Cephalopods – the name means head-foot, because their heads typically have tentacles, which seem like feet – cephalopods today are represented by octopuses, squids, and cuttlefish, plus the chambered nautilus.
Nautiloids lived in a shell, but unlike a snail or clam, where the animal lives in essentially a single hollow space, even if it is complex, nautiloids’ shells had multiple chambers in which the animal lived, with the segments interconnected by a thin tissue called a siphuncle. As the animal grows, more segments are added, and each one is separated by a distinct layer called a septum. The boundary layers, the septa, eventually became incredibly complex, with a fractal-like appearance.
|Ordovician nautiloid from Kentucky|
Nautiloids survived multiple mass extinctions until the Late Cenozoic, only about 10 or 20 million years ago. They’ve declined to the point that there are only six species today, compared to 2,500 fossil species known.
—Richard I. Gibson
Photo by Mark Wilson, public domain. Ordovician of Kentucky; an internal mold showing siphuncle and half-filled camerae, both encrusted.
Photograph of the fossil nautiloid Trilacinoceras taken by Dlloyd, used under GNU free documentation license.