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Terreneuvian – Holocene, 542–0 Ma
|The X-ray tetra (Pristella maxillaris) is one of the few chordates with a visible backbone. The spinal cord is housed within its backbone.|
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A chordate is any animal belonging to the phylum Chordata, possessing a , a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail for at least some period of its life cycle. The Chordata, together with sister clade Ambulacraria, form the deuterostomes as in the embryo development stage the anus forms before the mouth.
Taxonomically, the phylum includes the subphyla Vertebrata, which includes fish, amphibians, reptiles, birds, and mammals; Tunicata, which includes salps and sea squirts; and Cephalochordata, comprising the lancelets.
Hemichordata, which includes the acorn worms, has been presented as a fourth chordate subphylum, but it now is usually treated as a separate phylum. It, along with the Echinodermata, which includes starfish, sea urchins, sea cucumbers, and crinoids, are the chordates' closest taxon Ambulacraria. Fossil chordates are known from at least as early as the Cambrian explosion.
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