Biology:Chordate

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Short description: Phylum of animals having a dorsal nerve cord

Chordates
Temporal range: Cambrian Stage 3–Present, 525–0 Ma[1]
(Possible Ediacaran record, 555 Ma[2])
Chordata.png
Example of chordates: Branchiostoma lanceolatum (Cephalochordata), Polycarpa aurata (Tunicata), as well as a tiger shark and a Siberian tiger (Vertebrata).
Scientific classification e
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Haeckel, 1874[3][4]
Subgroups

And see text

A chordate (/ˈkɔːrdeɪt/ Template:Respelling) is a deuterostomic animal belonging to the phylum Chordata (/kɔːrˈdeɪtə/ Template:Respelling). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail. The name "chordate" comes from the first of these synapomorphies, the notochord, which plays a significant role in chordate body plan structuring and movements. Chordates are also bilaterally symmetric, have a coelom, possess a closed circulatory system, and exhibit metameric segmentation.

In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two conserved signature indels (CSIs) in their proteins: cyclophilin-like protein and inner mitochondrial membrane protease ATP23, which are exclusively shared by all vertebrates, tunicates and cephalochordates.[5] These CSIs provide molecular means to reliably distinguish chordates from all other metazoans.

Chordates are divided into three subphyla: Craniata or Vertebrata (fish, amphibians, reptiles, birds and mammals); Tunicata or Urochordata (sea squirts, salps and relatives, and larvaceans); and Cephalochordata (lancelets). The Craniata and Tunicata compose the clade Olfactores, which is sister to Cephalochordata (see diagram under Phylogeny). Extinct taxa such as Vetulicolia and Conodonta are Chordata, but their internal placement is less certain. Hemichordata (which includes the acorn worms) was previously considered a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. The Chordata and Ambulacraria, together and possibly with the Xenacoelomorpha, are believed to form the superphylum Deuterostomia, although this has recently been called into doubt.[6]

Chordate fossils have been found from as early as the Cambrian explosion, 539 million years ago.[7] Cladistically (phylogenetically), vertebrates – chordates with the notochord replaced by a vertebral column during development – are a subgroup of the clade Craniata, which consists of chordates with a skull. Of the more than 81,000[8] living species of chordates, about half are ray-finned fishes that are members of the class Actinopterygii and the vast majority of the rest are tetrapods (mostly birds and mammals).

Anatomy

The glass catfish (Kryptopterus vitreolus) is one of the few chordates with a visible backbone. The spinal cord is housed within its backbone.

Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features:[9]

  • A notochord, a stiff but elastic rod of glycoprotein wrapped in two collagen helices, which extends along the central axis of the body. Among members of the subphylum Vertebrata (vertebrates), the notochord gets replaced by hyaline cartilage or osseous tissue of the spine, and notochord remnants develop into the intervertebral discs, which allow adjacent spinal vertebrae to bend and twist relative to each other. In wholly aquatic species, this helps the animal swim efficiently by flexing its tail side-to-side.
  • A hollow dorsal nerve cord, also known as the neural tube, which develops into the spinal cord, the main communications trunk of the nervous system. In vertebrates, the rostral end of the neural tube enlarges into several vesicles, which give rise to the brain during embryonic development.
  • Pharyngeal slits. The pharynx is the part of the throat immediately behind the mouth. In fish, the slits are modified to form gills, but in some other chordates they are part of a filter-feeding system that extracts food particles from ingested water. In tetrapods, they are only present during embryonic stages of the development.
  • A post-anal tail. A muscular tail that extends backwards behind the anus. In some chordates such as humans, this is only present in the embryonic stage.
  • An endostyle. This is a groove in the ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus.[10] It also stores iodine, and may be a precursor of the vertebrate thyroid gland.[9]

There are soft constraints that separate chordates from other biological lineages, but are not part of the formal definition:

  • All chordates are deuterostomes. This means that, during the embryo development stage, the anus forms before the mouth.
  • All chordates are based on a bilateral body plan.[11]
  • All chordates are coelomates, and have a fluid-filled body cavity called a coelom with a complete lining called peritoneum derived from mesoderm (see Brusca and Brusca).[12]
1 = bulge in spinal cord ("brain")
2 = notochord
4 = post-anal tail
5 = anus
6 = digestive canal
9 = space above pharynx
11 = pharynx
12 = vestibule
13 = oral cirri
14 = mouth opening
15 = gonads (ovary / testicle)
16 = light sensor
17 = nerves
18 = metapleural fold
19 = hepatic caecum (liver-like sack)
Anatomy of the cephalochordate Amphioxus. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.

