Biology

Ammonoids were soft-bodied organisms with an external shell. 

The Hildoceras bifrons shell is a flattened spiral shape with evolute whorls.

Internally the shell is divided up into chambers - each chamber would have been filled with gas to act as a buoyancy aid for the animal. 

As ammonoids are extinct and soft body parts are rarely preserved, little is known about their way of life. However, some evidence suggests that they had an active predatory lifestyle, catching prey in their tentacle-like arms.

Size

A complete adult macroconch varies between 95mm and 175mm diameter, whereas an adult microconch varies between 24mm and 41.5mm diameter.

Glossary

Nekton is a free-swimming animals

Evolute and involute are geometrical terms relating to the shape of a curve

Lappets are spoon shaped extensions from the margin of the aperture

Arcuate is curved

Striae are fine lines of ornament

Ventro-lateral keels are ridges (keels) located on the sides of the venter

Tricarinate and bisulcate are two grooves separating the has three ridges (keels)of the venter.  

Venter is  the shell edge around the circumference