Death by ammonite: Early Jurassic fish choked to death on an ammonite shell!

A fantastic catch

Fossilized stomach contents are rare in the fossil record but can offer incredibly important insights into the life and death of extinct animals through deep time. While studying fossil fishes in the collections of the Stuttgart Museum, I (Sam Cooper, PhD student, Stuttgart State Museum of Natural History) identified a unique predator-prey association from the Early Jurassic Posidonia Shale (Posidonienschiefer) of southern Germany, consisting of a large bony fish named Pachycormus preserving a large ammonite shell inside of its stomach. The ammonite shell is undigested and well preserved, indicating that the shell was swallowed immediately prior to the fish’s death. Working together with Dr. Erin Maxwell (Stuttgart State Museum of Natural History), we concluded that this unique feeding event was fatal, with the ingestion of the ammonite shell being directly responsible for the fish’s death. Either the shell became trapped in the throat causing the fish to choke and drown, or the sharp edge of the shell tore open the stomach wall causing catastrophic internal bleeding. Either way, it must had been a very unpleasant death.

Ammonites – tough shells to crack

Have you ever looked at an ammonite and wondered what it would taste like? No, me neither, but clearly some animals which inhabited the Jurassic seas did. Despite their high abundance as fossils from across the globe, there is little evidence to suggest ammonites were prey to Mesozoic vertebrates. Rare damaged shells with bite marks were likely bitten by marine reptiles but the potential for fishes to prey on ammonites had only been speculation. At first, we thought this association was a mere coincidence, with the dead fish simply lying on top of an ammonite shell. However, upon detailed examination we found conclusive evidence to prove that the shell is contained inside of the fish’s gut, thereby confirming this unique and unexpected predator-prey relationship.

All shell, no meat

The ammonite was empty at the time of ingestion as indicated by the absence of the aptychus – a jaw-like structure associated with the soft parts of the ammonite creature. This behaviour is highly unusual, as an empty ammonite shell would not provide any nutritional value to the fish, and therefore it was not ingested intentionally. Erin and I proposed two possible scenarios for why this happened:

(1) fatal attraction?

The empty shell was likely floating around in the Jurassic seas which flooded southern Germany at this time, with the movement of the shell mimicking a distressed smaller fish or squid which caught the eye of our Pachycormus. The fish strikes at the shell, unaware of what the prey actually was – engulfing it swiftly without hesitation. Unaware of this fatal mistake, or perhaps first attempting to dislodge the unwanted meal, the shell is swallowed. Or:

(2) bit off more than it could chew?

The fish was scavenging on the rotting ammonite creature as it hung limp from the body chamber, lifeless tentacles swaying in the water column, the aptychus already displaced. Something goes badly wrong and the shell became trapped in the mouth, and unable to dislodge it, the fish attempts to swallow it.

A fatal mistake?

Regardless of how the ammonite ended up inside of the fish’s gut, our research concludes that the feeding event was likely a fatal one, with the fish dying in only a few minutes to hours after this final meal. We know this for two reasons. firstly, the shell is remarkable well preserved, even better mind you than the majority of ammonites which have been collected from the Posidonienschiefer in more than 200 years of collecting. The ammonite could only had been exposed (if at all) to stomach acids for a very brief window, perhaps only few minutes, as otherwise one would expect the aragonite shell to rapidly deteriorate and dissolve if exposed to acids for a prolonged duration. Secondly, the size of the ammonite is very large in relation to the stomach of the fish, with its wide diameter and awkward shape likely causing a fatal blockage or on its path towards the stomach. It is difficult to provide a suitable analogy for this type of trauma, although I suspect it would be the near equivalent to you or I swallowing a small dinner plate – not something I would encourage unless you want to end up like Pachycormus.

Treasures in the basement

The specimen was donated to the Stuttgart State Museum of Natural History in the 1970s, where it was placed into a draw and practically forgotten about. It is remarkable to think that such an important fossil had gone unnoticed and unappreciated for almost half a century. It is a textbook example of why it is important to re-examine old museum specimens, even if the species they represent are relatively ‘common’. If you want to make fantastic discoveries in palaeontology, you don’t always need to visit a beach or quarry, rather just ask to take a look inside the draws at your local museum.