Ammonite bitten by a mosasaur
Late Cretaceous
Fox Hills Formation
South Dakota, USA.

Highly unusual 94mm ammonite on 124mm cannonball concretion that was bitten by a small mosasaur.
( Part and counterpart included.)

An ammonite could be bitten by a mosasaur during a predatory attack in the ancient oceans of the Late Cretaceous. Ammonites were marine mollusks with coiled, chambered shells, while mosasaurs were large, fast-swimming marine reptiles with powerful jaws and sharp, conical teeth designed to grasp and crush prey.

When a mosasaur attacked an ammonite, it likely targeted the soft body inside the shell. However, fossil evidence shows that mosasaurs often bit through the shell itself, leaving distinctive puncture marks, cracks, or even shattered shell fragments. These bite marks suggest that mosasaurs could crush the shells with considerable force, sometimes breaking through the coil to extract the flesh within.

Such fossilized interactions—ammonite shells with mosasaur bite marks—provide rare and direct evidence of predator-prey behavior in the Cretaceous seas, showing how even well-armored animals like ammonites were vulnerable to top predators like mosasaurs.