Temporal range: Latest Ediacaran - Recent
|A X-ray tetra is one of the few chordates with a visible backbone|
- Phylum Chordata
- Tunicata - (tunicates, formerly Urochordata; 3,000 species)
- Cephalochordata - (lancelets, 30 species)
- Vertebrata (vertebrates - animals with backbones; 57,739 species)
- Agnatha (jawless vertebrates; 100+ species)
- Gnathostomata (jawed vertebrates)
- †Placodermi (Paleozoic armoured forms)
- Chondrichthyes (cartilaginous fish; 300+ species)
- †Acanthodii (Paleozoic "spiny sharks")
- Osteichthyes (bony fishes; 30,000+ species)
- Tetrapoda (four-legged vertebrates; 18,000+ species)
Origin of chordates
A long-standing theory is that in transformed larvae of sea-squirts (tunicates) lies the origin of chordates. Adult tunicates are sessile, but their larvae are motile, and have some features found in early vertebrates. The process of paedomorphosis, where juvenile features are retained in the adult, is the proposed mechanism. Genome analysis does show that the tunicates are the closest living relatives of vertebrates.
Chordate Facts for Kids. Kiddle Encyclopedia.