fossil swimming crustacean of the Silurian and Devonian, 1874, from Greek eurys "broad, wide" (see eury-) + pteron "feather, wing" (from PIE root *pet- "to rush, to fly"); so called from their swimming appendages.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pattram "wing, feather, leaf," patara- "flying, fleeting;" Hittite pittar "wing;" Greek piptein "to fall," potamos "river, rushing water," pteron, pteryx "feather, wing," ptilon "soft feathers, down, plume;" Latin petere "to attack, assail; seek, strive after; ask for, beg; demand, require," penna "feather, wing;" Old Norse fjöðr, Old English feðer "feather;" Old Church Slavonic pero "feather;" Old Welsh eterin "bird."
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Definitions of eurypterid from WordNet
large extinct scorpion-like arthropod considered related to horseshoe crabs;