Orthrozanclus reburrus

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A tiny grazing animal covered with spines

Taxonomy:

3D animation of Orthrozanclus reburrus.

ANIMATION BY PHLESCH BUBBLE © ROYAL ONTARIO MUSEUM

Orthrozanclus reburrus (USNM 213695) – Part and counterpart. Specimen in dorsal view originally collected by Walcott but never published. Compare Walcott’s photograph (middle left) and its heavily retouched versions of both the part and counterpart (right) to enhance certain features, perhaps in preparation for a publication. Specimen length = 9 mm. Specimen wet – direct light. Walcott Quarry.

Orthrozanclus reburrus (USNM 213696) – Part and counterpart. Specimen in dorsal view originally collected by Walcott but never published. Specimen length = 9 mm. Specimen wet – direct light. Walcott Quarry.

Orthrozanclus reburrus (ROM 57835) – Part and counterpart. Complete specimen in dorsal view. Specimen length = 6 mm. Specimen wet – direct light (far left and far right), wet – polarized light (middle left and middle right). Walcott Quarry.

Orthrozanclus reburrus (ROM 57836) – Part and counterpart. Complete specimen in ventral view. Specimen length = 11 mm. Specimen wet – direct light (far left and far right), wet – polarized light (middle left and middle right). Walcott Quarry.

Kingdom: Animalia

Phylum: Mollusca

Class: Unranked clade halwaxiids (stem group molluscs)

Species name: Orthrozanclus reburrus

Remarks:

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).

Described by: Conway Morris and Caron

Description date: 2007

Etymology:

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.

Type Specimens: Holotype –ROM57197 in the Royal Ontario Museum, Toronto, Canada.

Other species:

Burgess Shale and vicinity: none.

Other deposits: none.

Age & Localities:

Period:

Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).

Principal localities:

The Walcott Quarry on Fossil Ridge.

History of Research:

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).

Description:

Morphology:

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.

Abundance:

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).

Maximum Size:

11 mm

Ecology:

Life habits: Epibenthic, Mobile

Feeding strategies: Herbivorous

Ecological Interpretations:

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.

References:

CARON, J.-B. AND D. A. JACKSON. 2008. Paleoecology of the Greater Phyllopod Bed community, Burgess Shale. Palaeogeography, Palaeoclimatology, Palaeoecology, 258: 222-256.

CONWAY MORRIS, S. AND J.-B. CARON. 2007. Halwaxiids and the early evolution of the lophotrochozoans. Science, 315: 1255-1258.

SIGWART, J. D. AND M. D. SUTTON. 2007. Deep molluscan phylogeny: synthesis of palaeontological and neontological data. Proceedings of the Royal Society B: Biological Sciences, 274: 2413-2419.