What is earth's biggest arthropod?

Answer 1

Spinosaurus

EDIT - Either Basilosaurus or Liopleurodon.

That's incorrect. Largest was megadolon. It was 16-20 meters and more then 50 tons. For me that's absolutly largest carnivore ever.

Answer 2

Historically, the largest arthropod ever known was the sea scorpion, which was sometimes over 8 feet long.

Currently, the largest arthropod is the Japanese spider crab, which can have a span over 12 feet.

Answer 3

Eurypterids, or sea scorpions, living from around 460 to 248 million years ago in the Paleozoicum. They typically cracked small armoured fish, our closest relatives at the time.