By Paul Rincon
BBC News Online science staff
Direct evidence has now been found to show that trilobites - among the most diverse of fossil animal groups - were eaten by other ancient sea creatures.
Trilobites may have been a major source of food for other animals
Scientists discovered cracked trilobite body parts in the gut of a 510-million-year-old fossil marine animal.
It was long suspected that the ubiquitous trilobites, which survived for about 300 million years, were a major food source for larger creatures.
New research in Biology Letters offers the first firm evidence for this.
Trilobites are an extinct group of arthropods, animals with an outer skeleton, a segmented body and jointed limbs.
They are probably the most common fossils of the Paleozoic Era (about 545-250 million years ago) and scientists use them to help date different layers of rock.
US, French and Chinese researchers identified fragments of the hard outer shell of trilobites in the fossilised gut contents of an unknown animal from the Kaili Formation, a sequence of rocks from southern China dating to Middle Cambrian times.
The broken up pieces of trilobite exoskeleton are each about 1mm in length and are stacked on top of each other.
But by using a scanning electron microscope, the researchers were able to identify features on the chewed-up shell fragments known as scrobicules, pitting on the surface of the shell, that are typical of so-called eodiscoid trilobites.
The only eodiscoid trilobites known from the Kaili Formation are from the family Pagetia and the researchers suggest these could be the ones that ended up as the Chinese meal.
"It's very interesting. It's about as direct evidence as you can get, even better than having coprolites - fossilised faeces," Professor Simon Conway Morris, a palaeobiologist at the University of Cambridge, UK, told BBC News Online.
TRILOBITES - ANCIENT ANIMALS
Lived before the dinosaurs
Biggest were a metre long
15,000 species now described
Can assist with dating of rocks
"Eodiscoid trilobites are normally thought to be swimmers that live up in the water column, which either means that they weren't - that they lived on the seabed - or that [this predator] is going up into the water column and chasing them."
Professor Conway Morris suggested that the find could be indicative of a complex ecology at Kaili in Middle Cambrian times.
The researchers say the animal which ate the trilobites is unlikely to have scavenged them because all the gut contents appear to belong to one animal.
"We're fairly confident this specimen provides actual evidence of predation," co-author Heyo Van Iten, associate professor of geology at Hanover College in Indiana, US, told BBC News Online.
"There are a variety of other explanations for the presence of the trilobite parts in the gut of this fossil. But they are all less probable than the simplest one, which was that they were actually eating these things."
They have also found dark-coloured nodules in the digestive canal of the creature, which appear to contain spherical bacteria.
The team suggests these nodules are well-developed digestive glands, which are present in other Cambrian arthropods with presumed predatory habits.
The gut contents include trilobite body parts
The gut contents belong to an arthropod which has some similarities to the creature Fuxianhuia, known from early Cambrian rocks in Chengjiang, China. But not enough of the creature remains for it to be named.
Other, indirect evidence of predation on trilobites has been known for some time.
Bite marks in the animals are common, with many more appearing on the right-hand side than the left-hand side - an observation that has not yet been fully resolved.
Many scientists also believe that the hard exoskeleton of trilobites evolved as a defence against predators.