Chordate

Taxobox
name = Chordates
fossil_range = Cambrian – Recent


image_caption = Yellowfin tuna, "Thunnus albacares"
domain = Eukaryota
regnum = Animalia
subregnum = Eumetazoa
unranked_phylum = Bilateria
superphylum = Deuterostomia
phylum = Chordata
phylum_authority = Bateson, 1885
subdivision_ranks = Classes
subdivision = See below

Chordates (phylum Chordata) are a group of animals that includes the vertebrates, together with several closely related invertebrates. They are united by having, at some time in their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail. The phylum Chordata consists of three subphyla: Urochordata, represented by tunicates; Cephalochordata, represented by lancelets;and Craniata, which includes Vertebrata. The Hemichordata have been presented as a fourth chordate subphylum, but they are now usually treated as a separate phylum. Urochordate larvae have a notochord and a nerve cord but these are lost in adulthood. Cephalochordates have a notochord and a nerve cord but no brain or specialist sense organs, and a very simple circulatory system. Craniates are the only sub-phylum whose members have skulls. In all craniates except for Hagfish, the dorsal hollow nerve cord has been surrounded with cartilaginous or bony vertebrae and the notochord generally reduced; hence hagfish are not regarded as vertebrates. The chordates and three sister phyla, the Hemichordata, the Echinodermata and the Xenoturbellida, make up the deuterostomes, one of the two superphyla which encompass all fairly complex animals.

Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses, but current consensus is that chordates are monophyletic, in other words contain all and only the descendants of a single common ancestor, and that craniates' nearest relatives are cephalochordates. 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 these are vertebrates. Because the fossil record of chordates is poor, only molecular phylogenetics offers a reasonable prospect of dating their emergence. However the use of molecular phylogeneticss for dating evolutionary transitions is controversial.

It has also proved difficult to prodice a detailed classification within the living chordates. Attempts to produced evolutionary "family trees" produce results that differ from traditional classes because several of those classes are not monophyletic. As a result vertebrate classification is in a state of flux.

Definition, sub-divisions and closest relatives

Definition

Annotated image | caption=Anatomy of the cephalochordate "Amphioxus"


image-top=0 | image-left=0 | float=right
annotations=

Chordates form a phylum - a grouping of animals with a shared bodyplan [cite book
last = Valentine | first = J.W. | year = 2004 | title = On the Origin of Phyla
publisher = University Of Chicago Press | location = Chicago | id = 0226845486
page=7"Classifications of organisms in hierarchical systems were in use by the seventeenth and eighteenth centuries. Usually organisms were grouped according to their morphological similarities as perceived by those early workers, and those groups were then grouped according to their similarities, and so on, to form a hierarchy."] - defined by having at some stage in their lives all of the following:cite journal
author=Rychel, A.L., Smith, S.E., Shimamoto, H.T., and Swalla, B.J. | date=2006
title=Evolution and Development of the Chordates: Collagen and Pharyngeal Cartilage
journal=Molecular Biology and Evolution | volume 23 | issue=3 | pages=541-549 | doi=10.1093/molbev/msj055 ]
*a notochord, in other words a fairly stiff rod of cartilage that extends along the inside of the body and helps the animal to swim by flexing its tail.
*a dorsal neural tube, which develops into the spinal cord, the main commmunications trunk of the nervous system, in fish and other vertebrates
*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 other chordates they are part of a filter feeding system that extracts particles of food from the water in which the animals live.
*a muscular tail that extends backwards behind the anus.
*an endostyle. This groove in the ventral wall of the pharynx produces mucus to gather food particles, helps in transporting food to the esophagus,cite journal
author=Ruppert, E. | journal=Canadian Journal of Zoology | volume=83 | pages=8–23 | date=2005
doi: 10.1139/Z04-158
title=Key characters uniting hemichordates and chordates: homologies or homoplasies?
url=http://article.pubs.nrc-cnrc.gc.ca/RPAS/RPViewDoc?_handler_=HandleInitialGet&articleFile=z04-158.pdf&journal=cjz&volume=83
accessdate=2008-09-22 ] and stores iodine. It may be a precursor of the vertebrate thyroid gland.

