What it was
A relative of sharks with an anvil on its back. The anvil sits where a back fin should be, and it is built from three pieces: a base plate, a spine and a brush of fine tubes packed side by side.
The teeth on top
Rows of enlarged skin teeth cover the top of the anvil, all pointing forward. There is a matching patch on top of the head. The best complete anvil-back we have is a male of a close relative, Akmonistion. It is 62 centimeters long, about the length of your arm.
Still a mystery
What the anvil did is an open question. Nobody has ever worked out what it was for. On the family tree, the anvil sits closer to the living ratfish than to modern sharks.