What it was
Charnia stood upright on the seafloor, a leaf-shaped frond about as long as your forearm. Its branches don't sit opposite each other. They alternate and meet down the middle in a zigzag seam, and every branch is built from smaller copies of itself.
It had no mouth, no eye and no gut anyone can find. Nobody knows for sure what kind of life it was.
How we know
Its fossils are molds: sand buried the soft body and minerals set around the outside before the inside rotted. So we have its outline in fine detail, and nothing of what was inside.