Orthrozanclus reburrus ("Dawn scythe with bristling hair") is a sea creature known from the Middle Cambrian (~) Burgess shale, about one centimeter long, with long spikes protruding from its armored body
The describers of this fossil animal, Simon Conway Morris and Jean-Bernard Caron, say Orthrozanclus may have formed a link between the halkieriid and the wiwaxiid families, uniting them tentatively in a group called "Halwaxiida", characterized by a similar type of body armor; these organisms might have been stem group molluscs, or fall as a stem group to the larger lophotrochozoan clade (containing molluscs, annelids and brachiopods). However, the status of the Halwaxiid grouping is not universally accepted.
The animal clearly lived on the sea-floor, and is thought to have had a muscular foot rather like that of a snail.
Orthrozanclus′ sclerites are very similar to those of its Burgess Shale contemporary Wiwaxia. Its shell is very similar to: one of the two Burgess Shale shell types labelled Oikozetetes; the forward shell of Halkieriids, most of which are dated to the Early Cambrian; and those of other Early Cambrian fossils such as Ocruranus and Eohalobia. These similarities suggest that Orthrozanclus was an intermediate form between Wiwaxia and the Halkieriids and that all three of these taxa formed a monophyletic clade, in other words a group that consists of a common ancestor and all of its descendants. However this draws Orthrozanclus into a complex debate that has gone on since 1990 about whether Wiwaxia is more closely related to molluscs or to polychaete worms, and therefore about the entire "family tree" of the Lophotrochozoa, a "super-phylum" that is thought to contain modern molluscs, annelids and brachiopods as well as some extinct groups. The main opponent of this view, Nicholas Butterfield, proposes that Wiwaxia is more closely related to annelids while Halkieriids are very close to molluscs.
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