A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa: a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail.
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. These CSIs provide molecular means to reliably distinguish chordates from all other animals.
Chordates are divided into three subphyla: Vertebrata (fish, amphibians, reptiles, birds and mammals), which possess a skull and whose notochords are replaced by a cartilaginous/bony axial endoskeleton (spine); Tunicata or Urochordata (sea squirts, salps, and larvaceans), which only retain the synapomorphies during their larval stage; and Cephalochordata (lancelets), which resemble jawless fish but have no gills or a distinct head. The vertebrates and tunicates compose the clade Olfactores (see diagram under Phylogeny), which is characterized by the presence of an advanced olfactory system and is sister to Cephalochordata. Extinct taxa such as the conodonts are chordates, 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, which is now thought to be closer to the echinoderms, and together they form the clade Ambulacraria. The sister phylum of the chordates, Ambulacraria, and possibly Xenacoelomorpha are believed to form the superphylum Deuterostomia, although this is called into doubt in a 2021 publication.
Chordata is the third-largest phylum of the animal kingdom (behind only the protostomal phyla Arthropoda and Mollusca) and is also one of the most ancient animal taxa. Chordate fossils have been found from as early as the Cambrian explosion, over 539 million years ago. Of the more than 81,000 living species of chordates, about half are ray-finned fishes (class Actinopterygii) and the vast majority of the rest are tetrapods, a terrestrial clade of lobe-finned fishes (Sarcopterygii) that evolved air-breathing using lungs.
Etymology 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 bilaterally symmetric, possess a coelom and a closed circulatory system, and exhibit metameric segmentation. 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.
Anatomy
Chordates form a phylum of animals that are defined by having, at some stage in their lives, all the following anatomical features:
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 communication trunk of the nervous system. In vertebrates, the rostral end of the neural tube enlarges into several vesicles during embryonic development, which give rise to the brain. 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 development. A post-anal tail. A muscular tail that extends backwards beyond the location of the anus. In some chordates, such as hominids, 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. It also stores iodine, and may be a precursor of the vertebrate thyroid gland.
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, during embryonic development, the anus forms before the mouth does. All chordates are based on a bilateral body plan. All chordates are coelomates and have a fluid-filled body cavity (coelom) with a complete serosal lining derived from mesoderm called mesothelium (see Brusca and Brusca).R.C.Brusca, G.J.Brusca. Invertebrates. Sinauer Associates, Sunderland Mass 2003 (2nd ed.), p. 47, .
Classification The following schema is from the 2015 edition of Vertebrate Palaeontology.Benton, M.J. (2004). Vertebrate Palaeontology, Third Edition. Blackwell Publishing. The classification scheme is available online The invertebrate chordate classes are from Fishes of the World. While it is generally 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; 32 species) Class Leptocardii (lancelets) Subphylum Tunicata (Urochordata) – (tunicates; 3,000 species) Class "Ascidiacea" (sea squirts; paraphyletic as thaliaceans are excluded) Class Thaliacea (salps, doliolids and pyrosomes) Class Appendicularia (larvaceans) Subphylum Vertebrata (Craniata) (vertebrates – animals with backbones; 66,100+ species) Infraphylum "Agnatha" paraphyletic (jawless vertebrates; 100+ species) Superclass Cyclostomata Class Myxinoidea or Myxini (hagfish; 85 species) Class Petromyzontida or Hyperoartia (lampreys; 53 species) Class †Conodonta Class †Myllokunmingiida Class †Pteraspidomorphi Class †Thelodonti Class †Anaspida Class †Cephalaspidomorphi 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; paraphyletic when tetrapods are excluded) Subclass Actinopterygii (ray-finned fish; about 30,000 species) Clade "Sarcopterygii" (35,100+ species; paraphyletic when tetrapods are excluded – 8 species of lobe-finned fish) Class Actinistia (2 species) Class Dipnoi (6 species) Superclass Tetrapoda (four-limbed vertebrates; 35,100+ species) Class Amphibia (amphibians; 8,100+ species) Class Sauropsida (reptiles (including birds); 21,300+ species – 10,000+ species of birds and 11,300+ species of reptiles) Class Synapsida (mammals; 5,700+ species) Genus Cathaymyrus Genus Pikaia
Subphyla
Lancelets (Cephalochordata)
The cephalochordates are small, "vaguely fish-shaped" animals that lack brains and clearly defined heads, as well as lacking specialized sensory organs. These burrowing filter-feeders compose the earliest-branching chordate subphylum.
Tunicates (Urochordata)
The tunicates have three distinct adult shapes. Each is a member of one of three monophyletic clades. All tunicate larvae have the standard chordate features, including long, tadpole-like tails. Their larvae also have rudimentary brains, light sensors, and tilt sensors.
The smallest of the three groups of tunicates is the Appendicularia. They retain tadpole-like shapes and active swimming all their lives and were for a long time regarded as larvae of the other two groups.
The other two groups, the sea squirts and the salps, lose the notochord, the nerve cord, and the post-anal tail as they metamorphose into adult forms. Both are soft-bodied filter feeders with multiple gill slits and feed on plankton.
Sea squirts are sessile organisms and consist mainly of water pumps and filter-feeding apparatus. Most attach firmly to the sea floor, where they remain in one place for life.
The salps float in mid-water and have a two-generation cycle in which one generation is solitary and the next forms chain-like colonies.
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.Oxford English Dictionary, Third Edition, January 2009: Urochordata The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.
Vertebrates (Craniata)
All craniate species have a distinct skull. However, not all of them possess vertebrae. The notochord is accompanied or replaced by the vertebral column in most craniates. It consists of a series of cylindrical, bony or cartilaginous vertebrae and projections that link the vertebrae. These generally have neural arches that protect the spinal cord.
Hagfish have incomplete braincases and lack vertebrae. Thus, they were regarded as non-vertebrate craniates. However, the cladistic exclusion of hagfish from the vertebrates is controversial, as they may have secondarily lost their vertebral columns, making them vertebrates.
Molecular phylogenetics, which uses DNA sequences to classify organisms, has generally supported the grouping of hagfish and lampreys (which have vertebrae) in a clade named Cyclostomata, suggesting that the hagfish are indeed vertebrates. Examination of fossil hagfish also supports this conclusion. Indicating that Vertebrata is the same group as Craniata, instead of the former being a subset of the latter.
Phylogeny
Overview
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 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 identified by DNA analysis as there is no physical trace of any chordate-like structures.
Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses. The current consensus is that chordates are monophyletic (meaning, all chordates are descend from a single, common ancestor, which can be considered a chordate) and that the vertebrates' nearest relatives are tunicates. In 2016, 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 provided strong evidence of the monophyly of Chordata.
All of the earliest chordate fossils have been found in the Early Cambrian Chengjiang fauna, and include three species that are regarded as fish, and hence 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 proven 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.
Diagram of the evolutionary relationships of chordates.