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 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:
Meet our newest potential weapon against malaria – a fungus loaded with a chemical found in scorpion venom. Metarhizium anisopliae is a parasitic fungus that infects a wide variety of insects, including the mosquitoes that spread malaria. Their spores germinate upon contact and the fungus invades the insect’s body, slowly killing it. Now, Weiguo Fang from the University of Maryland has modified the fungus to target the malaria parasites lurking inside the mosquitoes.
Fang loaded the fungus with two chemicals that attack the malaria parasite Plasmodium falciparum. The first is a protein called SM1 that prevents the parasites from attaching to the mosquito’s salivary glands. By blocking Plasmodium‘s path, SM1 stops the parasite from travelling down the mosquito’s mouthparts into the people it bites. The second chemical is scorpine – a toxic protein wielded by the emperor scorpion, which kills both bacteria and Plasmodium. This double whammy of biological weapons slashed the number of parasites in mosquito saliva by 98%.
In 1996, a loggerhead turtle called Adelita swam across 9,000 miles from Mexico to Japan, crossing the entire Pacific on her way. Wallace J. Nichols tracked this epic journey with a satellite tag. But Adelita herself had no such technology at her disposal. How did she steer a route across two oceans to find her destination?
Nathan Putman has the answer. By testing hatchling turtles in a special tank, he has found that they can use the Earth’s magnetic field as their own Global Positioning System (GPS). By sensing the field, they can work out both their latitude and longitude and head in the right direction.
“He relaxed and spread his two arms lazily across the seat back. He steered with an extra arm he’d recently fitted just beneath his right one to help improve his ski-boxing.” – Douglas Adams, The Hitchhiker’s Guide to the Galaxy
Zaphod Beeblebrox is just one of several characters from science-fiction and mythology to have extra arms. It’s a common enough trope, but would it actually work in real life? Could the human mind, which is so accustomed to controlling two arms, cope with a third or fourth one? According to Arvid Guterstam from the Karolinska Institute, the answer is yes. By placing a rubber right hand next to a person’s real one, and stroking both at the same time, Guterstam managed to convince people that they had a third arm.
Around 520 million years ago, a walking cactus roamed the Earth. Its body had nine segments, each bearing a pair of armour-plated legs, covered in thorns. It was an animal, but one that looked more like the concoction of a bad fantasy artist. Jianni Liu from Northwest University in Xi’an discovered this bundle of spines and named it Diania cactiformis – the “walking cactus from Yunnan”. And she thinks that it sits at the roots of the most successful group of animals on the planet.
If Liu is right, Diania is one of the earliest relatives of the arthropods – the group that includes insects, spiders, crabs, and more. These species all share a segmented body, a hard external skeleton and jointed legs. They are life’s winners, the most diverse of all animal groups.
To understand what made them so special, we need to look at where they came from. “Delving around the roots of arthropods might help us understand drivers of their current huge biodiversity,” says Michael Benton from the University of Bristol. For a start, how did they evolve from soft-bodied worm-like creatures into the armoured, legged animals we know today? Diania might help to provide some answers. If Liu is right, it was an animal that was “close to the point of becoming a true arthropod”.
In June 1935, the cane toad began its invasion of Australia. Sailors brought the animal over from Hawaii in an attempt to control the cane beetle that was ravaging Australia’s sugar cane crops. It was a mistake that the continent’s wildlife would pay for. The toad did nothing to stop the beetles. Instead, it launched its own invasion, spreading across the continent from its north-eastern point of entry. As it marched, it left plummeting populations of native species in its wake.
The toads are born conquerors. Females can lay 35,000 eggs many times a year, and each can develop into a new frog in less than 10 weeks. They’re unfussy eaters and they’ll munch away on bird eggs, smaller native frogs and more. And they have large glands behind their heads, which secrete a milky poison. Local predators (or domestic pets) that try to eat them tend to die.
Now, Daniel Florance from the University of Sydney has found a clever way of corralling the cane toad invasion. He realised that humans have continued to give the toad a hand, long after we first brought them to Australia. By creating dams and troughs, we provided the toad with watery staging grounds that allowed it to spread across otherwise impassably dry land.
Alexandrium is part of the sea’s collection of plankton. It’s a single-celled creature but it can create colonies by amassing together in long chains. At their most extreme, these colonies can form large swarms to produce harmful red tides.
As chains, Alexandrium swims and grows faster, but it is vulnerable to predators such as copepods – small relatives of crabs or shrimp. Erik Selander from theTechnical University of Denmark found that when the chains detect the chemical traces of copepods, they break apart. By turning back into single cells, they make themselves harder to find. They also swim at a slower pace to avoid creating telltale movements in the water. When threatened by predators, these plankton enter stealth mode.