Cephalopod body size and macroecology through deep time.

As actively swimming predators, cephalopods have played a key role regulating and engineering marine ecosystems for more than 500 million years and continue to do so. For the first time, we portray fluctuations of cephalopod body size including species from the Cambrian to today. For comparability, we determined the maximum body volumes of each species using various proxies, because classical measures like mantle length cannot be applied homologously to all groups. We separately examined Cephalopoda with orthoconic conchs (without ammonoids and coleoids), Nautilida, Ammonoida (without heteromorphs), and Neocoleoida (squids, octopuses and their ancestors). The long-term trajectories of these groups differ in their overall trends. Each of these groups reacted in other ways to the mass extinctions. All groups except the nautilids evolved species exceeding one meter in size, which belong to the marine megafauna. Nautilids and orthocones share a threshold volume of about 100 litres, while in ammonoids and neocoleoids, the limit lies closer to 500 litres.