Classification

The following schema is from the 2015 edition of Vertebrate Palaeontology.[13][14] The invertebrate chordate classes are from Fishes of the World.[15] While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy.

  • Phylum Chordata
    • Subphylum Cephalochordata (Acraniata) – (lancelets; 30 species)
      • Class Leptocardii (lancelets)
    • Clade Olfactores
      • Subphylum Tunicata (Urochordata) – (tunicates; 3,000 species)
      • Subphylum Vertebrata (Craniata) (vertebrates – animals with backbones; 66,100+ species)
        • Superclass 'Agnatha' paraphyletic (jawless vertebrates; 100+ species)
        • Infraphylum Gnathostomata (jawed vertebrates)
          • Class †Placodermi (Paleozoic armoured forms; paraphyletic in relation to all other gnathostomes)
          • Class Chondrichthyes (cartilaginous fish; 900+ species)
          • Class †Acanthodii (Paleozoic "spiny sharks"; paraphyletic in relation to Chondrichthyes)
          • Class Osteichthyes (bony fish; 30,000+ species)
          • Superclass Tetrapoda (four-limbed vertebrates; 35,100+ species) (The classification below follows Benton 2004, and uses a synthesis of rank-based Linnaean taxonomy and also reflects evolutionary relationships. Benton included the Superclass Tetrapoda in the Subclass Sarcopterygii in order to reflect the direct descent of tetrapods from lobe-finned fish, despite the former being assigned a higher taxonomic rank.)[16]

Subphyla

Cephalochordate: lancelet. Pictured species: Branchiostoma lanceolatum

Cephalochordata: Lancelets

Main page: Biology:Lancelet

Cephalochordates, one of the three subdivisions of chordates, are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs.[21] These burrowing filter-feeders compose the earliest-branching chordate subphylum.[22][23]

Tunicata (Urochordata)

Tunicates: sea squirts

Most tunicates appear as adults in two major forms, known as "sea squirts" and salps, both of which are soft-bodied filter-feeders that lack the standard features of chordates. Sea squirts are sessile and consist mainly of water pumps and filter-feeding apparatus;[24] salps float in mid-water, feeding on plankton, and have a two-generation cycle in which one generation is solitary and the next forms chain-like colonies.[25] However, all tunicate larvae have the standard chordate features, including long, tadpole-like tails; they also have rudimentary brains, light sensors and tilt sensors.[24] The third main group of tunicates, Appendicularia (also known as Larvacea), retain tadpole-like shapes and active swimming all their lives, and were for a long time regarded as larvae of sea squirts or salps.[26] The etymology of the term Urochordata (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail.[27] The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.[24]

Comparison of two invertebrate chordates
A. Lancelet, B. Larval tunicate, C. Adult tunicate
--------------------------------------------------------
1. Notochord, 2. Nerve chord, 3. Buccal cirri, 4. Pharynx, 5. Gill slit, 6. Gonad, 7. Gut, 8. V-shaped muscles, 9. Anus, 10. Inhalant syphon, 11. Exhalant syphon, 12. Heart, 13. Stomach, 14. Esophagus, 15. Intestines, 16. Tail, 17. Atrium, 18. Tunic

Craniata (Vertebrata)

Craniate: Hagfish

Craniates all have distinct skulls. They include the hagfish, which have no vertebrae. Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or possibly all deuterostomes, are by their tails".[28]