ub-divisions

There are three major groupings within the chordates:

Closest non-chordate relatives

Origins

The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes, and chordates are deuterostomes.cite journal|author=Erwin, Douglas H.; Eric H. Davidson|year=2002|title=The last common bilaterian ancestor|journal=Development|volume=129|pages=3021–3032|url=http://dev.biologists.org/cgi/content/full/129/13/3021|pmid=12070079] It seems very likely that ma|555|million years old "Kimberella" was a member of the protostomes.The Rise and Fall of the Ediacaran Biota|New data on "Kimberella", the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications|Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y.|157|179|12] cite journal
author = Butterfield, N.J.
year = 2006
title = Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale
journal = Bioessays
volume = 28
issue = 12
pages = 1161–6
doi = 10.1002/bies.20507
accessdate = 2007-05-21] If so, this means that the protostome and deuterostome lineages must have split some time before "Kimberella" appeared - at least ma|558, and hence well before the start of the Cambrian ma|Cambrian. The Ediacaran fossil "Ernettia", from about ma|549|543, may represent a deuterostome animal. [cite journal
author=Dzik , J. | title=Organic membranous skeleton of the Precambrian metazoans from Namibia
journal=Geology | date=June 1999 | volume=27 | issue=6 | pages=519-522
url=http://geology.geoscienceworld.org/cgi/content/abstract/27/6/519 | accessdate=2008-09-22 "Ernettia" is from the Kuibis formation, approximate date given by cite journal
author=Waggoner, B. | title=The Ediacaran Biotas in Space and Time
journal=Integrative and Comparative Biology | date=2003 | volume=43 | issue=1 | pages=104-113| doi=10.1093/icb/43.1.104
url=http://icb.oxfordjournals.org/cgi/content/full/43/1/104 | accessdate=2008-09-22 ]

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, ma|542.citation | contribution=Early skeletal fossils
author = Bengtson, S. | editor=Lipps, J.H., and Waggoner, B.M. | title=Neoproterozoic- Cambrian Biological Revolutions
year = 2004
journal = Palentological Society Papers
volume = 10
pages = 67-78
url = http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf | accessdate=2008-07-18 ] The Mid Cambrian fossil "Rhabdotubus johanssoni" has been interpreted as a pterobranch hemichordate. [cite journal
author=Bengtson, S., and Urbanek, A. | date=October 2007
title="Rhabdotubus", a Middle Cambrian rhabdopleurid hemichordate
journal=Lethaia | volume=19 | issue=4| pages=293-308 | doi=10.1111/j.1502-3931.1986.tb00743.x
url=http://www3.interscience.wiley.com/journal/120025616/abstract | accessdate=2008-09-23 ] Opinions differ about whether the Chengjiang fauna fossil "Yunnanozoon", from the earlier Cambrian, was a hemichordate or chordate.cite journal
author=Shu, D., Zhang, X. and Chen, L. | date=April 1996
title= Reinterpretation of Yunnanozoon as the earliest known hemichordate
journal=Nature| volume=380 | pages=428-430 | doi=10.1038/380428a0
url=http://www.nature.com/nature/journal/v380/n6573/abs/380428a0.html | accessdate=2008-09-23 ] cite journal
author=Chen, J-Y., Hang, D-Y., and Li, C.W.
title=An early Cambrian craniate-like chordate
journal=Nature | volume-402 | pages=518-522 | date=December 1999 | doi=10.1038/990080
url=http://www.nature.com/nature/journal/v402/n6761/abs/402518a0.html | accessdate=2008-09-23 ] Another Chenjiang 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. "Haikouichthys" and "Myllokunmingia", also from the Chenjiang fauna, are regarded as fish.cite journal
author=Shu, D-G., Conway Morris, S., and Han, J., "et al"
title=Head and backbone of the Early Cambrian vertebrate Haikouichthys
journal=Nature | volume=421 | page=526-529 | date=January 2003 | doi=10.1038/nature01264;
url=http://www.nature.com/nature/journal/v421/n6922/abs/nature01264.html | accessdate=2008-09-21 ] [cite journal
author=Shu, D-G., Conway Morris, S., and Zhang, X-L.
title=Lower Cambrian vertebrates from south China
journal=Nature | volume=402 | Date=November 1999
url=http://www.bios.niu.edu/davis/bios458/Shu1.pdf | accessdate=2008-09-23 ] "Pikaia", discovered much earlier but from the Mid Cambrian Burgess Shale, is also regarded as a primitive chordate. [cite journal
author=Shu, D-G., Conway Morris, S., and Zhang, X-L.
title=A "Pikaia"-like chordate from the Lower Cambrian of China
journal=Nature | volume=384 | pages= 157 - 158 | date=November 1996 | doi=10.1038/384157a0
url=http://www.nature.com/nature/journal/v384/n6605/abs/384157a0.html | accessdate=2008-09-23 ] On the other hand fossils of early chordates are very rare, since non-vertebrate chordates have no bones or teeth, and none have been reported for the rest of the Cambrian.

