Unranked clade (stem group polychaetes)
Canadia was briefly compared to modern chrysopetalids (Family: Palmyridae) but the similarities were thought to be too general to allow the inclusion of this species to this group (Conway Morris, 1979). Canadia is now regarded as a stem-group polychaete (Eibye-Jacobsen, 2004).
Canadia – from Canada, the country where the Burgess Shale is located.
spinosa – from the Latin spinosus, “full of spines,” reflecting its spiny appearance.
Lectotype –USNM57654 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: none.
Other deposits: One specimen known from the Spence Shale (Middle Cambrian of Utah) and described as Canadia sp. (Robison, 1969).
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott Quarry on Fossil Ridge.
Brief history of research:
Walcott (1911) described two different species of Canadia (C. setigera and C. spinosa) in his initial census of the Burgess Shale, and a more detailed description was produced from his notes after his death by Resser (Walcott, 1931) adding several additional species (C. grandis, C. irregularis, C. sparsa, C. dubia, and C. simplex). Conway Morris (1979) synonymised C. irregularis and C. grandis with C. spinosa, while the other species have all been reinterpreted as different genera. Nick Butterfield (1990) succeeded in isolating individual scales by dissolving fossils in acid. These scales were compared with the sclerites of Wiwaxia, suggesting a possible affinity between the two taxa. Wiwaxia is now regarded as a primitive mollusc (Caron et al., 2006) implying the scales of Canadia and sclerites of Wiwaxia are probably convergent. Like Burgessochaeta, Canadia has proven useful in calculating the extent of decay in fossil assemblages (Caron and Jackson, 2006).
Canadia is a bristled worm around 2 to 4 cm long and slightly dorsoventrally flattened. A long pair of smooth, tentacles protrudes from the front of its head. The variation in shape seen among these tentacles suggests that the organism could contract and extend them. The rest of the body consists of 20 to 22 trunk segments, each bearing a pair of lateral projections called parapodia. On the first segment the parapodia are simple (uniramous), while all the other segments have biramous parapodia (divided into two sections). All parapodia bear bristles called setae. In the second segment through to the last segment they form two main bundles, the notosetae (dorsal) and the neurosetae (lateral). Gills (branchiae) are situated between these two bundles of setae. The notosetae cover the organism asymmetrically, with the longest, widest setae closest to the midline. The lateral surface of the larger setae is serrated, and all the setae bear a finely spaced patterning of ridges, which may have given Canadia an iridescent lustre in life (Parker, 1998). The animal had a straight gut, and an eversible soft proboscis.
Canadia is relatively rare in the Walcott Quarry representing only 0.05% of the specimens counted in the community (Caron and Jackson, 2008).
Canadia probably lived close to the seafloor and could have swum by using its bristle-fans as paddles and by undulating its body. It would have used its tentacles primarily as sensory organs, and its proboscis for feeding on live or dead organisms.