Cambrian Chordate
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The Cambrian chordates are an extinct group of animals belonging to the phylum
Chordata A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (Apomorphy and synapomorphy, synapomorphies) th ...
that lived during the
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 ...
, between 538 and 485 million years ago. The first Cambrian chordate known is '' Pikaia gracilens'', a
lancelet The lancelets ( ), also known as amphioxi (: amphioxus ), consist of 32 described species of somewhat fish-like benthic filter feeding chordates in the subphylum Cephalochordata, class Leptocardii, and family Branchiostomatidae. Lancelets dive ...
-like animal from 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 ...
in
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. The discoverer,
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. He is famous for his discovery in 19 ...
, described it as a kind of worm (
annelid The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...
) in 1911, but it was later identified as a chordate. Subsequent discoveries of other Cambrian fossils from the Burgess Shale in 1991, and 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 China in 1991, which were later found to be of chordates, several Cambrian chordates are known, with some fossils considered as putative chordates. The Cambrian chordates are characterised by the presence of segmented muscle blocks called
myomere Myomeres are blocks of skeletal muscle tissue arranged in sequence, commonly found in aquatic chordates. Myomeres are separated from adjacent myomeres by fascia consisting of connective tissue, known as myosepta. Myomere counts are sometimes us ...
s and
notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...
, the two defining features of chordates. Before the full understanding of Cambrian fossils, chordates as members the most advanced phylum were believed to appear on Earth much later than the Cambrian. However, the better picture of Cambrian explosion in the light of Cambrian chordates, according to
Stephen Jay Gould Stephen Jay Gould ( ; September 10, 1941 – May 20, 2002) was an American Paleontology, paleontologist, Evolutionary biology, evolutionary biologist, and History of science, historian of science. He was one of the most influential and widely re ...
, prompted "revised views of evolution, ecology and development," and remarked: "So much for chordate uniqueness marked by slightly later evolution."


