FISHING IN ANCIENT SEAS

If you could cast a fishing line into our ancient seas, it is likely that you would hook an ammonite, not a fish. When we find them, it is their hugely varied fossilized shells that we see. 

Rarely is the very soft, squid-like fellow inside preserved so we can easily forget what the entire animal looked like. 

These marine cephalopods were predatory, squid-like creatures that lived inside the coil-shaped shells we find. Like other cephalopods, ammonites had sharp, beak-like jaws inside a ring of squid-like tentacles that extended from their shells. They used these tentacles to snare prey, — plankton, vegetation, fish and crustaceans — similar to the way a squid or octopus hunt today.

Catching a fish with your hands is no easy feat, as I am sure you know. But the Ammonites were skilled and successful hunters. They caught their prey while swimming and floating in the water column. 

Within their shells, they had a number of chambers, called septa, filled with gas or fluid that were interconnected by a wee air tube called a siphuncle. By pushing air in or out, they were able to control their buoyancy in the water column.

They lived in the last chamber of their shells, continuously building new shell material as they grew. As each new chamber was added, the squid-like body of the ammonite would move down to occupy the final outside chamber.

We find ammonite fossils, and plenty of them, in sedimentary rock from all over the world. They were prolific breeders that evolved rapidly. 

In some cases, we find rock beds where we can see evidence of a new species that evolved, lived and died out in such a short time span that we can walk through time, following the course of evolution using ammonites as a window into the past. 

For this reason, they make excellent index fossils. An index fossil is a species that allows us to link a particular rock formation, layered in time with a particular species or genus found there. Generally, deeper is older, so we use the sedimentary layers of rock to match up to specific geologic time periods, rather like the way we use tree rings to date trees.