Most craniates are vertebrates, in which the notochord is replaced by the vertebral column.[29] These consist of a series of bony or cartilaginous cylindrical vertebrae, generally with neural arches that protect the spinal cord, and with projections that link the vertebrae. However hagfish have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates,[30] but as members of the craniates, the group from which vertebrates are thought to have evolved.[31] However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may be degenerate vertebrates who have lost their vertebral columns.[32]

The position of lampreys is ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish.[33] However, molecular phylogenetics, which uses biochemical features to classify organisms, has produced both results that group them with vertebrates and others that group them with hagfish.[34] If lampreys are more closely related to the hagfish than the other vertebrates, this would suggest that they form a clade, which has been named the Cyclostomata.[35]

Phylogeny

Overview

Haikouichthys, from about 518 million years ago in China, may be the earliest known fish.[36]

There is still much ongoing differential (DNA sequence based) comparison research that is trying to separate out the simplest forms of chordates. As some lineages of the 90% of species that lack a backbone or notochord might have lost these structures over time, this complicates the classification of chordates. Some chordate lineages may only be found by DNA analysis, when there is no physical trace of any chordate-like structures.[37]

Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses. The current consensus is that chordates are monophyletic, meaning that the Chordata include all and only the descendants of a single common ancestor, which is itself a chordate, and that craniates' nearest relatives are tunicates. Recent identification of two conserved signature indels (CSIs) in the proteins cyclophilin-like protein and mitochondrial inner membrane protease ATP23, which are exclusively shared by all vertebrates, tunicates and cephalochordates also provide strong evidence of the monophyly of Chordata.[5]

All of the earliest chordate fossils have been found in the Early Cambrian Chengjiang fauna, and include two species that are regarded as fish, which implies that they are vertebrates. Because the fossil record of early chordates is poor, only molecular phylogenetics offers a reasonable prospect of dating their emergence. However, the use of molecular phylogenetics for dating evolutionary transitions is controversial.

It has also proved difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "family trees" shows that many of the traditional classes are paraphyletic.

Deuterostomes
 
Ambulacraria
 

Hemichordates Ptychodera flava in New Caledonia, Encyclopaedia Britannica (1911) rotated.png

 
 

Echinoderms Starfish (PSF).png

 
 
Chordates
 

Cephalochordates Branchiostoma cultellus.jpg

 
Olfactores
 

Tunicates PSM V44 D080 Salpa.jpg

 
 

Craniates (vertebrates) Eptatretus cirrhatus (New Zealand hagfish).gif

 
 
 
 
 
Diagram of the evolutionary relationships of chordates[10]

While this has been well known since the 19th century, an insistence on only monophyletic taxa has resulted in vertebrate classification being in a state of flux.[38]

The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes, the latter of which contains chordates.[39] It seems very likely the 555 million-year-old Kimberella was a member of the protostomes.[40][41] If so, this means the protostome and deuterostome lineages must have split some time before Kimberella appeared—at least 558 million years ago, and hence well before the start of the Cambrian 541 million years ago.[39] The Ediacaran fossil Ernietta, from about 549 to 543 million years ago, may represent a deuterostome animal.[42]

A skeleton of the blue whale, the largest animal, extant or extinct, ever discovered. Mounted outside the Long Marine Laboratory at the University of California, Santa Cruz. The largest blue whale ever reliably recorded measured 98ft (30m) long.
A peregrine falcon, the world's fastest animal. Peregrines use gravity and aerodynamics to achieve their top speed of around 242mph (390km/h), as opposed to locomotion.

Fossils of one major deuterostome group, the echinoderms (whose modern members include starfish, sea urchins and crinoids), are quite common from the start of the Cambrian, 542 million years ago.[43] The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate.[44] Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate.[45][46] Another fossil, Haikouella lanceolata, also from the Chengjiang fauna, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes—although it also had short tentacles round its mouth.[46] Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish.[36][47] Pikaia, discovered much earlier (1911) but from the Mid Cambrian Burgess Shale (505 Ma), is also regarded as a primitive chordate.[48] On the other hand, fossils of early chordates are very rare, since invertebrate chordates have no bones or teeth, and only one has been reported for the rest of the Cambrian.[49]

