Cambrian predator had killer eyes
View to a kill A fearsome predator that swam in the Cambrian oceans was in fact a metre-long arthropod with killer vision, say researchers.
Palaeontologist Dr John Paterson, from the University of New England, and colleagues, have discovered the fossilised remains of compound eyes from a creature called anomalocaris.
"The fact that each eye in anomalocaris would have had over 16,000 lenses, means it would have very, very good resolution," says Paterson, whose work is published today in the journal Nature.
"When you consider that a modern housefly, for example, has about 3000 lenses, it's pretty impressive that an animal half a billion years old already has remarkable vision like this."
Around 500 million years ago, during the Cambrian, large alien-like creatures began to resemble modern animals, says Paterson.
"But you still had the odd freak swimming around," he says, referring to the metre-long free-swimming anomalocaris.
"It's an animal that's on its way to becoming an arthropod from an evolutionary point of view," says Paterson.
The creature would have propelled itself through the water by moving flaps down the length of its body in a wave-like pattern, he says.
It had a soft shrimp-like exoskeleton and fierce grasping claws and would have dined on soft-bodied organisms such as worms.
But anomalocaris has presented a bit of puzzle to scientists because while it looked like it might be an arthropod, it was missing certain attributes.
Arthropods include animals with jointed legs such as insects, spiders, crustaceans and millipedes.
"But when you look at anomalocaris, it doesn't actually have any jointed legs," says Paterson.
Now, the discovery of fossilised compound eyes belonging to the animal has confirmed it is indeed an arthropod.
"Compound eyes are actually a definitive arthropod characteristic or trait," says Paterson.
The fossilised eyes were discovered alongside remains of the grasping claws and the body flaps in shale at Emu Bay on the northern coast of Kangaroo Island, off South Australia.
The soft tissue of the compound eyes were slowly replaced by a mineral called pyrite in a low oxygen sediment, says Paterson.
The discovery supports the idea that compound eyes evolved very early on in arthropod evolution, before the evolution of jointed legs or hardened exoskeletons.
The surprisingly sophisticated eyes of anomalocaris are rivalled only by modern dragonflies, which can have up to about 28,000 lenses in each eye, says Patterson.
"Dragonflies are known for their exceptional vision," he says.
Lenses in a compound eye can be likened to pixels on a computer screen and the more lenses you have the better the resolution.
The small size of the lenses in the anomalocaris eyes suggests the animal would have lived in well-lit waters and had a major advantage over its prey.
"The little that we do know about the contemporaries of anomalocaris, they either had fairly poor eyesight or they were completely blind," says Paterson.
He says the presence of anomalocaris would have driven the development of protective adaptations in prey animals, and in turn, counter adaptations in predators.
Such an escalatory 'arms race' would have seen, for example, the evolution of such adaptations in prey as shells, camouflage and burrowing into sediments.
Paterson now hopes to find the remains of even more Cambrian creatures in the Emu Bay Shale.
"It would be great to find a hybrid that looked like anomalocaris but it had walking legs. That would be a beautiful missing link," he says.