This species lived in the Cretaceous when the dinosaurs were alive. They live today. They are living fossils. However, they are not true crabs. We are talking about the animal called the "HORSESHOE CRAB".
If you have visited a shop which sells sea shells, chances are you have also see a horseshoe crab.
Here is a little on the taxonomy of the horse shoe crab (Limulus polyphemus):
Superkingdom: Eukaryota All animals, plants,
fungi, and protists.
Kingdom: Animalia Animals
Subkingdom: Metazoa Multicellular animals
Phylum: Arthropoda Jointed leg invertebrates
Sub-Phylum: Cheilcerata — animals with no jaws
Class: Merostomata — mouth surrounded by legs
Subclass: Xiphosura — from the Greek Xiphos meaning sword and ura meaning tail
Order: Xiphosurida — sword-tailed animals
Family: Limulidae — one living member, Limulus
Genus: Limulus — from the Latin, meaning somewhat oblique, odd, or askew and referring to the sideways placement of the compound eyes
Species: polyphemus — from the Greek, meaning one-eyed giant and referring to the simple eyes on the front of the shell
Authority: NOAA Sea Grant
Common names: Horseshoe crab