Opalescent Ammonite with Mosasaur Bite Marks

sphenodiscus lenticularis

General

Ammonites get their name from the Egyptian deity Amon-Rê, who was always depicted with ram's horns attached to its head. 

The shells are the 'houses' of a species of squid that first appeared on earth more than 350 million years ago and died out about 66 million years ago, at the same time as the collapse of the dinosaurs.

The animal itself lived in the last and largest chamber of the shell. Whenever that became too small for its body, it would build a new, larger chamber onto the old one and relocatethere. On the outside of an ammonite, one can usually see the lines that indicate where a new chamber begins. 

The chambers also had another function. By filling them partially or completely with gas (mainly nitrogen that they produced themselves), ammonites could rise to the surface like a submarine. And that was necessary because that is where reproduction took place. It is also most probably the reason why ammonites became extinct. Acid rain and dust clouds caused by the meteorite impact at the end of the Cretaceous Period resulted in very poor conditions for the eggs and young to develop properly.

The largest ammonite ever found had a diameter of over 2.5 meter. 

Specific

This impressive specimen has been expertly prepared and polished to reveal the internal chamber structure, with septal divisions infilled in contrasting dark mineralization, creating a striking graphic pattern across the whorl.

The exterior shows vivid opalescent and iridescent flashes—subtle greens, reds, and amber tones—characteristic of mineral replacement within the original nacreous shell layers. These chromatic effects animate the surface under changing light, lending the fossil a sculptural presence beyond its scientific importance.

Most compelling are a series of rounded punctures and associated fractures along the outer whorl, interpreted as mosasaur bite marks. The damage, subsequently healed by sedimentary infill and mineralization, preserves a dramatic moment of ecological interaction from approximately 66 million years ago.

Ammonites from the Fox Hills Formation represent some of the final flourishing lineages before the end-Cretaceous extinction. This specimen, combining aesthetic opalescence, substantial scale, and documented predation marks, stands as both a natural artwork and a paleobiological narrative.