Unranked clade halwaxiids (stem group molluscs)
The classification of this animal remains ambiguous but is thought to belong to the halwaxiids, a group including Wiwaxia and halkierids (Conway Morris and Caron, 2007). Two alternative positions were proposed for the halwaxiids, as either stem-group lophotrochozoans (a group which includes molluscs, annelids, and brachiopods) or as stem-group molluscs (see also Sigwart and Sutton, 2007).
Conway Morris and Caron
Orthrozanclus – from the Greek orthros, “dawn,” referring to its ancestral position, and zanclon, “sickle-like,” in reference to its long sclerites.
reburrus – from the Latin reburrus, “with bristling hair,” in reference to its hairy appearance.
Holotype – ROM 57197 in the Royal Ontario Museum, Toronto, Canada.
Burgess Shale and vicinity: none.
Other deposits: none.
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott Quarry on Fossil Ridge.
Brief history of research:
Two specimens were collected by Walcott but were never described. Between 1994 and 2000, the Royal Ontario Museum collected nine additional specimens from the Walcott Quarry. These were recognized as an unknown sclerite-bearing animal (Type C, Caron and Jackson 2006) allowing for a formal description of this new species (Conway Morris and Caron, 2007).
Specimens vary from 6 mm to 11.3 mm in length. The ventral side is flat, whereas the dorsal surface is rounded in cross-section, bearing three zones of sclerites and one anterior shell. A set of relatively small sclerites is present around the margins of the body. These appear flat and slightly curved in one direction. Above this marginal set is a second set of much longer sclerites that seem to originate from a narrow zone along the entire length of the body. These are circular in cross section, appear to have a larger base, and tend to be curved and pointing upwards; they are probably hollow and bear one or two ridges. The presence of kinked sclerites suggests a lack of mineralization. A third set of much smaller sclerites covers the convex dorsal side of the body, but these are not clearly preserved. The shell, which was presumably mineralized, is triangular in outline with the pointed end towards the front. Fine striations along the shell are probably growth lines, indicating that growth occurred from the front to the back.
Very rare; all 11 known specimens come from the Walcott Quarry only, where it represents a tiny fraction of the community (0.02%) (Caron and Jackson, 2008).
Orthrozanclus is similar in overall aspect and probably in ecology to the better known Wiwaxia corrugata.Orthrozanclus, like Wiwaxia, was probably herbivorous and would have crept along the seafloor in search for food.