March 17. Ordovician cephalopods

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.

nautiloid Trilacinoceras

Like so many other groups, cephalopods saw a huge diversification and even a period of dominance during the Ordovician. The most common type from that time is the nautiloids.

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

They grew to be pretty big – Endoceras, an Ordovician type, has been measured at 11 feet long – you can imagine that with a mass of grasping tentacles at the front –  and it’s possible that there were even longer nautiloids. Earlier specimens tended to be straight, but some from the Ordovician are curved as well. They were predators, and probably amounted to the terror of the seas during much of the Paleozoic Era.

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.