Further reading:

Reconstructing fossil cephalopods: Endoceras

Retro vs Modern #17: Ammonites

Hammering Away at Hamites

An endocerid [picture by Entelognathus - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=111981757]:



An ammonite fossil:



A hamite ammonoid that looks a lot like a paperclip [picture by Hectonichus - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=34882102]:



Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

When you think about cephalopods, if that’s a word you know, you probably think of octopuses and squid, maybe cuttlefish. But those aren’t the only cephalopods, and in particular in the past, there used to be even more cephalopods that are even weirder than the ones we have today.

Cephalopods are in the family Mollusca along with snails and clams, and many other animals. The first ancestral cephalopods date back to the Cambrian, and naturally we don’t know a whole lot about them since that was around 500 million years ago. We have fossilized shells that were only a few centimeters long at most, although none of the specimens we’ve found are complete. By about 475 million years ago, these early cephalopod ancestors had mostly died out but had given rise to some amazing animals called Endocerids.

Endocerids had shells that were mostly cone-shaped, like one of those pointy-ended ice cream cones but mostly larger and not as tasty. Most were pretty small, usually only a few feet long, or less than a meter, but some were really big. The largest Endoceras giganteum fossil we have is just under 10 feet long, or 3 meters, and it isn’t complete. Some scientists estimate that it might have been almost 19 feet long, or about 5.75 meters, when it was alive.

But that’s just the long, conical shell. What did the animal that lived in the shell look like? We don’t know, but scientists speculate that it had a squid-like body. The head and arms were outside of the shell’s opening, while the main part of the body was protected by the front part of the shell. We know it had arms because we have arm impressions in sections of fossilized sea floor that show ten arms that are all about the same length. We don’t know if the arms had suckers the way many modern cephalopods do, and some scientists suggest it had ridges on the undersides of the arms that helped it grab prey, the way modern nautiluses do. It also had a hood-shaped structure on top of its head called an operculum, which is also seen in nautiluses. This probably allowed Endoceras giganteum to pull its head and arms into its shell and use the operculum to block the shell’s entrance.

We don’t know what colors the shells were, but some specimens seem to show a mottled or spotted pattern. The interior of Endoceras giganteum's shell was made up of chambers, some of which were filled with calcium deposits that helped balance the body weight, so the animal didn’t have trouble dragging it around.

3D models of the shells show that they could easily stick straight up in the water, but we also have trace fossils that show drag marks of the shell through sediment. Scientists think Endoceras was mainly an ambush predator, sitting quietly until a small animal got too close. Then it would grab it with its arms. It could also crawl around to find a better spot to hunt, and younger individuals that had smaller shells were probably a lot more acti...