What it was
A placoderm, a name that means "plated skin." Its head and chest were locked inside a shell of bone, like a helmet and a breastplate. It was about three and a half meters long, built like a barrel. Older books drew it up to ten meters, because the soft body behind the armor never made it into the rock.
The bite
No teeth. Just two blades of bone that wore each other sharper as they slid past, like scissors that sharpen as they cut. Its helmet tipped up as the jaw swung down, so the mouth snapped open in a fraction of a second. The bite closed with more than 5,000 newtons: roughly a horse, standing on one knife edge.
What happened to it
About 359 million years ago, the deep water lost its oxygen while ice spread and the seas fell. Every placoderm on Earth disappeared.