Discovery

''Pikaia gracilens'' was the first Cambrian chordate known. It was discovered 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. He is famous for his discovery in 19 ...
from 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 ...
in the mountains of
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. Walcott reported the specimen with taxonomic description in 1911. He used the series of transverse body segments as a feature of
annelid The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...
worms and classified it as a
polychaete Polychaeta () is a paraphyletic class of generally marine Annelid, annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called c ...
worm. He gave the name after
Pika Peak Pika Peak is a mountain located between Mount Richardson and Ptarmigan Peak in Banff National Park, Alberta, Canada The mountain was named in 1928 after the pika, the small "rock rabbit" that inhabits alpine regions. The Lake Louise Mountain ...
, a mountain in
Alberta Alberta is a Provinces and territories of Canada, province in Canada. It is a part of Western Canada and is one of the three Canadian Prairies, prairie provinces. Alberta is bordered by British Columbia to its west, Saskatchewan to its east, t ...
, Canada. However, he was also aware of the unique differences from living worms that he prudently remarked: "I am unable to place it within any of the families of the Polychaeta, owing to the absence of
parapodia In invertebrates, the term parapodium ( Gr. ''para'', beyond or beside + ''podia'', feet; : parapodia) refers to lateral outgrowths or protrusions from the body. Parapodia are predominantly found in annelids, where they are paired, unjointed late ...
aired protrusions on the sides of polychaete wormson the body segments back of the fifth."
University of Cambridge The University of Cambridge is a Public university, public collegiate university, collegiate research university in Cambridge, England. Founded in 1209, the University of Cambridge is the List of oldest universities in continuous operation, wo ...
palaeontologist Harry B. Whittington and his student
Simon Conway Morris Simon Conway Morris (born 1951) is an English palaeontologist, evolutionary biologist, and astrobiologist known for his study of the fossils of the Burgess Shale and the Cambrian explosion. The results of these discoveries were celebrated ...
re-analysed the specimens and came to an opinion in 1977 that ''Pikaia'' was obviously a chordate: the body segments were similar to muscle blocks in chordates, and a rod-like structure spans the body length, an indication of notochord, a defining structure of chordates. Their article in the ''
Scientific American ''Scientific American'', informally abbreviated ''SciAm'' or sometimes ''SA'', is an American popular science magazine. Many scientists, including Albert Einstein and Nikola Tesla, have contributed articles to it, with more than 150 Nobel Pri ...
'' in 1979 affirmed the chordate status, saying: "The chordates are represented in the Burgess Shale by the genus ''Pikaia'' and the single species ''P. gracilens''." Conway Morris published the formal classification as a chordate in 1979. Harvard University palaeontologist
Stephen Jay Gould Stephen Jay Gould ( ; September 10, 1941 – May 20, 2002) was an American Paleontology, paleontologist, Evolutionary biology, evolutionary biologist, and History of science, historian of science. He was one of the most influential and widely re ...
popularised ''Pikaia'' as an ancestral species of chordates in his 1989 book '' Wonderful Life: The Burgess Shale and the Nature of History,'' from which ''Pikaia'' became known as the "most famous early chordate fossil," or the earliest chordate, or the oldest ancestor of humans. The second Burgess shale chordate was also discovered by Walcott but left unexamined. The date of discovery was not recorded. Italian palaeontologist Alberto M. Simonetta, while working at the US
National Museum of Natural History The National Museum of Natural History (NMNH) is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. With 4.4 ...
, where Walcott's collections are maintained, became the first to analyse the specimen in 1960. He and Emilio Insom at the
University of Camerino The University of Camerino () is a university located in Camerino, Italy. It is the best university of Italy among those with fewer than 10,000 students, according to the Guida Censis Repubblica 2011 and 2012 ranking. It claims to have been foun ...
published the classification in 1993, giving the name '' Metaspriggina walcotti'', an animal with unknown identity. Conway Morris recovered additional specimen with which he described the species as that of chordate in 2008. In 1991, Hou Xian-guang (of the
Chinese Academy of Sciences The Chinese Academy of Sciences (CAS; ) is the national academy for natural sciences and the highest consultancy for science and technology of the People's Republic of China. It is the world's largest research organization, with 106 research i ...
), Lars Ramsköld and Jan Bergström (of the
Swedish Museum of Natural History The Swedish Museum of Natural History (), in Stockholm, is one of two major museums of natural history in Sweden, the other one being located in Gothenburg. The museum was founded in 1819 by the Royal Swedish Academy of Sciences, but goes bac ...
) reported a series of discoveries of Cambrian fossils from the
Maotianshan Shales 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 ...
in Chengjiang County,
Yunnan Yunnan; is an inland Provinces of China, province in Southwestern China. The province spans approximately and has a population of 47.2 million (as of 2020). The capital of the province is Kunming. The province borders the Chinese provinces ...
Province,
China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
. One specimen which they named '' Yunnanozoon lividum'' was a worm-like animal that could not be easily fit into any known type of animals. They remarked:
We are unaware of any described animal similar to ''Yunnanozoon'' gen. n., and are unable to refer it to a particular phylum. This is, however, no valid reason to regard it as presenting an unknown phylum. It may be noted that a thick cuticle with repetitive pattern is characteristic of some
aschelminth The Aschelminthes (Aeschelminthes or Nemathelminthes), closely associated with the Platyhelminthes, are an obsolete phylum of pseudocoelomate and other similar animals that are no longer considered closely related and have been promoted to phyla i ...
groups, but of virtually no other extant animals apart from
arthropods Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
.
They speculated that the animal was most related to roundworms ( aschelminthes). Four years later, Ramsköld and his team reanalysed the anatomical details and concluded that it was a chordate. The Maotianshan Shales have yielded other Cambrian chordates and chordate-related animals including ''
Cathaymyrus ''Cathaymyrus'' is a genus of Early Cambrian chordate known from the Chengjiang biota in Yunnan Province, China. Both species have a long segmented body with no distinctive head. The segments resemble v-shaped muscle blocks found in cephalochor ...
'' species (''C. diadexus'' and ''C. haikoensis''), ''
Haikouella ''Yunnanozoon lividum'' (Yunnan + Greek ζῷον ''zôion'' (animal), with species name Latin ''lividum''; (lead-coloured), referring to preserved colour of specimens) is an extinct species of bilaterian animal from the Lower Cambrian Chengj ...
'' species (''H. lanceolata'' and ''H. jianshanensis'', but possibly a type,
synonym A synonym is a word, morpheme, or phrase that means precisely or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are a ...
, of ''Yunnanozoon''), '' Myllokunmingia fengjiaoa'', ''Shankouclava anningense'', '' Zhongjianichthys rostratus, Zhongxiniscus intermedius,'' and a group called vetulicolians of uncertain classification.


