Why Cephalopods Lost Their Protective Shells

In a dibranchiate episode of PBS Eons, host Hank Green explains the long evolutionary process in which ancestral cephalopods eventually lost their distinctive shells. Except for the nautilus, which as a living fossil, remains protected under its hard covering.

The ancestors of modern, squishy cephalopods like the octopus and the squid all had shells. In ancient times, their shell was their greatest asset but it eventually proved to be their biggest weakness.