Fossils have uncovered a prehistoric giant marine creature called anomalocaridids dating back 443 to 485 million years ago.
The researchers say the creatures were two metres in length, fed on plankton and had flaps which have some resemblance to the walking limbs and gill flaps see in modern sea animals.
Examination of anomalocaridid fossils provides insights into the evolution of modern arthropods.
The large size of this filter feeder suggests they were well-adapted to take advantage of a presumably rich source of plankton during the Ordovician period, the authors note.
The find is reported in the journal Nature by Peter Van Roy of Yale University and colleagues.
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