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 動物 belong to this group as well as marine 動物 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 very similar to a modern creature called Branchiostoma, a small, transparent creature that lives in the sand at the bottom of the sea.

As it lacks a bony skeleton, paired fins, and jaws, Pikaia is not really
a fish.

Pikaia is a もっと見る complex creature than many other 動物 found in the Burgess Shale. It suggests that other complex creatures must have lived before it, although there is no fossil evidence for this.

The head of Pikaia was very primitive with a pair of tentacles, a mouth, and a simple brain for processing information.

Pikaia swam in a zigzag fashion similar to a sea snake.