Canadaspis ("Shield of Canada") is an extinct group of Cambrian marine arthropods with two-shell bodies. Fossils of Canadaspis have been found in North America and China. These creatures likely lived on the ocean floor and moved mainly by walking. They may have used their special limbs to stir up mud in search of food. Canadaspis is classified in the group Hymenocarina, which includes other Cambrian arthropods with two-shell bodies.

Description

The bivalved carapaces of Canadaspis perfecta are usually 8–52 millimetres (0.31–2.05 in) long, narrowing toward the front. The head had two small eyes on short stalks. Between the eyes was a forward-pointing spine and a pair of short antennae that lacked segments. These antennae are similar to those of Waptia and may be related to the hemi-ellipsoid bodies of crustaceans, suggesting they were used for sensing odors. The head also had a second pair of larger, segmented antennae, likely with more than 12 segments. The segments grew longer toward the end, with slender, forward-facing spines on the front of each segment. The head had a pair of mandibles and maxillae, both modified limbs used for eating. The mandible had a palp covered in setae (hair-like structures) and a toothed edge. The head also had two pairs of cephalothoracic legs with prominent endites (projections on the underside) and a terminal claw at the end. It is unclear if these limbs had one branch (uniramous) or two branches (biramous). The body had more than a dozen segments, divided into a front thorax with legs covered by the carapace and a rear, legless abdomen. The thorax had eight pairs of biramous legs. The endopods (lower branches) were segmented, likely with 13–14 segments, and ended in a claw. The exopods (outer branches) were lobe-shaped, with 9 or 10 rays extending outward. The abdomen ended with a telson, which had two spinose projections on its lower edge, each with one large spine and five smaller spines.

The bivalved carapace of C. laevigata is similar to that of C. perfecta, though smaller. The head had two stalked eyes and a pair of segmented, uniramous antennae. The body had 19 ring-like tergites (segments). There were ten pairs of biramous appendages, with the first pair on the head and the remaining nine along the body. The first five pairs were roughly equal in size, while the others gradually became smaller toward the rear. The biramous limbs were similar in structure. The endopods were strong and ended in claws. The exopods were flat and rounded. The body ended with a telson, which was longer than that of C. perfecta and had one large and one small pair of spines projecting backward.

Ecology

Canadaspis was probably a creature that lived on the seafloor, walking along the bottom. C. perfecta had claws at the ends of its limbs, which may have been used to mix sediment or scrape off a top layer that scientist Derek Briggs suggested might contain helpful microbes. Large particles disturbed by these claws were likely caught by spines inside its legs. These spines directed the food particles toward its mouth, where it used its mandibles to crush larger pieces.

Its antennae helped it sense its surroundings. Spines on the head of C. perfecta likely protected its eyes from predators. Its limbs probably moved in a wave-like pattern, creating a rippling motion. Even though Canadaspis likely did not swim, this movement may have helped it move across soft sediments. Its limbs also created water currents, which aided in feeding and breathing.

Members of C. perfecta seem to have shed their old exoskeletons in synchronized groups.

Classification

Canadaspis perfecta was first named by Charles Doolittle Walcott in 1912 as Hymenocaris perfecta. In 1960, Novozhilov moved it to a new genus called Canadaspis.

At first, scientists thought Canadaspis was a type of crustacean. However, this idea is no longer accepted. Some researchers now believe it belongs to a group called stem-group euarthropods. Currently, it is considered part of the group Hymenocarina, which are classified as mandibulates. Some scientists also suggest that Canadaspis laevigata might belong to a different genus.

Phylogeny of Hymenocarina after Izquierdo-López and Caron (2024)

Fossil occurrences

There are 4,525 examples of Canadaspis perfecta found in the Greater Phyllopod bed of the Burgess Shale in British Columbia, Canada. These examples make up 8.6% of the community in that location. Other examples of Canadaspis, which are thought to be closely related or part of C. perfecta, are also found in the Spence Shale of western Utah and the Pioche Shale of Nevada. Canadaspis laevigata is found in the Chengjiang biota of Yunnan, China, and is about 10 million years older than Canadaspis perfecta.