Eurypterus tetragonophthalmus

Eurypterus is a genus that lived in the Silurian period, 420 million years ago. Eurypterus is often described as a sea scorpion, as it had a sharp needle on its tail, which it probably used in combat. However, it is a member of the superclass Merostomata, a group of arthropods that currently includes only the horseshoe crab and is not directly related to scorpions, which belong to a different class, arachnids.