What they are
The crabs you know. A true crab scuttles sideways on legs you can see. A horseshoe crab is not one of them, and true crabs appear about 160 million years after this point in the story.
A clue in the shell
A crab's shell is made of chitin, the same tough stuff a fungus uses for its walls. That is how scientists tested whether the giant Prototaxites was a fungus. It had none.