1) Arthropod 
Eurypterids (sea scorpions) are an extinct group of arthropods that are related to arachnids and include the largest known arthropods to have ever lived. They are members of the extinct order Eurypterida (Chelicerata); which is the most diverse Paleozoic chelicerate order in terms of species. The name Eurypterida comes from the Greek words eury- (...
member of an extinct order (Eurypterida) of unusual arthropods rarely preserved as fossils. The eurypterids appeared at the beginning of the ...
[n] - large extinct scorpion-like arthropod considered related to horseshoe crabs
noun large extinct scorpion-like arthropod considered related to horseshoe crabs
any aquatic arthropod of the extinct order Eurypterida, from the Paleozoic Era, closely related to trilobites and scorpions.