''Canadaspis'' ("Shield of Canada") is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of bivalved

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

marine arthropod, known from North America and China. They are thought to have been

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

feeders that moved mainly by walking and possibly used its

biramous The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, ...

appendage An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...

s to stir mud in search of food. They have been placed within the Hymenocarina, which includes other bivalved Cambrian arthropods.

Description

''Canadaspis'' ''perfecta''

The bivalved carapaces of ''Canadaspis'' ''perfecta'' are typically in length, which taper towards the front end. The head had a small pair of eyes borne on short stalks. Between the eyes is a forward pointing spine, as well as a pair of short antennae, which appear to lack segmentation. Similar antennae are known from '' Waptia'', and are probably homologous to the hemi-ellipsoid bodies of crustaceans, and thus likely have an

olfactory The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, it ...

function. The head also has another pair of larger, segmented antennae, probably with more than 12 segments, the segments increased in length toward the end of the antenna, with the front end of the segments bearing slender, forward-facing spines. The head had a pair of mandibles and maxillae. The mandible bore a mandibular palp, which was fringed with

setae In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae ...

(hair-like structures), with the mandible having a toothed margin. The head had two pairs of cephalothoracic legs which have prominently developed endites (structures which project from the underside of the limb), with the legs ending in a terminal claw. It is unclear whether these limbs were uniramous (one branched) or

(divided into two branches). The body had over a dozen segments divided into an

anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

thorax with legs, covered by the carapace, and a posterior legless exposed abdomen. The thorax had 8 associated pairs of biramous legs. The limb endopods (the lower, leg-like branches) were segmented, probably with 13-14 segments, and also ended in a terminal claw. The exopods (the outer limb branches) were lobe-shaped, with 9 or 10 rays radiating outwards from their edges. The abdomen terminated with a

telson The telson () is the hindmost division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment (biology), segment of the arthropod body, or an additional division that is not a true segm ...

, which bore a pair of spinose projections directed posteriorly on its lower edge, each spinose projection consisted of one large spine and 5 smaller spines.

''Canadaspis laevigata''

The bivalved carapace of ''C. laevigata'' is similar to that of ''C. perfecta,'' though typically smaller in size. The head has a pair of stalked eyes and a pair of segmented uniramous antennae. The body has 19 ring-like tergites (segments). There are ten pairs of biramous appendages, the first of which appear to be located on the head, which the remaining nine run along the body. The first five pairs are roughly equal in size, while the remaining pairs gradually decrease in size posteriorly. The biramous limbs are all relatively similar in morphology. The endopods are robust, and end in claws. The exopod is flat and rounded. The body ends a telson, which is proportionally longer than that of ''C. perfecta'', which bore one large and one small pair of spines, projecting posteriorly.

Ecology

''Canadaspis'' was likely a

animal that lived walking along the seafloor. ''C. perfecta'' had claws on the end of its appendages which may have been used to stir up sediment, or to scrape off the top layer, which

Derek Briggs Derek Ernest Gilmor Briggs (born 10 January 1950) is an Irish palaeontologist and taphonomist based at Yale University. Briggs is one of three palaeontologists, along with Harry Blackmore Whittington and Simon Conway Morris, who were key in ...

suggested may have been a nutritious layer of microbes. Large particles it stirred up would have been captured by spines on the inside of its legs; these spines would have directed the food particles to the organism's mouth, where it used its mandibles to grind larger particles. Its antennae served a sensory function. The spines on the head of ''C. perfecta'' probably served to protect its vulnerable eyes from predators. Its limbs probably moved in a metachronal sequence to produce a rippling motion. Although ''Canadaspis'' probably did not swim, this could have helped propel the organism from under soft sediments. The appendages also produced currents which would have helped with feeding and respiration. Members of ''C. perfecta'' appear to have engaged in synchronised group

moulting In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is a process by which an animal casts off parts of its body to serve some beneficial purpose, either at ...

Classification

''Canadaspis perfecta'' was originally described by Charles Doolittle Walcott in 1912 as ''Hymenocaris perfecta.'' It was placed into the new separate genus ''Canadaspis'' in 1960 by Novozhilov. ''Canadaspis'' was historically interpreted as a

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

, but this interpretation is now rejected. It has alternatively been suggested to be a stem-group euarthropod. It is currently thought to be a member of the group Hymenocarina, which are interpreted as mandibulates. Some scientists believe that ''Canadaspis laevigata'' should be placed in a separate genus. Phylogeny of Hymenocarina after Izquierdo-López and Caron (2024)

Fossil occurrences

4525 specimens of ''Canadaspis'' ''perfecta'' are known from the Greater Phyllopod bed, of the

Burgess Shale The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fos ...

British Columbia British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...

, Canada, where they comprise 8.6% of the community. Other specimens of ''Canadaspis,'' considered closely related or belonging to ''C. perfecta'', are also found in the Spence Shale of western

Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

as well as the Pioche Shale of

Nevada Nevada ( ; ) is a landlocked state in the Western United States. It borders Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the seventh-most extensive, th ...

. ''Canadaspis laevigata'' comes from the

Chengjiang biota The Maotianshan Shales () are a series of Early Cambrian sedimentary deposits in the Chiungchussu Formation or Heilinpu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized orga ...

of Yunnan, China and is thus some 10 million years older than ''Canadaspis perfecta.''

References

External links

""Canadaspis perfecta"
. Burgess Shale Fossil Gallery. Virtual Museum of Canada. 2011. (Burgess Shale species 32) * {{Taxonbar, from=Q2731469 Burgess Shale fossils Cambrian arthropods Cambrian genus extinctions Prehistoric arthropod genera Burgess Shale animals Maotianshan shales fossils Wheeler Shale Hymenocarina