This week's Fossil Friday is a tale of two twirly-shelled sea dwellers - The Ammonite and the Nautilus. Both ancient, both tentacled, both spiral-shelled… so how do you tell them apart?
One of the easiest ways is to look at the direction of their internal chambers. In ammonites, the walls (called septa) are convex, with a 'wiggle', and not a simple curve. In nautiloids, those walls curve the other way, and are concave from the opening. It’s a subtle difference, but a key one!
Another great clue is the siphuncle - the tube that runs through the shell to help control buoyancy (basically, their built-in scuba gear). In Ammonites, it hugs the outer edge (ventral) of the coil, while in Nautiloids, it runs right through the middle like a central backbone. Handy if you’re ever eyeballing a cross-section and wondering who you’re dealing with. Just keep in mind though that when Ammonites are cut in half, the width of the saw blade typically cuts right through this 'tube' and so it's not often visible. This is often the same with Nautilus, but more frequently their siphuncle remains visible in one or the other half.
I’ve popped in a sketch showing both of the two shells, and how their chamber directions compare - plus one imagining how these guys may have looked when they were alive and cruising the prehistoric oceans like slightly baffled squid in fancy shells! While both creatures had tentacles and used 'jet propulsion' to move around, Ammonites are all extinct, while Nautiloids still exist today!
Fun fact: Some Ammonites got big. Like, car tyre big. The largest ever found was around 2 metres across – Not something you'd find in the aquatic section of your local pet shop!
