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Chordate Sunday, June 28, 2009

Chordates
Fossil range: Early Cambrian – Recent, 530–0 Ma
An 'X-ray fish' (Pristella tetra), its translucent body making visible the nerve cord that defines the chordates
An 'X-ray fish' (Pristella tetra), its translucent body making visible the nerve cord that defines the chordates
Scientific classification
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
(unranked): Bilateria
Superphylum: Deuterostomia
Phylum: Chordata
Bateson, 1885
Classes

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 that encompass all fairly complex animals.

Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses, but the current consensus is that chordates are monophyletic, in other words contain all and only the descendants of a single common ancestor which is itself a chordate, 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 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" give 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

Anatomy of the cephalochordate Amphioxus
1 = bulge in spinal cord ("brain")
4 = post-anal tail
5 = anus
9 = space above pharynx


11 = pharynx
13 = oral cirri
14 = mouth opening
16 = light sensor
17 = nerves
19 = hepatic caecum (liver-like sack)
Anatomy of the cephalochordate Amphioxus

Chordates form a phylum — a grouping of animals with a shared body plan[1] — defined by having at some stage in their lives all of the following:[2]

  • a notochord, in other words a fairly stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine, and in wholly aquatic species this helps the animal to swim by flexing its tail.
  • a dorsal neural tube. In fish and other vertebrates this develops into the spinal cord, the main communications trunk of the nervous system.
  • 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 particles of food from the water in which the animals live.
  • a muscular tail that extends backwards behind the anus.
  • 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.[3] It also stores iodine, and may be a precursor of the vertebrate thyroid gland.[2]

Sub-divisions

There are three major groupings within the chordates:

Craniates have distinct skulls. Michael J. Benton comments that "craniates are characterized by their heads, just as chordates, or possibly all deuterostomes, are by their tails."[4] Most are vertebrates, in which the notochord is replaced by the spinal column.[5] This consists of a series of bony or cartilaginous cylindrical vertebrae, generally with neural arches that protect the spinal chord and with projections that link the vertebrae. Hagfish have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates,[6] but as members of the craniates, the group from which vertebrates are thought to have evolved.[7] The position of lampreys is ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish.[8] 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.[9]
Craniate: Hagfish
Cephalochordates are small, "vaguely fish-shaped" animals that lack brains, clearly-defined heads and specialized sense organs.[10] These burrowing filter-feeders may be either the closest living relatives of craniates or surviving members of the group from which all other chordates evolved.[11][12]
Cephalochordate: Lancelet
Most tunicates appear as adults in two major forms, both of which are bags of jelly that lack the standard features of chordates: "sea squirts" are sessile and consist mainly of water pumps and filter feeding apparatus;[13] 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.[14] However all tunicate larvae have the standard chordate features, including long, tadpole-like tails; they also have rudimentary brains, light sensors and tilt sensors.[13] 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.[15] Because of their larvae's long tails tunicates are also called urochordates ("tail chordates").[13]
Tunicates: sea squirts
Tunicates: floating colony of salps

Closest non-chordate relatives

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 probosces 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 inter-connected. 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 tiny hacksaw blades, lived in tubes similar to those of pterobranchs.[16]
Enteropneust hemichordate: Balanoglossus
Pterobranch hemichordate: Rhabdopleura
Echinoderms differ from chordates' other relatives in three conspicuous ways: instead of having bilateral symmetry they have radial symmetry, like wheels; their bodies are supported by skeletons made of calcite, a material not used by chordates, and these skeleton enclose their bodies but are also covered by a thin skin; they have tube feet. The feet are powered by another unique feature of echinoderms, a water vascular system of canals that also function as a "lung" and are 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.[17]
Echinoderm: starfish
Echinoderm: crinoid

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.[18] It seems very likely that 555 million years old Kimberella was a member of the protostomes.[19][20] If so, this means that 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 542 million years ago.[18] The Ediacaran fossil Ernettia, from about 549 to 543 million years ago, may represent a deuterostome animal.[21]

Haikouichthys, from about 518 million years ago in China, may be the earliest known fish.[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, 542 million years ago.[23] The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate.[24] Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate.[25][26] 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.[26] Haikouichthys and Myllokunmingia, also from the Chenjiang fauna, are regarded as fish.[22][27] Pikaia, discovered much earlier but from the Mid Cambrian Burgess Shale, is also regarded as a primitive chordate.[28] On the other hand fossils of early chordates are very rare, since non-vertebrate chordates have no bones or teeth, and only one has been reported for the rest of the Cambrian.[29]

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.[3] 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, hemichordates and chordates evolved.[31] 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.[3][32] 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.[30]

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 900 million years ago and the earliest chordates around 896 million years ago.[32] However molecular estimates of dates often disagree with each other and with the fossil record,[32] and their assumption that the molecular clock runs at a known constant rate has been challenged.[33][34]

Classification

Taxonomy

The following schema is from the third edition of Vertebrate Palaeontology.[35] While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy.

