What it was
A low dome about the size of a plum, with three arms curling out from the top. Three. Not two, not four. Nothing alive today is built on three.
How it ate
It had no mouth that anyone can see. Scientists put a model of it into simulated flowing water: its three arms slowed the current just above the surface, and food dropped into three small pits on top of the dome, whichever way the water came from. A filter made from nothing but shape.