The evolutionary relationships between the chordate groups and between chordates as a whole and their closest deuterostome relatives have been debated since 1890. Studies based on anatomical, embryological, and paleontological data have produced different "family trees". Some closely linked chordates and hemichordates, but that idea is now rejected.[10] Combining such analyses with data from a small set of ribosome RNA genes eliminated some older ideas, but opened up the possibility that tunicates (urochordates) are "basal deuterostomes", surviving members of the group from which echinoderms, hemichordates and chordates evolved.[50] Some researchers believe that, within the chordates, craniates are most closely related to cephalochordates, but there are also reasons for regarding tunicates (urochordates) as craniates' closest relatives.[10][51]

Since early chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by molecular phylogenetics techniques—by analyzing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before 900 million years ago and the earliest chordates around 896 million years ago.[51] However, molecular estimates of dates often disagree with each other and with the fossil record,[51] and their assumption that the molecular clock runs at a known constant rate has been challenged.[52][53]

Traditionally, Cephalochordata and Craniata were grouped into the proposed clade "Euchordata", which would have been the sister group to Tunicata/Urochordata. More recently, Cephalochordata has been thought of as a sister group to the "Olfactores", which includes the craniates and tunicates. The matter is not yet settled.

A specific relationship between Vertebrates and Tunicates is also strongly supported by two CSIs found in the proteins predicted exosome complex RRP44 and serine palmitoyltransferase, that are exclusively shared by species from these two subphyla but not Cephalochordates, indicating Vertebrates are more closely related to Tunicates than Cephalochordates.[5]

Cladogram

Phylogenetic tree of the chordate phylum. Lines of the cladogram show probable evolutionary relationships between both extinct taxa, which are denoted with a dagger (†), and extant taxa. Relatives of vertebrates are invertebrates. The positions (relationships) of the lancelets, tunicates, and craniates/vertebrates are based on the following studies:[54][55][56][57]

Chordata
Cephalochordata
 

Amphioxiformes (lancelets) Branchiostoma lanceolatum (Amphioxus lanceolatus).png

 
 
Olfactores
 

†Haikouella

 
Tunicata
 

Appendicularia (formerly Larvacea)

 
 

"Ascidiacea" (polyphyletic; sea squirts) Natural History - Mollusca - Ascidia virginea.png

 
 

Thaliacea (salps) PSM V44 D080 Salpa.jpg

 
 
Vertebrata/
 
Cyclostomata
 

Myxini (hagfishes) Myxine glutinosa Gervais.jpg

 
 

Hyperoartia/Petromyzontida (lampreys) Flussneunauge.jpg

 
(Agnathans)
 
 
 

†Conodonta ConodontZICA.png

 
 

†Pteraspidomorphi (includes †Arandaspida, †Astraspida and †Heterostraci) Astraspis desiderata.png

 
 

†Cephalaspidomorphi (includes †Galeaspida, †Osteostraci and †Pituriaspida) Tremataspis NT small.jpg

 
 
Gnathostomata
 

†"Placodermi" (paraphyletic; includes †Antiarchi, †Petalichthyida, †Ptyctodontida and †Arthrodira) D Terrelli.png

 
Crown
 
 

†"Acanthodii" (paraphyletic) Acanthodes BW.jpg

 
 

Chondrichthyes/Cartilaginous fishes (sharks, rays, rat fish) White shark (Duane Raver).png

 
 
Euteleostomi/
 

Actinopterygii (ray-finned fishes) Common carp (white background).jpg

 
 
Sarcopterygii
 

Actinistia (coelacanths) Coelacanth flipped.png

 
 
 

Dipnoi (lungfishes) Chinle fish Arganodus cropped cropped.png

 
 
 Tetrapoda 
 

 Amphibia Salamandra salamandra (white background).jpg

 
 Amniota 
 Synapsida 

 Mammalia Phylogenetic tree of marsupials derived from retroposon data (Paucituberculata).png

 
 Sauropsida 
 
 

 Lepidosauromorpha (lizards, snakes, tuatara, and their extinct relatives) British reptiles, amphibians, and fresh-water fishes (1920) (Lacerta agilis).jpg

 
 
 
 

 Archosauromorpha (crocodiles, birds, and their extinct relatives) Deinosuchus riograndensis.png

 
 
 
 

  Testudinata (turtles and their extinct relatives)

 
 
 
 
(four‑limbed vertebrates)
 
 
(lobe‑finned fishes)
 
Osteichthyes
Gnathostomata
 
 
 
Craniata
 
 

Closest nonchordate relatives

Acorn worms or Enteropneusts are example of hemichordates.

