A chordate (/ˈkɔːrdeɪt/ KOR-dayt) is an animal belonging to the phylum Chordata (/kɔːrˈdeɪtə/ kor-DAY-tə). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These five synapomorphies include a notochord, dorsal hollow nerve cord, endostyle or thyroid, pharyngeal slits, and a post-anal tail. The name "chordate" comes from the first of these synapomorphies, the notochord, which plays a significant role in chordate structure and movement. Chordates are also bilaterally symmetric, have a coelom, possess a circulatory system, and exhibit metameric segmentation.
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 mitochondrial inner membrane protease ATP23, which are exclusively shared by all vertebrates, tunicates and cephalochordates.[5] These CSIs provide molecular means to reliably distinguish chordates from all other Metazoa.
Chordates are divided into three subphyla: Craniate or Vertebrate (fish, amphibians, reptiles, birds, and mammals); Tunicata or Urochordata (sea squirts, salps and relatives, and larvaceans); and Cephalochordata (which includes lancelets). The Craniata and Tunicata compose the clade Olfactores, which is sister to Cephalochordata. (See diagram under Phylogeny.) Extinct taxa such as Vetulicolia and Conodonta are Chordata, 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: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. The Chordata and Ambulacraria, together and possibly with the Xenacoelomorpha, are believed to form the superphylum Deuterostomia, although this has recently been called into doubt.[6]
Chordate fossils have been found from as early as the Cambrian explosion, 539 million years ago.[7] Cladistically (phylogenetically), vertebrates – chordates with the notochord replaced by a vertebral column during development – are a subgroup of the clade Craniata, which consists of chordates with a skull. Of the more than 81,000[8] living species of chordates, about half are ray-finned fishes that are members of the class Actinopterygii and the vast majority of the rest are tetrapods (mostly birds and mammals).
Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features:[9]
There are soft constraints that separate chordates from other biological lineages, but are not part of the formal definition: