Squids In!

Scientist used to think that the extinct Ammonites were closely related to the modern Nautilus. Both are Cephalopod molluscs and both have a shell. An especially well-preserved Ammonite fossil was, however, discovered by Neville Hollingworth in 1998. This fossil was exceptional, as remnants of the animal's internal organs were preserved (https://www.theguardian.com/science/2021/dec/08/ammonites-were-jet-setters-of-the-mesozoic-era-say-scientists). The Hollingworth fossil has now been subjected to Computer-assisted Tomography and neutron scans by geologists at Cardiff University. These scientists were, consequently, able to make a 3-D reconstruction of the Ammonite's internal organs. To their surprise, they found that Ammonites, unlike the Nautilus, had a hyponome. A hyponome is a siphon-like structure, used by Octopii and Squids (modern Cephalopods), to jet-propel themselves through the water. The Octopus and the Squid don't have external shells but the Ammonite's internal muscles indicate they were able to defensively retract into theirs. It looks as if the evolution of Ammonites will have to be substantially re-evaluated. It is still uncertain why they died out at approximately the same time as the non-Avian dinosaurs.