Living Fossils: The Ancient Beauty of Nautiloids

Full Nautiloid Fossil

These sea creatures belong to the class Cephalopoda, which also includes squid, octopus & cuttlefish, like them Nautiloids were carnivores. They likely preyed on trilobites, small fish & other marine organisms using their tentacles and beak-like jaws.

Nautiloid fossils show the shell with various chambers but of course the fleshy tentacles were too soft to fossilise, this sometimes makes it difficult to visualise what they were like when alive. Their chambers filled with gas for buoyancy, and they were able to adjust their air to water ratio in each chamber through the siphuncle (tube) that ran through all of these chambers, enabling them to stay floating through the ocean. They also used Jet propulsion for movement, they had a hyponome, which was a muscular, funnel-like structure used to expel water, they could use this to move backwards at faster speeds when needed (probably to escape predators)

Inside of a Nautiloid fossil, showing central tube

Nautiloids can sometimes be mistaken for ammonites, however there a few distinct differences between the two. Nautiloids have smooth shells with straight or curved sutures (lines) dividing each chamber, whereas ammonites are more intricately detailed. Plus, Nautiloids have a siphuncle tube in the centre of their shell, whereas an Ammonites is around the edge of the shell and usually isn’t visible.

	Nautiloid	Ammonite
Geological Age	Late Cambrian (500 million years ago), still living today (e.g., Nautilus).	Devonian (400 million years ago), went extinct at the end of the Cretaceous (66 million years ago).
Suture Pattern	Simple sutures—smooth, gently curved lines.	Complex sutures—highly frilled or wavy patterns.
Chamber Function	Chambers filled with gas for buoyancy control via a central siphuncle.	Same function, but the siphuncle runs along the outer shell margin, not centrally.
Shell Ornamentation	Usually plain or slightly striated.	Often ribbed, ridged or spined.

Modern day Nautilus

First appearing in the Late Cambrian period over 500 million years ago, Nautiloids still have a relative alive today - The Nautilus – they can be found in deep ocean waters, often referred to as living fossils as they can be traced back 500 million years.

Nautiloid shells were mainly made from aragonite, a minerally stable form of calcium carbonate, when fossilising these shells can sometimes form into calcite which is another form of calcium carbonate or can be replaced by minerals around them such as pyrite.