Fossil pits show how ammonites turned parasites into pearls

Ammonites – the ancient relatives of squid and octopuses – left behind some of the most common and beautiful fossils. But look closely at their elegant, spiral shells and you might be able to spot a sinister secret. Some of them are dotted with small pits along their inner walls. Kenneth de Baets from the University of Zurich thinks that the remains of parasitic worms that infested the ammonites and were eventually trapped and killed.

The pits were first described by Michael House in 1960 and they’re known as “Housean pits” in his honour.  They’re also called “pearls” since House suggested that these precious stones once resided in the pits. We associate pearls with oysters but any shelled mollusc can produce them, from ammonites to clams. If parasites or irritating particles get inside the shell, the animal protects itself by sealing off the intruder in a mineral sphere. Pearls may be pretty but they’re also a defensive prison.

Now, de Baets, together with Christian Klug and Dieter Korn, has confirmed House’s ideas by studying a large sample of ammonite shells, which he uncovered in Morocco. He thinks that the pits were indeed once filled by pearls. These precious prisons were created to trap parasitic worms.

The critical piece of evidence was a series of thin tubes within each of the pits, which led to the shell’s outer surface. De Baets discovered these tubes by looking carefully at cross-sections of ammonite shells, and he thinks that they’re tunnels created by parasites.

Certainly, they’re not normal parts of the ammonite’s shell – you can’t find them in all specimens and they’re seldom arranged symmetrically. And they’re not the result of objects penetrating or boring through the shell from outside, because no such marks have been found. Instead, their shape and size suggest that they were created by a living thing. De Baets thinks that they’re a close match to the tubes created by modern flukes or trematodes – a group that commonly infects snails, cephalopods and other molluscs.

The trematodes could have swum through the gap between the shell and the ammonite’s body. Alternatively, they could have wormed their way towards the shell from an internal organ, after being swallowed. Once inside, they would have fed upon the host’s tissues until they were sealed in by the overgrowing shell and trapped within a pearl.

If de Baets is right, these parasites have been infecting their cephalopod hosts for around 400 million years. To understand their evolution, he compared the different types of pits and built a family tree that revealed their evolutionary relationships. The tree shows that over time, the parasites went from creating a few big pits to several, smaller ones. The tree provides a tantalising glimpse into the battles between parasite and host, with some trematodes giving up soon after infecting ammonites, others jumping from host to host and yet others co-evolving with the same partner for around 15 million years.

Studying prehistoric parasites isn’t easy. They’re usually small, they tend to live inside their hosts, and their typically soft bodies don’t fossilise well. Often, the only clues to their presence at the traces of the damage they caused. Like the Housean pits of ammonite shells, many of these signs are obscure to the untrained eye, including cysts on the surface of shells and holes created by driller-killers.

I’ve written about two of the most compelling examples, discovered in recent years. David Hughes from Harvard University claimed that small scars on the veins of fossil leaves were left behind by ants that were infected by a lethal fungus. In their deaths, the ants gripped the leaves in their jaws while the fungus fatally erupted from their heads. And Ewan Wolff from the University of Wisconsin thinks that small pits in the jaws of Tyrannosaurus rexsmall pits in the jaws of Tyrannosaurus rex were caused by a parasite whose relatives cause trichomonosis in modern birds. This “plague of tyrants” would have created ulcers throughout the dinosaur’s jaw and eroded its bone.

In the most exciting examples, parasites are preserved together with their hosts in mutual death throes. Witness, for example, a mite that was trapped while sucking the blood from a spider. George Poinar Jr, the scientist who popularised the Jurassic Park idea of extracting DNA from insects trapped in amber, has found several such cases. In pieces of amber, he has discovered: a sand fly whose body is riddled with a parasite similar to those that cause sleeping sickness; several parasitic worms actually bursting out of their hosts; a parasite feeding on another parasite, growing on the world’s oldest mushroom; and many more.

Reference: Klug, C. (2010). Devonian pearls and ammonoid-endoparasite co-evolution Acta Palaeontologica Polonica DOI: 10.4202/app.2010.0044

More on parasites:

• Pocket science – ancient death-grip scars
• The plague of tyrants – a common bird parasite that infected Tyrannosaurus
• Body-snatching, not socialising, drove the evolution of bigger-brained insects
• Parasitic worms paint warning colours on their hosts using glowing bacteria
• Disfiguring disease caused by an alliance between three parasites