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Chordates are a diverse group of animals that includes vertebrates, tunicates, and lancelets. They have a hollow dorsal nerve cord, notochord, pharyngeal slits/pouch, endostyle, and post-anal tail. The earliest chordates are found in the Chengjiang fauna of fossil beds in China, dated to 520-530 million years ago. Chordates evolved a central nerve cord, which became a backbone, allowing for greater neurological complexity and motor control. Chordates have colonized land and sea and include fish, amphibians, reptiles, birds, and mammals, including humans.
Chordates include vertebrates and a pair of close cousins, tunicates (ascidites) and lancelets, primitive aquatic animals. In scientific classification, Chordata is one of the 38 animal phyla, one of the “big nine” together with Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda and Echinodermata. The precise origin of chordates is unknown, but fish- or lancet-shaped fossils from the Cambrian show that they existed at least 530 million years ago, during the so-called Cambrian explosion of biodiversity. Distinctive features of chordates are a hollow dorsal nerve cord, notochord, pharyngeal slits/pharyngeal pouch, endostyle, and post-anal tail.
The earliest chordates are found in the Chengjiang fauna of fossil beds in China, dated to 520-530 million years ago. Eight possible chordates are found in these fossil beds. The most famous is Myllokunmingia, a primitive fish, probably agnathan (no jaw), the first vertebrate to appear in the fossil record. He possessed a skull and what appears to be a primitive notocord. Vaguely resembling a modern hagfish, Myllokunmingia was about 2.8 cm (1.1 in) long and 6 mm tall, quite small. A related chordate is Haikouichthys ercaicunensis, another primitive fish-like animal. Another enigmatic fossil, Yunnanozoon lividum (“Yunnan livid animal”) is a chordate or hemichordate, a closely related phylum.
Chordates became one of the most successful animal phyla, colonizing land and sea and becoming the fourth most diverse phylum, after arthropods, nematodes and molluscs, with over 100,000 species. The major innovation of chordates was a central nerve cord, which evolved into a backbone. This provided a designated nerve highway, allowing for greater neurological complexity and motor control. The backbone has provided a way for animals to get more robust without using an exoskeleton.
Some familiar chordates include fish and eels, including jawless fish such as lampreys, sharks, rays and rays, amphibians, reptiles, birds, and mammals, including humans. There are a number of extinct groups in the phylum Chordata, including spiny sharks, armored fish called placoderms, the dinosaurs, and numerous ancient tetrapods including some of the first animals to colonize land, thus the first animals to colonize the continental interior.
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