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Chordata

(Phylum)

Overview

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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 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

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Definition

Anatomy of the cephalochordate Amphioxus
1 = bulge in spinal cord ("brain")
2 = notochord
3 = dorsal nerve cord
4 = post-anal tail
5 = anus
6 = digestive canal
7 = circulatory system
8 = atriopore
9 = space above pharynx
10 = pharyngeal slit (gill)
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

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

Sub-Divisions

There are three major groupings within the chordates:

Closest Non-Chordate Relatives

Origins

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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 none have been reported for the rest of the Cambrian.

A consensus family tree of the chordates[3][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.[30] 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][31] 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.[29]

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.[31] However molecular estimates of dates often disagree with each other and with the fossil record,[31] and their assumption that the molecular clock runs at a known constant rate has been challenged.[32][33]

Classification

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taxonomy

The following schema is from the third edition of Vertebrate Palaeontology.[34] While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy.

Phylogeny

Notes:

Photos

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Taxonomy

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The Phylum Chordata is a member of the Superphylum Panarthropoda. Here is the complete "parentage" of Chordata:

The Phylum Chordata is further organized into finer groupings including:

Classes

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Acanthodii

[more]

Actinopterygii

The Actinopterygii (the plural form of Actinopterygius) constitute the class of the ray-finned fishes. [more]

Agnatha

[more]

Amphibia

Amphibians (class Amphibia), such as frogs, toads, salamanders, newts, and gymnophiona, are cold-blooded animals that metamorphose from a juvenile, water-breathing form to an adult, air-breathing form. Typically, amphibians have four limbs. Unlike other land animals (amniotes), amphibians lay eggs in water, as their fish ancestors did. Amphibians are superficially similar to reptiles. [more]

Appendicularia

Larvaceans (Class Appendicularia) are solitary, free-swimming tunicates found throughout the world's oceans. Like most tunicates, appendicularians are filter feeders. Unlike other tunicates, appendicularians live in the pelagic zone, specifically in the upper sunlit portion of the ocean (photic zone) or sometimes deeper. Superficially, the appendicularian body shape (morphology) resembles that of the tadpole-like larvae of most tunicates. Like a common tunicate larva, the adult appendicularian form has a discrete trunk and tail. Evolution by retaining juvenile traits as an adult is known as neoteny. [more]

Ascidiacea

Ascidiacea (commonly known as the ascidians or sea squirts) is a class in the Tunicata subphylum of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" made of the polysaccharide tunicin, as compared to other tunicates which are less rigid. [more]

Aves

[more]

Cephalaspidomorphi

Cephalaspidomorphs are a taxon of jawless fishes named for the cephalaspids, a group of osteostracans. Most of the members of this group are extinct; however, it interests modern biologists because it may include the lampreys. If so, the lampreys extend the known range of the group from the Silurian and Devonian periods to the present day. [more]

Cephalochordata

The lancelets (subphylum Cephalochordata, traditionally known as amphioxus) are a group of primitive chordates. They are usually found buried in sand in shallow parts of temperate or tropical seas. In Asia, they are harvested commercially for food for humans and domesticated animals. They are an important object of study in zoology as they provide indications about the origins of the vertebrates. Lancelets serve as an intriguing comparison point for tracing how vertebrates have evolved and adapted. Although lancelets split from vertebrates more than 520 million years ago, its genome holds clues about evolution, particularly how vertebrates have employed old genes for new functions. [more]

Chondrichthyes

Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nostrils, scales, two-chambered hearts, and skeletons made of cartilage rather than bone. They are divided into two subclasses: Elasmobranchii (sharks, rays and skates) and Holocephali (chimaera, sometimes called ghost sharks, which are sometimes separated into their own class). [more]

Conodonta

Elasmobranchii

Holocephali

Leptocardii

[more]

Mammalia

Mammals (formally Mammalia) are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, which they use in feeding their young. They are also characterized by the possession of sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain. [more]

Osteichthyes

Osteichthyes (IPA: ), also called bony fish, are a taxonomic group of fish that includes the ray-finned fish (Actinopterygii) and lobe finned fish (Sarcopterygii). The split between these two classes occurred around 440 mya. [more]

