Five traits at some life stage make a chordate
Chordates are bilaterians that, at least briefly, show a notochord, dorsal hollow nerve cord, pharyngeal slits, endostyle or thyroid, and post-anal tail. The phylum ranks third in animal richness after arthropods and molluscs, with Cambrian fossils older than 539 million years.
Three subphyla make up the group. Vertebrates, over 66,000 species, have skulls and swap the notochord for a spine; tunicates, about 3,000 species of sea squirts, salps and larvaceans, show the defining traits mainly as larvae; and the 32 species of lancelets are brainless, headless burrowing filter-feeders that branched off earliest. Vertebrates and tunicates together form a clade marked by advanced smell. Acorn worms, once counted as a fourth subphylum, now sit closer to starfish and sea urchins.
The notochord is a stiff but springy rod of glycoprotein wrapped in two collagen helices; in vertebrates it is replaced by cartilage or bone, and its remnants become the discs between vertebrae. The hollow nerve cord along the back becomes the spinal cord, with its front end swelling into the brain. Throat slits serve as gills in fish or filters in other chordates, appearing only in embryos of land vertebrates, and the endostyle, a mucus groove that also stores iodine, may be the forerunner of the thyroid.
Sequencing adds two molecular markers, signature insertions or deletions in a cyclophilin-like protein and in the mitochondrial protease ATP23, found in every chordate and no other animal. All chordates are also deuterostomes, meaning the anus forms before the mouth in the embryo.
Of more than 81,000 living species, roughly half are ray-finned fish and most of the rest are tetrapods. Sea squirts, which spend adult life cemented to the seabed as little more than pumps and filters, lose their notochord, nerve cord and tail at metamorphosis, whereas larvaceans stay tadpole-shaped swimmers for life.
Source: Chordate