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. Combining such analyses with data from a small set of ribosome RNA genes eliminated some older ideas, but open the possibility that tunicates (urochordates) are "basal deuterostomes", in other words surviving members of the group from which echinoderms, hemochordates and the other chordates evolved.cite journal
author=Winchell, C.J., Sullivan, J., Cameron, C.B., Swalla, B.J., and Mallatt, J.
title=Evaluating Hypotheses of Deuterostome Phylogeny and Chordate Evolution with New LSU and SSU Ribosomal DNA Data
journal=Molecular Biology and Evolution | volume=19 | pages=762-776 | date=2002
url=http://mbe.oxfordjournals.org/cgi/content/full/19/5/762#MBEV-19-05-09-SWALLA1
accessdate=2008-09-23 ] Most researchers agree that, within the chordates, craniates are most closely related to cephalochordates, but there also reasons for regarding tunicates (urochordates) as craniates' closest relatives.cite journal
author= Blair, J.E., and S. Blair Hedges, S.B.
title=Molecular Phylogeny and Divergence Times of Deuterostome Animals journal=Molecular Biology and Evolution | date=2005 | volume=22 | issue=11 | pages=2275-2284 | doi=10.1093/molbev/msi225
url=http://mbe.oxfordjournals.org/cgi/content/full/22/11/2275 | accessdate=2008-09-23 ] One other phylum, Xenoturbellida, appears to be basal within the deuterostomes, in other words closer to the original deuterostomes than to the chordates, echinoderms and hemichordates.Perseke M, Hankeln T, Weich B, Fritzsch G, Stadler PF, Israelsson O, Bernhard D, Schlegel M. (2007) "The mitochondrial DNA of "Xenoturbella bocki": genomic architecture and phylogenetic analysis". "Theory Biosci". 126(1):35-42. Available on-line at [http://www.bioinf.uni-leipzig.de/Publications/PREPRINTS/07-009.pdf] ]

Since chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by molecular phylogenetics techniques, in other words by analysing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before ma|900 and the earliest chordates around ma|896. However molecular estimates of dates often disagree with each other and with the fossil record, and their assumption that the molecular clock runs at a known constant rate has been challenged.cite journal |author= Ayala, F.J.|year=1999 |title=Molecular clock mirages |journal=BioEssays |volume=21|issue=1 |pages=71–75|url=http://www3.interscience.wiley.com/cgi-bin/abstract/60000186/ABSTRACT?CRETRY=1&SRETRY=0 |doi=10.1002/(SICI)1521-1878(199901)21:1<71::AID-BIES9>3.0.CO;2-B ] cite journal |author=Schwartz, J. H. and Maresca, B. |year=2006 |title=Do Molecular Clocks Run at All? A Critique of Molecular Systematics |journal=Biological Theory |volume=1 |pages=357–371|url= |doi=10.1162/biot.2006.1.4.357]

