Asked by: Richard Barker, Leamington Spa
Ammonites are a group of extinct marine molluscs and they were able to swim. Their shells are divided into chambers and most of the animal’s body sat in the outermost chamber, with just a single tube, called a siphuncle, extending backwards into the older chambers. The siphuncle diffused gas in and out of the shell chambers to adjust the buoyancy so ammonites could float in the mid-ocean.
Ammonites are related to modern squid and cuttlefish and probably swam backwards by squirting water from a siphon. The modern nautilus has a similar shell layout and lifestyle, although they aren’t closely related.