The closest relatives of the Chordates are believed to be the Hemichordates and Echinodermata, which together form the Ambulacraria. The Chordata and Ambulacraria together form the superphylum Deuterostomia.

Hemichordates

Main page: Biology:Hemichordate

Hemichordates ("half chordates") have some features similar to those of chordates: branchial openings that open into the pharynx and look rather like gill slits; stomochords, similar in composition to notochords, but running in a circle round the "collar", which is ahead of the mouth; and a dorsal nerve cord—but also a smaller ventral nerve cord.

There are two living groups of hemichordates. The solitary enteropneusts, commonly known as "acorn worms", have long proboscises and worm-like bodies with up to 200 branchial slits, are up to 2.5 metres (8.2 ft) long, and burrow though seafloor sediments. Pterobranchs are colonial animals, often less than 1 millimetre (0.039 in) long individually, whose dwellings are interconnected. Each filter feeds by means of a pair of branched tentacles, and has a short, shield-shaped proboscis. The extinct graptolites, colonial animals whose fossils look like tiny hacksaw blades, lived in tubes similar to those of pterobranchs.[58]

Echinoderms

A red knob sea star, Protoreaster linckii is an example of Asterozoan Echinoderm.
Main page: Biology:Echinoderm

Echinoderms differ from chordates and their other relatives in three conspicuous ways: they possess bilateral symmetry only as larvae – in adulthood they have radial symmetry, meaning that their body pattern is shaped like a wheel; they have tube feet; and their bodies are supported by skeletons made of calcite, a material not used by chordates. Their hard, calcified shells keep their bodies well protected from the environment, and these skeletons enclose their bodies, but are also covered by thin skins. The feet are powered by another unique feature of echinoderms, a water vascular system of canals that also functions as a "lung" and surrounded by muscles that act as pumps. Crinoids look rather like flowers, and use their feather-like arms to filter food particles out of the water; most live anchored to rocks, but a few can move very slowly. Other echinoderms are mobile and take a variety of body shapes, for example starfish, sea urchins and sea cucumbers.[59]

History of name

Although the name Chordata is attributed to William Bateson (1885), it was already in prevalent use by 1880. Ernst Haeckel described a taxon comprising tunicates, cephalochordates, and vertebrates in 1866. Though he used the German vernacular form, it is allowed under the ICZN code because of its subsequent latinization.[4]

