Why is the horseshoe crab considered a living fossil?

The American horseshoe crab is a common sight on Florida's beaches. Many people have seen horseshoe crabs but do not realize they are looking at one of the oldest animals on the planet. read more

Horseshoe crabs are invertebrates meaning that they don't have a backbone or a spine but, instead, have a hard shell to protect them. Horseshoe crabs are more closely related to spiders, ticks, and scorpions, than they are to crabs. Source. Horseshoe crabs typically swim upside-down and at about a 30-degree angle to the horizon. read more

The horseshoe crab isn’t really a crab but it certainly has been around a long, long, time. Horseshoe crabs are common in the fossil record about 300 million years ago and ancestors of the horseshoe crab back go back more than 400 million years. read more