Spriggina.

Spriggina (sprig-EE-nuh) is an odd genus of animal from the Ediacaran Period (about 600 million years ago). No one really knows what kind of animal Spriggina really was--if it was an annelid, arthropod, sea pen, or maybe even something else. Spriggina had a horseshoe-shaped head and many tapering segments that formed a body that looked like a flattened worm.

Spriggina could have had eyes and antennae on the head, but no one really knows if it did have these. On top, Spriggina had overlapping plates of cuticle (cuticle is a hard layer that is not quite a shell), and possibly a mouth in the middle of the head.

Spriggina was probably not an arthropod if it didn't have eyes, antennae, or legs. And also probably not an annelid because it didn't have chaeta, which are bristles.

Spriggina was found in Australia and is 3 cm long.

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Tribrachidium.

Tribrachidium was a strange genus of ediacara which has been found in Russia, Ukraine, and Australia. 

Tribrachidium has been described as a member of many groups. It probably lived on the bottom of the ocean filtering food. Like many animals from the Ediacaran Period, Tribrachidium was mysterious and little is known about it. 

Tribrachidium had trilateral symmetry, which means its three sides were alike. Tribrachidium's three arms were possibly hollow and could inflate and deflate themselves, possibly to move up and down in the water. It was about 2" or 5 cm. 

Tribrachidium is the only animal that has been discovered so far that is trilaterally symmetrical. So Tribrachidium may have been in its own group with no descendants.