A small, wormlike creature called Pikaia is thought to be the ancestor of all backboned animals.
Its fossil remains were found in the 530- million-year-old mudstone deposits of the Burgess Shale in Canada.
Pikaia was the first known chordate. All backboned 动物 belong to this group as well as marine 动物 called tunicates and acraniates.
Pikaia was 2 inches long, with a notochord running around its body-- a kind of primitive spine that gave its body flexibility.
The notochord also allowed the animal's simple muscles to work against it, and the animal's body organs to hang from it.
Pikaia is...
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