3D animation of Canadaspis perfecta.
Animation by Phlesch Bubble © Royal Ontario Museum
Hymenocarines were early arthropods with bivalved carapaces and mandibles, forming the bulk of the first mandibulates (represented today by myriapods, crustaceans and insects) Canadaspis was originally classified as a Malacostracan crustacean (Walcott 1912; Briggs 1978), but this has since been refuted (Briggs 1992; Legg et al. 2012; Aria and Caron 2017).
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.”
perfecta – from the Latin perfectus, “complete.”
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). Indeterminate species of Canadaspis have also been recovered from the Guzhangian Fulu Biota of China (Peng et al. 2020), the Wuliuan Kaili Biota (Wang et al. 2009) and the middle Cambrian Pioche Shale (Kimmig et al. 2019).
The Walcott Quarry on Fossil Ridge.
Originally referred to as Hymenocaris by Charles Walcott (1912), the genus Canadaspis was erected by Novozhilov (Orlov 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, has been 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. An additional species, Canadaspis laevigata was later described from the early Cambrian Chengjiang biota (Hou and Bergström 1997). As with other hymenocarines, the crustacean affinity of Canadaspis was challenged by later workers (Briggs 1992; Hou and Bergström 1997; Waloszek 1999), and a position as stem-group euarthropods argued instead (Budd 2002, 2008; Ortega-Hernández et al. 2017). The two pairs of antennae described by Briggs (1978), for example, have been re-interpreted as one pair of antennulae and one pair more frontal, unsegmented appendages (Ortega-Hernández and Budd 2016). The discovery of mandibles in the hymenocarines Branchiocaris (Aria and Caron 2017) and Waptia (Vannier et al. 2018) prompted the classification of Canadaspis as an early mandibulate. Mandibles had originally been observed in Canadaspis by Briggs (1978), but their identity had not been widely accepted (Edgecombe 2017). Further details of these structures, including a teeth-bearing surface and a potential three-segmented palp have since been uncovered (Izquierdo-López and Caron 2022a), further strengthening the mandibulate position of Canadaspis. Thanks to its long history of research, abundance, and comprehensive anatomical reconstruction, Canadaspis has been included in different morphometric (Zeng et al. 2020; Izquierdo-López and Caron 2022b) and palaeoecological analyses (Briggs 1978). Carapace assemblages also suggest that Canadaspis performed coordinated moulting: multiple individuals moulting at the same time and place (Haug et al. 2013).
The carapace of Canadaspis covers half of its total body length. Carapace valves are suboval in outline and taper towards the anterior, more pronounced than in the closely related Perspicaris. Valves connect dorsally, and do not form an anterior process, but often form a small posterior process. The length of the bivalved carapaces ranges in size from 0.8-5.2 cm in length. The head protrudes slightly from behind the carapace valves, and its outline is generally rounded. Canadaspis has one pair of small, elongate eyes, as well as one pair of elongate unsegmented appendages located between the eyes. Other head appendages include a pair of antennulae, a pair of mandibles, a possible pair of maxillae and two pairs of limbs incorporated into the head (cephalothorax). The antennulae are short and thick and have at least 12 segments, all fringed with long spines. The mandibles are rounded and have a dentate inner edge. A following cephalic appendage may represent a small three-segmented palp fringed with setae. The morphology of the maxillae is currently not clear. The thorax has 8 segments, each with a two-branched limb (biramous). Each limb has a multisegmented base (basipod). In thoracic legs, each segment bears a small spine. The basipod bears an inner walking branch (endopod), divided into 7 segments, with the terminal one ending in a complex three-clawed structure. The outer branch (exopod) is a large flap with lamellae, interpreted to be gills. Posterior to the thorax there are 7 segments that do not bear legs (the abdomen). The edge of each segment is spinose. The last segment is highly spinose and bears a pair of larger spinose processes. 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 closed to 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 have also propelled Canadaspis through the water column. 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 or aid in tearing and manipulating other sources of food. The synchronized moulting strategy of Canadaspis (Haug et al. 2013) is part of an increasing evidence of the complexity of behaviours in Cambrian bivalved arthropods, which also includes the ability to take care of eggs seen in Waptia (Caron and Vannier 2016) or Chuandianella (Ou et al. 2020), or the chain-like associations seen in Synophalos (Hou et al. 2009).