See also

References

  1. ↑ Yang, Chuan; Li, Xian-Hua; Zhu, Maoyan; Condon, Daniel J.; Chen, Junyuan (2018). "Geochronological constraint on the Cambrian Chengjiang biota, South China" (in en). Journal of the Geological Society 175 (4): 659–666. doi:10.1144/jgs2017-103. ISSN 0016-7649. Bibcode: 2018JGSoc.175..659Y. http://nora.nerc.ac.uk/id/eprint/521412/1/2018-JGS-Chuan%20Yang%20et%20al.pdf. 
  2. ↑ Fedonkin, M. A.; Vickers-Rich, P.; Swalla, B. J.; Trusler, P.; Hall, M. (2012). "A new metazoan from the Vendian of the White Sea, Russia, with possible affinities to the ascidians". Paleontological Journal 46 (1): 1–11. doi:10.1134/S0031030112010042. Bibcode: 2012PalJ...46....1F. 
  3. ↑ Haeckel, E. (1874). Anthropogenie oder Entwicklungsgeschichte des Menschen. Leipzig: Engelmann.
  4. ↑ 4.0 4.1 Nielsen, C. (July 2012). "The authorship of higher chordate taxa". Zoologica Scripta 41 (4): 435–436. doi:10.1111/j.1463-6409.2012.00536.x. 
  5. ↑ 5.0 5.1 5.2 Gupta, Radhey S. (January 2016). "Molecular signatures that are distinctive characteristics of the vertebrates and chordates and supporting a grouping of vertebrates with the tunicates". Molecular Phylogenetics and Evolution 94 (Pt A): 383–391. doi:10.1016/j.ympev.2015.09.019. ISSN 1055-7903. PMID 26419477. 
  6. ↑ Kapli, Paschalia; Natsidis, Paschalis; Leite, Daniel J.; Fursman, Maximilian; Jeffrie, Nadia; Rahman, Imran A.; Philippe, Hervé; Copley, Richard R. et al. (2021-03-19). "Lack of support for Deuterostomia prompts reinterpretation of the first Bilateria" (in en). Science Advances 7 (12): eabe2741. doi:10.1126/sciadv.abe2741. ISSN 2375-2548. PMID 33741592. Bibcode: 2021SciA....7.2741K. 
  7. ↑ "Stratigraphic Chart 2022". International Stratigraphic Commission. February 2022. https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf. 
  8. ↑ "Chordates". https://eol.org/pages/694#:~:text=Chordata%20(Chordates)%20is%20a%20phylum,Vertebrates%2C%20Cephalochordates%2C%20and%20Tunicates.. 
  9. ↑ 9.0 9.1 Rychel, A.L.; Smith, S.E.; Shimamoto, H.T.; Swalla, B.J. (March 2006). "Evolution and Development of the Chordates: Collagen and Pharyngeal Cartilage". Molecular Biology and Evolution 23 (3): 541–549. doi:10.1093/molbev/msj055. PMID 16280542. 
  10. ↑ 10.0 10.1 10.2 10.3 Ruppert, E. (January 2005). "Key characters uniting hemichordates and chordates: homologies or homoplasies?". Canadian Journal of Zoology 83: 8–23. doi:10.1139/Z04-158. http://article.pubs.nrc-cnrc.gc.ca/RPAS/RPViewDoc?_handler_=HandleInitialGet&articleFile=z04-158.pdf&journal=cjz&volume=83. Retrieved 2008-09-22. 
  11. ↑ Valentine, J.W. (2004). On the Origin of Phyla. Chicago: University of Chicago Press. p. 7. ISBN 978-0-226-84548-7. 
  12. ↑ R.C.Brusca, G.J.Brusca. Invertebrates. Sinauer Associates, Sunderland Mass 2003 (2nd ed.), p. 47, ISBN:0-87893-097-3.
  13. ↑ Benton, M.J. (2004). Vertebrate Palaeontology, Third Edition. Blackwell Publishing. The classification scheme is available online
  14. ↑ Benton, Michael J. (2014). Vertebrate Palaeontology (4th ed.). John Wiley & Sons. ISBN 978-1-118-40764-6. https://books.google.co.uk/books?id=qak-BAAAQBAJ&pg=PP1. 
  15. ↑ Nelson, J. S. (2006). Fishes of the World (4th ed.). New York: John Wiley and Sons, Inc. ISBN 978-0-471-25031-9. 
  16. ↑ Benton, M.J. (2004). Vertebrate Paleontology. 3rd ed.. Blackwell Science Ltd. 
  17. ↑ Frost, Darrel R.. "ASW Home". American Museum of Natural History, New York. http://research.amnh.org/herpetology/amphibia/index.html. 
  18. ↑ "Reptiles face risk of extinction". 15 February 2013. https://www.bbc.co.uk/nature/21458115. 