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[36] in the scientific journal Nature.

References

  1. ^ Valentine, J.W. (2004). On the Origin of Phyla. Chicago: University Of Chicago Press. p. 7. ISBN 0226845486. "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."
  2. ^ a b Rychel, A.L., Smith, S.E., Shimamoto, H.T., and Swalla, B.J. (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.
  3. ^ a b c d Ruppert, E. (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 on 2008-09-22.
  4. ^ Benton, M.J. (2000). Vertebrate Palaeontology: Biology and Evolution. Blackwell Publishing. p. 13. ISBN 0632056142. http://books.google.co.uk/books?hl=en&id=PQuKO7xqjNQC&dq=vertebrate&printsec=frontcover&source=web&ots=sEzOFBiKPq&sig=widuILMby4C27tPyeiv5545LBhw&sa=X&oi=book_result&resnum=3&ct=result#PPA13,M1. Retrieved on 2008-09-22.
  5. ^ "Morphology of the Vertebrates". University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/vertebrates/vertmm.html. Retrieved on 2008-09-23.
  6. ^ "Introduction to the Myxini". University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html. Retrieved on 2008-10-28.
  7. ^ Campbell, N.A. and Reece, J.B. (2005). Biology (7th ed.). San Francisco, CA: Benjamin Cummings.
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  21. ^ 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. http://geology.geoscienceworld.org/cgi/content/abstract/27/6/519. Retrieved on 2008-09-22. 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. http://icb.oxfordjournals.org/cgi/content/full/43/1/104. Retrieved on 2008-09-22.
  22. ^ a b Shu, D-G., Conway Morris, S., and Han, J., et al. (January 2003). "Head and backbone of the Early Cambrian vertebrate Haikouichthys". Nature 421: 526–529. doi:10.1038/nature01264; (inactive 2009-03-14). http://www.nature.com/nature/journal/v421/n6922/abs/nature01264.html. Retrieved on 2008-09-21.
  23. ^ Bengtson, S. (2004), Early skeletal fossils, in Lipps, J.H., and Waggoner, B.M., "Neoproterozoic-Cambrian Biological Revolutions" (PDF), Palentological Society Papers 10: 67–78, http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf, retrieved on 2008-07-18
  24. ^ Bengtson, S., and 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. Retrieved on 2008-09-23.
  25. ^ Shu, D., Zhang, X. and Chen, L. (April 1996). "Reinterpretation of Yunnanozoon as the earliest known hemichordate". Nature 380: 428–430. doi:10.1038/380428a0. http://www.nature.com/nature/journal/v380/n6573/abs/380428a0.html. Retrieved on 2008-09-23.
  26. ^ a b Chen, J-Y., Hang, D-Y., and Li, C.W. (December 1999). "An early Cambrian craniate-like chordate". Nature 402: 518–522. doi:10.1038/990080. http://www.nature.com/nature/journal/v402/n6761/abs/402518a0.html. Retrieved on 2008-09-23.
  27. ^ Shu, D-G., Conway Morris, S., and Zhang, X-L. (November 1999). "Lower Cambrian vertebrates from south China" (PDF). Nature 402: 42. doi:10.1038/46965. http://www.bios.niu.edu/davis/bios458/Shu1.pdf. Retrieved on 2008-09-23.
  28. ^ Shu, D-G., Conway Morris, S., and Zhang, X-L. (November 1996). "A Pikaia-like chordate from the Lower Cambrian of China". Nature 384: 157–158. doi:10.1038/384157a0. http://www.nature.com/nature/journal/v384/n6605/abs/384157a0.html. Retrieved on 2008-09-23.
  29. ^ 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, http://jpaleontol.geoscienceworld.org/cgi/content/extract/82/2/424, retrieved on 2009-04-28
  30. ^ a b 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 online at [1]
  31. ^ Winchell, C.J., Sullivan, J., Cameron, C.B., Swalla, B.J., and Mallatt, J. (01 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. PMID 11961109. http://mbe.oxfordjournals.org/cgi/content/full/19/5/762#MBEV-19-05-09-SWALLA1. Retrieved on 2008-09-23.
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  35. ^ Benton, M.J. (2004). Vertebrate Palaeontology, Third Edition. Blackwell Publishing, 472 pp. The classification scheme is available online
  36. ^ The amphioxus genome and the evolution of the chordate karyotype, Nicholas H. Putnam, et al. Nature vol 453 p. 1064–1071, June 19, 2000

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