What it is
Not a crab. A horseshoe crab walks forward, with every leg tucked under one smooth shield. It has a curved front shield, a hinged back plate and a long spike for a tail.
How old
The oldest horseshoe crab fossil is 445 million years old. It was found in Canada and is only about four centimeters long, and it already carries the plan. About 150 million years ago, one left nearly ten meters of footprints in a lagoon in what is now Germany, with the animal lying at the end of its trail.
Did it stop evolving?
No. Its genes kept changing at an ordinary pace. What held still was the design. At least five times, some horseshoe crabs moved into fresh water and tried new shapes. Every one of those branches died out.
Why it's still here
It kept a kind of home that never went away: shallow coasts, river mouths and muddy bottoms. It eats almost anything it can dig up and copes with near-freezing water and summer heat. One line walked the edge of the sea through all five mass extinctions.