  19. ↑ "New Study Doubles the Estimate of Bird Species in the World". Amnh.org. https://www.amnh.org/about-the-museum/press-center/new-study-doubles-the-estimate-of-bird-species-in-the-world. 
  20. ↑ "Species Statistics Aug 2019". http://www.reptile-database.org/db-info/SpeciesStat.html. 
  21. ↑ Benton, M.J. (14 April 2000). Vertebrate Palaeontology: Biology and Evolution. Blackwell Publishing. p. 6. ISBN 978-0-632-05614-9. https://books.google.com/books?id=PQuKO7xqjNQC&q=vertebrate. Retrieved 2008-09-22. 
  22. ↑ Gee, H. (19 June 2008). "Evolutionary biology: The amphioxus unleashed". Nature 453 (7198): 999–1000. doi:10.1038/453999a. PMID 18563145. Bibcode: 2008Natur.453..999G. 
  23. ↑ "Branchiostoma". Lander University. http://lanwebs.lander.edu/faculty/rsfox/invertebrates/branchiostoma.html. 
  24. ↑ 24.0 24.1 24.2 Benton, M.J. (14 April 2000). Vertebrate Palaeontology: Biology and Evolution. Blackwell Publishing. p. 5. ISBN 978-0-632-05614-9. https://books.google.com/books?id=PQuKO7xqjNQC&q=vertebrate. 
  25. ↑ "Animal fact files: salp". BBC. https://www.bbc.co.uk/nature/blueplanet/factfiles/jellies/salp_bg.shtml. 
  26. ↑ "Appendicularia". Australian Government Department of the Environment, Water, Heritage and the Arts. http://www.environment.gov.au/biodiversity/abrs/publications/electronic-books/pubs/tunicates/05-appendicularia.pdf. 
  27. ↑ Oxford English Dictionary, Third Edition, January 2009: Urochordata
  28. ↑ Benton, M.J. (14 April 2000). Vertebrate Palaeontology: Biology and Evolution. Blackwell Publishing. pp. 12–13. ISBN 978-0-632-05614-9. https://books.google.com/books?id=PQuKO7xqjNQC&q=vertebrate. Retrieved 2008-09-22. 
  29. ↑ "Morphology of the Vertebrates". University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/vertebrates/vertmm.html. 
  30. ↑ "Introduction to the Myxini". University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html. 
  31. ↑ Campbell, N.A.; Reece, J.B. (2005). Biology (7th ed.). San Francisco, CA: Benjamin Cummings. ISBN 978-0-8053-7171-0. 
  32. ↑ Janvier, P. (2010). "MicroRNAs revive old views about jawless vertebrate divergence and evolution". Proceedings of the National Academy of Sciences 107 (45): 19137–19138. doi:10.1073/pnas.1014583107. PMID 21041649. Bibcode: 2010PNAS..10719137J. "Although I was among the early supporters of vertebrate paraphyly, I am impressed by the evidence provided by Heimberg et al. and prepared to admit that cyclostomes are, in fact, monophyletic. The consequence is that they may tell us little, if anything, about the dawn of vertebrate evolution, except that the intuitions of 19th century zoologists were correct in assuming that these odd vertebrates (notably, hagfishes) are strongly degenerate and have lost many characters over time". 
  33. ↑ "Introduction to the Petromyzontiformes". University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/vertebrates/basalfish/petro.html. 
  34. ↑ Shigehiro Kuraku, S.; Hoshiyama, D.; Katoh, K.; Suga, H; Miyata, T. (December 1999). "Monophyly of Lampreys and Hagfishes Supported by Nuclear DNA-Coded Genes". Journal of Molecular Evolution 49 (6): 729–735. doi:10.1007/PL00006595. PMID 10594174. Bibcode: 1999JMolE..49..729K. 
  35. ↑ Delabre, Christiane (2002). "Complete Mitochondrial DNA of the Hagfish, Eptatretus burgeri: The Comparative Analysis of Mitochondrial DNA Sequences Strongly Supports the Cyclostome Monophyly". Molecular Phylogenetics and Evolution 22 (2): 184–192. doi:10.1006/mpev.2001.1045. PMID 11820840. 