Burgess Shale chordates


''Pikaia gracilens''

''Pikaia'' ''gracilens'' was a primitive chordate having a
lancelet The lancelets ( ), also known as amphioxi (: amphioxus ), consist of 32 described species of somewhat fish-like benthic filter feeding chordates in the subphylum Cephalochordata, class Leptocardii, and family Branchiostomatidae. Lancelets dive ...
-like body that lacked a well-defined head components and averaged about 3.8 cm in length. On each side of its head is a pair of large, antenna-like tentacles similar to those of snails. There are a series of short appendages on either side of the underside of the head just after the mouth, and their exact nature or function is unknown. The
pharynx The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...
is associated with six pairs of slits with tiny filaments that could be used for respiratory apparatus. A hollow tubular structure running from its anterior part of the body to the tail was earlier believed to be an indication of the presence of a
notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...
, a defining feature of all chordates from protochordates to mammals. However, a reanalysis in 2012 indicated that the hollow tube is not the notochord and instead described it as a dorsal organ, purported to be a storage tube. The notochord was reinterpreted to run underneath the dorsal organ a thin rod-like filament. The body segments are anatomically blocks of
skeletal muscles Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...
, called the
myomere Myomeres are blocks of skeletal muscle tissue arranged in sequence, commonly found in aquatic chordates. Myomeres are separated from adjacent myomeres by fascia consisting of connective tissue, known as myosepta. Myomere counts are sometimes us ...
s, which are found in
vertebrates Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...
only. No eyes can be observed. The muscle orientation and flat shaped body indicate that ''Pikaia'' was an active and free swimmer. It would have swum by throwing its body into a series of S-shaped, zigzag curves, as do living eels. However, its myomere arrangement suggests that ''Pikaia'' could not be a fast swimmer.


''Metaspriggina walcotti''

''Metaspriggina Walcotti'' is fish-like and measures up to in length and in breadth. It possesses a notochord along with seven pairs of pharyngeal bars, possibly made of
cartilage Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...
. The pharyngeal bars were formed of multiple separate pairs of bones. The first two pairs are larger than the others and do not support any gills, a characteristic that suggests a distant relationship to
gnathostomata Gnathostomata (; from Ancient Greek: (') 'jaw' + (') 'mouth') are jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all extant vertebrates, including all living bony fishes (both ray-finned ...
ns (jawed vertebrates from fish to humans). It lacked
fin A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. F ...
s and it had a weakly developed
cranium The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...
, but possessed two well-developed upward-facing eyes with
nostril A nostril (or naris , : nares ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called turbinates ...
s behind them. Unlike in ''Pikaia'' in which myomeres are numerous and V-shaped, the myomeres have a W-shaped configuration and are 40 in number.