Classification

Taxonomy

The following schema is from the third edition of "Vertebrate Palaeontology". [Benton, M.J. (2004). "Vertebrate Palaeontology", Third Edition. Blackwell Publishing, 472 pp. [http://palaeo.gly.bris.ac.uk/benton/vertclass.html The classification scheme is available online] ] 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 Tunicata (Urochordata)— (tunicates, 3,000 species)
** Subphylum Cephalochordata (Acraniata)— (lancelets, 30 species)
** Subphylum Vertebrata (Craniata) (vertebrates — animals with backbones; 57,674 species)
*** Class 'Agnatha'Paraphyletic (jawless vertebrates; 100+ species)
**** Subclass Myxinoidea (hagfish; 65 species)
**** Subclass Petromyzontida (Lampreys)
**** Subclass Conodonta
**** Subclass Pteraspidomorphi (Paleozoic jawless fish)
***** Order Anaspida
***** Order Thelodonti (Paleozoic jawless fish)
*** Infraphylum Gnathostomata (jawed vertebrates)
**** Class Placodermi (Paleozoic armoured forms)
**** Class Chondrichthyes (cartilaginous fish; 900+ species)
**** Class Acanthodii (Paleozoic "spiny sharks")
**** Class Osteichthyes (bony fishes; 30,000+ species)
***** Subclass Actinopterygii (ray-finned fish; about 30,000 species)
***** Subclass Sarcopterygii (lobe-finned fish)
**** Superclass Tetrapoda (four-legged vertebrates; 18,000+ species)
***** Class Amphibia (amphibians; 6,000 species)
***** Series Amniota (with amniotic egg)
****** Class Sauropsida — (reptiles; 8,225+ species)
****** Class Aves (birds; 8,800–10,000 species)
****** Class Synapsida (mammal-like "reptiles"; 4,500+ species)
****** Class Mammalia (mammals; 5,800 species)

Phylogeny

clade| style=font-size:85%;line-height:85%
label1=Chordata
1=clade
label1= Cephalochordata
1=clade
1= Amphioxus
label2=
2=clade
label1=Tunicata
1=clade
1= Appendicularia (formerly Larvacea)
2= Thaliacea
3= Ascidiacea
label2= Craniata
2=clade
1=Myxini
label2= Vertebrata
2=clade
1= Conodonta†
2= Cephalaspidomorphi†
3= Hyperoartia
4= Pteraspidomorphi†
label5= Gnathostomata
5=clade
1= Placodermi†
2= Chondrichthyes
label3= Teleostomi
3=clade
1= Acanthodii†
label2= Osteichthyes
2=clade
1= Actinopterygii
label2= Sarcopterygii
2=clade
label1=void
1=clade
label1= Tetrapoda
1=clade
1= Amphibia
label2= Amniota
2=clade
label1= Synapsida
1=clade
label1=void
1=clade
1= Mammalia
label2= Sauropsida
2=clade
label1=void
1=clade
1= Aves

Notes:
* Lines show probable evolutionary relationships, including extinct taxa, which are denoted with a dagger, †. Some are invertebrates. Chordata is a phylum.
* The positions (relationships) of the Lancelet, Tunicate, and Craniata clades are as reported [The amphioxus genome and the evolution of the chordate karyotype, Nicholas H. Putnam, et al. Nature vol 453 p. 1064-1071, June 19, 2000] in the scientific journal "Nature".

References

External links

* [http://www.globaltwitcher.com/taxa_class.asp?phylaid=1 Chordate on GlobalTwitcher.com]
* [http://tolweb.org/Chordata/2499 Chordate node at Tree Of Life]
* [http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=7711 Chordate node at NCBI Taxonomy]