  36. ↑ 36.0 36.1 Shu, D-G.; Conway Morris, S.; Han, J. (January 2003). "Head and backbone of the Early Cambrian vertebrate Haikouichthys". Nature 421 (6922): 526–529. doi:10.1038/nature01264. PMID 12556891. Bibcode: 2003Natur.421..526S. 
  37. ↑ Josh Gabbatiss (15 August 2016), Why we have a spine when over 90% of animals don't, BBC, http://www.bbc.com/earth/story/20160812-why-we-have-a-spine-when-over-90-of-animals-dont 
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  39. ↑ 39.0 39.1 Erwin, Douglas H.; Eric H. Davidson (1 July 2002). "The last common bilaterian ancestor". Development 129 (13): 3021–3032. doi:10.1242/dev.129.13.3021. PMID 12070079. http://dev.biologists.org/cgi/content/full/129/13/3021. 
  40. ↑ {{{1}}} (2007), "{{{2}}}", in Vickers-Rich, Patricia; Komarower, Patricia, The Rise and Fall of the Ediacaran Biota, Special publications, 286, London: Geological Society, pp. {{{3}}}–{{{4}}}, doi:10.1144/SP286.{{{5}}}, ISBN 9781862392335, OCLC 156823511 
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  42. ↑ Dzik, J. (June 1999). "Organic membranous skeleton of the Precambrian metazoans from Namibia". Geology 27 (6): 519–522. doi:10.1130/0091-7613(1999)027<0519:OMSOTP>2.3.CO;2. Bibcode: 1999Geo....27..519D. Ernettia is from the Kuibis formation, approximate date given by Waggoner, B. (2003). "The Ediacaran Biotas in Space and Time". Integrative and Comparative Biology 43 (1): 104–113. doi:10.1093/icb/43.1.104. PMID 21680415. 
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  44. ↑ Bengtson, S.; Urbanek, A. (October 2007). "Rhabdotubus, a Middle Cambrian rhabdopleurid hemichordate". Lethaia 19 (4): 293–308. doi:10.1111/j.1502-3931.1986.tb00743.x. http://www3.interscience.wiley.com/journal/120025616/abstract. 
  45. ↑ Shu, D.; Zhang, X.; Chen, L. (April 1996). "Reinterpretation of Yunnanozoon as the earliest known hemichordate". Nature 380 (6573): 428–430. doi:10.1038/380428a0. Bibcode: 1996Natur.380..428S. 
  46. ↑ 46.0 46.1 Chen, J-Y.; Hang, D-Y.; Li, C.W. (December 1999). "An early Cambrian craniate-like chordate". Nature 402 (6761): 518–522. doi:10.1038/990080. Bibcode: 1999Natur.402..518C. 
  47. ↑ Shu, D-G.; Conway Morris, S.; Zhang, X-L. (November 1999). "Lower Cambrian vertebrates from south China". Nature 402 (6757): 42. doi:10.1038/46965. Bibcode: 1999Natur.402...42S. http://www.bios.niu.edu/davis/bios458/Shu1.pdf. Retrieved 2008-09-23. 
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  49. ↑ Conway Morris, S. (2008). "A Redescription of a Rare Chordate, Metaspriggina walcotti Simonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada". Journal of Paleontology 82 (2): 424–430. doi:10.1666/06-130.1. Bibcode: 2008JPal...82..424M. http://jpaleontol.geoscienceworld.org/cgi/content/extract/82/2/424. Retrieved 2009-04-28. 
  50. ↑ Winchell, C. J.; Sullivan, J.; Cameron, C. B.; Swalla, B. J.; Mallatt, J. (1 May 2002). "Evaluating Hypotheses of Deuterostome Phylogeny and Chordate Evolution with New LSU and SSU Ribosomal DNA Data". Molecular Biology and Evolution 19 (5): 762–776. doi:10.1093/oxfordjournals.molbev.a004134. PMID 11961109. 
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  56. ↑ "A phylogenomic framework and timescale for comparative studies of tunicates". BMC Biology 16 (1): 39. April 2018. doi:10.1186/s12915-018-0499-2. PMID 29653534. 
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  59. ↑ Cowen, R. (2000). History of Life (3rd ed.). Blackwell Science. p. 412. ISBN 978-0-632-04444-3. 

External links

Wikidata ☰ Q10915 entry



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