Maotianshan Shale chordates


''Yunnanozoon lividum''

''Yunnanozoon lividum'' is small, measuring 2.5 to 4 cm long. It is described as a
deuterostome Deuterostomes (from Greek: ) are bilaterian animals of the superphylum Deuterostomia (), typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia comprises three phyla: Chordata, Echinodermata, ...
being either a
hemichordate Hemichordata ( ) is a phylum which consists of triploblastic, eucoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and incl ...
or
chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...
. An analysis in 2022 affirmed it to be one of the earliest members ( basal) of the vertebrate family tree. It has a general appearance of that of ''Pikaia''. There are no indications of structures such as a heart, gills, etc., which are seen in well-preserved specimens of ''
Haikouella ''Yunnanozoon lividum'' (Yunnan + Greek ζῷον ''zôion'' (animal), with species name Latin ''lividum''; (lead-coloured), referring to preserved colour of specimens) is an extinct species of bilaterian animal from the Lower Cambrian Chengj ...
''. An analysis in 2015 concluded that ''
Haikouella ''Yunnanozoon lividum'' (Yunnan + Greek ζῷον ''zôion'' (animal), with species name Latin ''lividum''; (lead-coloured), referring to preserved colour of specimens) is an extinct species of bilaterian animal from the Lower Cambrian Chengj ...
'' is a junior synonym of ''Yunnanozoon'', and that its two species (''H. lanceolata'' and ''H. jianshanensis'') are members of the genus ''Yunnanozoon''.


''Myllokunmingia fengjiaoa''

''Myllokunmingia fengjiaoa'' is 2.8 cm long and 0.6 cm high. It is among the oldest possible
craniate A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the mu ...
s. It appears to have a
skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...
and skeletal structures made of
cartilage Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...
. It has a distinct head and trunk with a forward sail-like dorsal fin and a
ventral fin Pelvic fins or ventral fins are paired fins located on the ventral (belly) surface of fish, and are the lower of the only two sets of paired fins (the other being the laterally positioned pectoral fins). The pelvic fins are homologous to the hind ...
fold. There are five or six gill pouches with hemibranchs on its head. 25 myomeres with rearward-facing chevrons are arranged on its trunk. It has a distinct notochord, a pharynx and a
digestive tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the Digestion, digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascula ...
that may run all the way to the rear tip of the animal. Mouth is not identified.


''Zhongjianichthys rostratus''

''Zhongjianichthys rostratus'' is generally regarded as an early fish, and therefore as one of the first vertebrates. It has an eel-like body, 1.1 cm in length, and covered with a thick
integument In biology, an integument is the tissue surrounding an organism's body or an organ within, such as skin, a husk, Exoskeleton, shell, germ or Peel (fruit), rind. Etymology The term is derived from ''integumentum'', which is Latin for "a coverin ...
. It has no visible myomeres. Its ventral fin is low and runs much of the body's length. It has reduced fins that indicates that it was mainly bottom-dwelling and did not swim much.


''Zhongxiniscus intermedius''

''Zhongxiniscus intermedius'' had a small, broad and short, fish-like body that was roughly 10 mm in length. It has S-shaped myomeres, dorsal and ventral fins. It is tentatively interpreted as an intermediate form between ''Cathaymyrus'' and two vertebrates ''Haikouichthys'' and ''Myllokunmingia.''


''Haikouichthys ercaicunensis''

''Haikouichthys ercaicunensis'' looks like ''Myllokunmingia'' with which it was discovered from the same
beds A bed is a piece of furniture that is used as a place to sleep, rest, and relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many be ...
. The body about long and is narrower than ''Myllokunmingia.'' The head and tail are distinct at the opposite ends. There are at least six to nine probable
gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...
s. The myomeres are with rear directed chevrons in the tail. It has a long
notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...
along its body length. It has a prominent dorsal fin with fin radials but not as well developed in those of
hagfish Hagfish, of the Class (biology), class Myxini (also known as Hyperotreti) and Order (biology), order Myxiniformes , are eel-shaped Agnatha, jawless fish (occasionally called slime eels). Hagfish are the only known living Animal, animals that h ...
and
lamprey Lampreys (sometimes inaccurately called lamprey eels) are a group of Agnatha, jawless fish comprising the order (biology), order Petromyzontiformes , sole order in the Class (biology), class Petromyzontida. The adult lamprey is characterize ...
s.''''


References

{{Reflist Cambrian fossil record Burgess Shale fossils Cambrian first appearances Cambrian China Maotianshan shales fossils Paleontology in Yunnan Paleontology in British Columbia