Nepidae - an insect, commonly known as a "water scorpion"
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In fact, no spider that large has ever been found: at the time the series began production, the sea scorpion Megarachne had been mistakenly interpreted as a spider.
Less ambiguous than the above ichnogenera, are the traces left behind by invertebrates such as Hibbertopterus, a giant "sea scorpion" or eurypterid of the early Paleozoic era.