Unranked clade (stem group arthropods)
Canadaspis was originally classified as a Malacostracan crustacean (Walcott, 1912; Briggs, 1978), but this has been widely debated (e.g. Hou and Bergström, 1997; Boxshall, 1998; Walossek, 1999; Butterfield, 2002). It has also been placed in the upper euarthropod stem-lineage (Edgecombe, 2010), forming a clade with other bivalved arthropods such as Perspicaris (Bergström and Hou, 1998; Waloszek et al., 2007), and possibly including Fuxianhuia (Budd, 2002; Budd and Telford, 2009).
Canadaspis – from the country Canada, whose name derives from the Saint-Lawrence Iroquoian kanata, “settlement” or “land,” and the Greek aspis, “ shield.”
perfecta – from the Latin perfectus, “complete.”
Lectotype –USNM57703 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: none.
Other deposits: C. laevigata from the Lower Cambrian Chengjiang biota (Hou and Bergström, 1991, 1997). Further material of Canadaspis cf. perfecta has been recovered from additional localities in the USA (Robison and Richards, 1981; Lieberman, 2003; Briggs et al., 2008).
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott Quarry on Fossil Ridge.
Brief history of research:
Originally referred to as Hymenocaris by Charles Walcott (1912), the genus Canadaspis was erected by Novozhilov (1960). Four species were designated by Simonetta and delle Cave (1975), but two of them, Canadaspis ovalis and Canadaspis dictynna have since been redescribed within Perspicaris dictynna (Briggs, 1977). A third species, Canadaspis obesa was redescribed within Canadaspis perfecta (Briggs, 1978). A full study of the fourth and only valid species, Canadaspis perfecta was published by Briggs (1978), who posited a crustacean affinity. This was rebuffed by later workers (Hou and Bergström, 1997; Boxshall, 1998; Walossek, 1999), and a close relationship with other bivalved Burgess Shale taxa in the arthropod stem lineage, including Branchiocaris, Perspicaris and Odaraia, was suggested (Budd, 2002; Budd, 2008).
Canadaspis is composed of a bivalved carapace covering a body with an appendage-bearing head region, an abdomen of 8 segments with associated limbs that are segmented and branch into two (biramous), and a thorax of 7 segments with a spiny telson or tail. The length of the bivalved carapaces ranges in size from 0.8-5.2 cm. The carapace valves are suboval in outline and taper towards the anterior, with a straight hinge line connecting them towards the back of the upper surface (dorsally).
The head has two pairs of antennae, small eyes, spiny mouth parts and two pairs of biramous appendages. The first antennae are short and unsegmented, while the second antennae are much longer, have at least 12 segments and are fringed with long spines. The small eyes were borne on short, blunt stalks. A series of spines behind the antennae are interpreted as mandibles, arthropod mouth parts used for cutting food. The ten pairs of biramous limbs of the head and abdomen consist of a segmented inner walking limb, and a large outer flap with lamellae, interpreted to be gills. The segmented abdomen does not bear appendages, and ends in a spiny telson. The gut of Canadaspis is sometimes preserved, with mid-gut glands giving it a segmented appearance (Butterfield, 2002).
Canadaspsis is abundant, with over 5,000 specimens known; it comprises 8.6% of the Walcott Quarry community (Caron and Jackson, 2008).
Canadaspis was likely to have lived on the sea floor, walking on its biramous appendages by moving them in a rippling motion. This would also waft water past the gills that form the outer branches of its biramous limbs, allowing for respiration. This movement may also propelled Canadaspis through the water column. The biramous appendages on Canadapsis' head are tipped with a pair of claws that were probably used in feeding. The inner surfaces of its legs were covered with spines that would have assisted in feeding by directing food particles to the organism's mouth. The mandibles would have been used to help consume the coarse particles found on the sediment surface. Canadapsis' spiny head-shield probably protected it from predators.