It may come as no surprise that the giant squid (Architeuthis dux) is the largest cephalopod, a group that includes squid, octopuses, cuttlefish and nautiluses.
Researchers determine squid length by measuring the creatures' mantle (body length). The mantle of A. dux is more than 6.6 feet long, but the animal's tentacles make the squid a whopping 59 feet long in total, said Michael Vecchione, a curator of cephalopods at the Smithsonian Institution National Museum of Natural History in Washington, D.C.
The largest extinct cephalopod by shell length was the Cameroceras, which was roughly 20 feet long. However, if you're looking at largest shell by diameter, that honor goes to the extinct Parapuzosia seppenradensis, an ammonite with a coiled shell that was 8.2 feet in diameter, Vecchione said.