxiida clade.
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.
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. Orthrozanclus, first described in 2007, 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, reported in 2006, put this animal into play as well – despite its lack of sclerites or shells, its feeding apparatus looks very like Wiwaxia’s. The siphogonuchitids, 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.
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