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''Canadaspis'' ("Shield of Canada") is an extinct
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of bivalved Cambrian arthropod, known from North America and China. They are thought to have been benthic 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, plu ...
appendages to stir mud in search of food. They have been placed within the
Hymenocarina Hymenocarina is an order of extinct arthropods known from the Cambrian. They possess bivalved carapaces, typically with exposed posteriors. Members of the group are morphologically diverse and had a variety of ecologies, including as filter feede ...
, 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 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 In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...
and maxillae. The mandible bore a mandibular palp, which was fringed with
setae In biology, setae (singular seta ; from the Latin word for " bristle") are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Annelid setae are stiff bristles present on the body. ...
, with the mandible having a toothed margin. The head had two pairs of cephalothoracic legs which have prominently developed endites, with the legs ending in a terminal claw. it is unclear whether these limbs were
uniramous 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, plu ...
or
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, plu ...
. The body had over a dozen segments divided into an anterior 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 were segmented, probably with 13-14 segments, and also ended in a terminal claw. The exopods were lobe-shaped, with 9 or 10 rays radiating outwards from their edges. The abdomen terminated with a
telson The telson () is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment of the arthropod body, or an additional division that is not a true segment on accou ...
, 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 A ''tergum'' (Latin for "the back"; plural ''terga'', associated adjective tergal) is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the 'base' and posterior edge is called the 'apex' or 'mar ...
. 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 poseriorly. 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 benthic 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 t ...
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 In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...
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.


Classification

''Canadaspis perfecta'' was originally described by
Charles Doolittle Walcott Charles Doolittle Walcott (March 31, 1850February 9, 1927) was an American paleontologist, administrator of the Smithsonian Institution from 1907 to 1927, and director of the United States Geological Survey. Wonderful Life (book) by Stephen Jay G ...
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 (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...
, but interpretation is now rejected. It has alternatively suggested to be a stem-group
euarthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, oft ...
. It is currently thought to be a member of the group
Hymenocarina Hymenocarina is an order of extinct arthropods known from the Cambrian. They possess bivalved carapaces, typically with exposed posteriors. Members of the group are morphologically diverse and had a variety of ecologies, including as filter feede ...
, which are interpreted as mandibulates. Some scientists believe that ''Canadaspis laevigata'' should be placed in a separate genus.


Fossil occurrences

4525 specimens of ''Canadaspis'' ''perfecta'' are known from the Greater
Phyllopod bed The Phyllopod bed, designated by USNM locality number 35k, is the most famous fossil-bearing member of the Burgess Shale fossil ''Lagerstätte''. It was quarried by Charles Walcott from 1911–1917 (and later named Walcott Quarry), and was t ...
, 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 fo ...
in
British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...
, 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 state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...
as well as the Pioche Shale of
Nevada Nevada ( ; ) is a state in the Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the 7th-most extensive, ...
. ''Canadaspis laevigata'' comes from the Chengjiang biota of Yunnan, China and thus some 10 million years older than ''Canadapsis 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