Placodermi

The Placodermi were a class of armoured prehistoric fishes, known from fossils, which lived from the late Silurian to the end of the Devonian Period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fishes; their jaws likely evolved from the first of their gill arches. A 380 million year old fossil of one species represents the oldest-known example of live birth. [more]

Reptilia

Reptiles, or members of the class Reptilia, are air-breathing, cold-blooded vertebrates that have skin covered in scales as opposed to hair or feathers. They are tetrapods (having or having descended from vertebrates with four limbs) and amniotes, whose embryos are surrounded by an amniotic membrane. Modern reptiles inhabit every continent with the exception of Antarctica, and four living orders are currently recognized: [more]

Sarcopterygii

Sauropsida

Sauropsida ("lizard-face") is a group of amniotes that includes reptiles, dinosaurs, and birds. Among amniotes, sauropsida is distinguished from theropsida ("beast-face"), also called synapsids. [more]

Synapsida

[more]

Thaliacea

The Thalicea comprise a class of marine animals within the subphylum Urochordata. Unlike their bottom-dwelling relatives the ascidians, thaliaceans are free-floating for their entire lifespan. There are three orders of Thaliacea: Pyrosomida, Doliolida, and Salpida. Pyrosomes are colonies of tiny zooids arranged around a central chamber called the cloaca. As the water exhaled by the zooids exits the cloaca through a common opening, the water movement slowly propels the pyrosome through the sea. Doliolids and salps alternate between solitary and colonial life stages. Salp colonies can be several meters in length. Doliolids and salps rely on muscular action to propel themselves through surrounding sea water, whereas the ciliary feeding currents of pyrosomes are their source of locomotion. All of the thaliceans are filter feeders. They have a barrel-shaped body through which they pump water, propelling them through the sea and from which they extract food. [more]

At least 106 species and subspecies belong to the Class Thaliacea.

More info about the Class Thaliacea may be found here.

References

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  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 groupe d 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. 
  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:10.113 9/Z04-158. http://article.pubs.nrc-cnrc.gc.ca/RPAS/RPViewDoc?_handler_=HandleInitialGet&articleFile=z04-158.pdf&journal=cjz&volume=83. Retrieved on 22 September 2008. 
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  29. ^ 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 on-line at [1]
  30. ^ Winchell, C.J., Sullivan, J., Cameron, C.B., Swalla, B.J., and Mallatt, J. (2002). "Evaluating Hypotheses of Deuterostome Phylogeny and Chordate Evolution with New LSU and SSU Ribosomal DNA Data". Molecular Biology and Evolution 19: 762–776. http://mbe.oxfordjournals.org/cgi/content/full/19/5/762#MBEV-19-05-09-SWALLA1. Retrieved on 23 September 2008. 
  31. ^ a b c Blair, J.E., and S. Blair Hedges, S.B. (2005). "Molecular Phylogeny and Divergence Times of Deuterostome Animals". Molecular Biology and Evolution 22 (11): 2275–2284. doi:10.1093/molbev/msi225. http://mbe.oxfordjournals.org/cgi/content/full/22/11/2275. Retrieved on 23 September 2008. 
  32. ^ Ayala, F.J. (1999). "Molecular clock mirages". BioEssays 21 (1): 71–75. doi:10.1002/(SICI)1521-1878(199901)21:1<71::AID-BIES9>3.0.CO;2-B. http://www3.interscience.wiley.com/cgi-bin/abstract/60000186/ABSTRACT?CRETRY=1&SRETRY=0. 
  33. ^ Schwartz, J. H. and Maresca, B. (2006). "Do Molecular Clocks Run at All? A C ritique of Molecular Systematics". Biological Theory 1: 357–371. doi:10.1162/biot.2006.1.4.357. 
  34. ^ Benton, M.J. (2004). Vertebrate Palaeontology, Third Edition. Blackwell Publishing, 472 pp. The classification scheme is available online
  35. ^ The amphioxus genome and the evolution of the chordate karyotype, Nicholas H. Putnam, et al. Nature vol 453 p. 1064-1071, June 19, 2000

Sources

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Last Revised: March 17, 2009