Sea Scorpions (The Eurypterids)
The sea scorpions were members of the phylum Arthropoda, the same group that our modern insects, spiders, centipedes, millipedes, crabs, lobsters, crayfish and horseshoe crabs belong to. Within the arthropod phylum, sea scorpions are further assigned to the class Merostomata, of which the horseshoe crabs are the only extant (still living) members, and the order Eurypterida.
The Eurypterids have been commonly called the "sea scorpions" due to their resemblance to our modern, land-living scorpions. They are not actually true scorpions but are rather closely related to them. In fact, some species of Eurypterid had a scorpion-like stinger in their tails, which may have actually contained a venom they used to subdue or kill their prey.
Like other members of the Arthropoda, the Eurypterids were invertebrates whose bodies were covered with an exoskeleton made of chitin, which they would periodically moult in order to grow larger. Like the insects, their bodies were composed of three parts: a prosoma or head, an opisthosoma and a telson or tail. The prosoma also held the Eurypterid's six pairs of legs. The foremost pair of legs had grasping appendages, the next four pairs were used for walking and the last pair of legs usually formed large paddles for swimming. The opisthosoma had gills for breathing on the underside of the segments. The telson could be a spine or could be modified into a tail fin (such as in the Pterygotidae) or into cercal blades (such as in Megalograptus).
The Eurypterids lived in both fresh water and marine environments, but they seem to have preferred estuaries or brackish (salty) water.
The earliest known Eurypterids are from the Lower Cambrian Period (570 to 500 million years ago). Paleomerus has characteristics of both the horseshoe crabs and the sea scorpions. Even during the Upper Ordovician Period (500 to 435 million years ago) Brachyopterus still had characteristics of both the horseshoe crabs and the Eurypterids.
Some of the Eurypterids grew to over six feet (2 meters) in length, but other species were relatively small -- only 2-4 inches (5-10 cm) in length. Their strong grasping legs and strong "teeth" suggest that they ate a diet of hard-shelled invertebrates. Coprolites of Eurypterids have been found which contain the body parts of other Eurypterids, suggesting that they may have also fed on each other, or the body parts may have been the result of eating their own moulted exoskeletons, which would have been rich in nutrients.
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