World Library  
Flag as Inappropriate
Email this Article

Chordates

Article Id: WHEBN0000518167
Reproduction Date:

Title: Chordates  
Author: World Heritage Encyclopedia
Language: English
Subject: Human development (biology), Notochord, Pannexin
Collection:
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Chordates

This article is about the animal phylum. For the leaf shape, see Cordate.
Chordata
Temporal range: Early Cambrian – Recent, 540–0Ma
The X-ray tetra (Pristella maxillaris) is one of the few chordates with a visible backbone. The spinal cord is housed within its backbone.
Classes

See below

Chordates are members of the phylum Chordata, deuterostome animals possessing a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail for at least some period of their life cycles. Taxonomically, the phylum includes the subphyla Vertebrata, including mammals, fish, amphibians, reptiles, birds; Tunicata, including salps and sea squirts; and Cephalochordata, comprising the lancelets.

The phylum Hemichordata including the acorn worms has been presented as a fourth chordate subphylum, but it now is usually treated as a separate phylum. It, along with the echinoderm phylum, including starfish, sea urchins, and sea cucumbers and their kin, are the chordates' closest relatives. Primitive chordates are known from at least as early as the Cambrian explosion.

There are more than 75,000 living species of chordates, about half of which are bony fish of the class osteichthyes. Both the world's largest and fastest animal, the blue whale and peregrine falcon respectively, are chordates, as are humans.[1]

Overview of affinities

Tunicate larvae have both a notochord and a nerve cord which are lost in adulthood. Cephalochordates have a notochord and a nerve cord (but no brain or specialist sensory organs) and a very simple circulatory system. Craniates are the only subphylum whose members have skulls. In all craniates except for hagfish, the dorsal hollow nerve cord is surrounded with cartilaginous or bony vertebrae and the notochord is generally reduced; hence, hagfish are not universally regarded as vertebrates, though recent DNA comparisons suggest that they are in fact vertebrates. The chordates and two sister phyla, the Hemichordata and the Echinodermata, make up the deuterostomes, one of the two superphyla that encompass all fairly complex animals. An article by Nakano et al. demonstrates that the Xenoturbellida are not deuterostomes.[2]

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 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 they 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.

For a recent concise review of chordate relationships, see Holland, N. D. 2005, Chordates. Curr. Biol. 15: R911-R914.

Origin of name

Although the name Chordata is often 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.[3]

Definition

1 = bulge in spinal cord ("brain")
4 = post-anal tail
5 = anus
9 = space above pharynx
11 = pharynx
12 = vestibule
13 = oral cirri
14 = mouth opening
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.

Chordates form a phylum of creatures that are based on a bilateral body plan,[4] and is defined by having at some stage in their lives all of the following:[5]

  • 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.
  • Post-anal tail. 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.[6] It also stores iodine, and may be a precursor of the vertebrate thyroid gland.[5]

Subdivisions

Craniata

Main article: Craniata


Craniates, one of the three subdivisions of chordates, have distinct skulls - including hagfish, which have no vertebrae. Michael J. Benton comments, "craniates are characterized by their heads, just as chordates, or possibly all deuterostomes, are by their tails". [7]

Most are vertebrates, in which the notochord is replaced by the spinal column,[8] which consists 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. Hagfish have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates,[9] but as members of the craniates, the group from which vertebrates are thought to have evolved.[10] The position of lampreys is ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish.[11] 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.[12]

Urochordata: tunicates

Main article: Tunicate


Most tunicates appear as adults in two major forms, 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;[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] The etymology of the term Urochorda(ta) (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail.[16] The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.[13]

Cephalochordata: lancelets

Main article: Lancelet


Cephalochordates are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs.[17] These burrowing filter-feeders comprise the earliest-branching chordate sub-phylum.[18][19]

Closest nonchordate relatives

Hemichordates

Main article: Hemichordate


Hemichordates ("half (½) 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.[20]

Echinoderms

Main article: Echinoderm

Echinoderms differ from chordates and their other relatives in three conspicuous ways: they possess bilateral symmetry as larvae and in adulthood they have radial symmetry, meaning 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 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.[21]

Origins

The majority of animals more complex than


Fossils of one major deuterostome group, the

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.[6] 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", surviving members of the group from which echinoderms, hemichordates and chordates evolved.[35] 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.[6][36] One other phylum, Xenoturbellida, has been thought for a while to be basal within the deuterostomes, closer to the original deuterostomes than to the chordates, echinoderms and hemichordates.[34] But an article by Nakano et al. ("Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha", Nature Communications, 2013; 4: 1537 DOI: 10.1038/ncomms2556) demonstrates that the Xenoturbellida are not deuterostomes.

Since chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by

Classification

Taxonomy


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.

The following schema is from the third edition of Vertebrate Palaeontology.[39] The invertebrate chordate classes are from Fishes of the World.[40] 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

Chordates


Cladogram of the Chordate phylum. Lines show probable evolutionary relationships, including extinct taxa, which are denoted with a dagger, †. Some are invertebrates. The positions (relationships) of the Lancelet, Tunicate, and Craniata clades are as reported[41] in the scientific journal Nature.
Chordata 
Cephalochordata

Amphioxus



Olfactores

Haikouella


Tunicata

Appendicularia (formerly Larvacea)



Thaliacea



Ascidiacea



Craniata

Myxini


Vertebrata

Myllokunmingia fengjiaoa



Zhongjianichthys rostratus



Conodonta



Cephalaspidomorphi



Hyperoartia (Petromyzontida)(Lampreys)



Pteraspidomorphi



osteostracan


Gnathostomata

Placodermi† (?paraphyletic)



Chondrichthyes


Teleostomi

Acanthodii† (?paraphyletic)


Osteichthyes

Actinopterygii


Sarcopterygii
void
 Tetrapoda 

 Amphibia


 Amniota 

 Mammalia


 Sauropsida 
void

 Lepidosauromorpha (lizards, snakes, tuatara, and their extinct relatives)





 Archosauromorpha (crocodiles, birds, and their extinct relatives)















See also

References

External links

  • Chordate on GlobalTwitcher.com
  • Chordate node at Tree Of Life
  • Chordate node at NCBI Taxonomy
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.