DUBLIN: A remarkably preserved fossil from South Australia has revealed that the top predator in the Cambrian oceans, over 500 million years ago, was equipped with complex eyes and excellent vision.
The alien-like eyes of this fearsome metre-long hunter consisted of thousands of tiny lenses and sat on protruding stalks. The optical design indicates that the creature, Anomalocaris, pursued prey in well-lit waters.
“What was most surprising is the huge number of lenses in each eye,” said lead author John Paterson from the University of New England of the study published in Nature today. With thousands of lenses in each eye, that “was something I never would have predicted, particularly of an animal of this age”.
Studying an ancient killer
The body of Anomalocaris was segmented and had several pairs of swimming flaps; its head had formidable clasping claws and a mouth surrounded by toothed plates. The body had a tendency to break into different parts, and these fossils were described as different animals for many years.
“It wasn’t until the 1980s that it was realised that all the different body parts belonged to one strange animal,” said Allison Daley, a researcher at the Natural History Museum in London who is studying Anomalocaris and its kin.
The details of the eyes remained a mystery until now. Each oval eye is up to 3 cm in length and contains over 16,000 lenses. Its acute vision rivals or exceeds that of most living insects and was probably comparable to predatory dragonflies today.
The fossilised eyes were discovered in the Emu Bay Shale of South Australia, dating from approximately 515 million years ago. “I had a pretty good idea of what they were straight away. We had a film crew with us at the time, so I quietly passed it over to Jim Jago [a researcher at the South Australian Museum], who took one look with his hand lens and gave me the classic raised eyebrow,” Paterson said.
The compound nature of the eye indicates that Anomalocaris is a close arthropod relative. Arthropods are the group which includes insects, crustaceans, spiders and centipedes.
Having direct evidence of the compound eye helps us pick apart the sequence by which arthropods built up their body plan, said Gregory Edgecombe, a co-author at the Natural History Museum in London. It shows the arthropod eye became elaborate early in the evolution of the group, originating before characteristics such as a hardened exoskeleton or jointed walking legs.
The acute vision of Anomalocaris gave it a distinct advantage over competing predators and prey, as many Cambrian animals either had poor vision or were completely blind. “This advantage would have placed considerable selective pressure on prey – that is, a case of survival of the fittest – so prey animals had to adapt and rapidly evolve defensive armour such as shells or face extinction,” Paterson said.
Anomalocaris has been blamed for bite marks on trilobite fossils. It seems likely that it would have eaten almost anything it could find, in a mixture of scavenging and predatory behaviour, said Daley, who compares its position in the food chain to today’s great white shark.
Paleobiologist Peter Van Roy, affiliated to Yale University and Ghent University in Belgium, described the size and complexity of the eyes as “amazing”.
“Predators are one of the factors driving the evolution of ecosystems, and having such a well-equipped apex predator in the Cambrian would likely have been a significant driver of the ‘arms race’ between predators and prey that really took off with the Cambrian explosion,” he said.
Emu Bay comes up trumps
“The pleasing thing is that Emu Bay Shale has turned up another trump card, commented palaeontologist Patrick Orr of University College Dublin in Ireland, and this is an exciting sequel to a report by this research team earlier this year. “But this is bigger and better material in terms of its potential impact.”
“This find, like all fossils from Cambrian Lagerstätten (the fancy term to describe fossil sites where soft parts are preserved), is incredible when you consider that the fossils are over 500 million years old and are recording the earliest complex marine animal ecosystems in the history of the planet,” added Daley.
“Despite the age, sites such as Emu Bay Shale preserve in amazing detail the soft parts of animals that wouldn’t normally be preserved,” she said, such as eyes, skin, gut and gills.
Paterson believes it is only a matter of time before they find the complete body of Anomalocaris in the Emu Bay Shale. “The Emu Bay Shale is now a force to be reckoned with in Cambrian research,” he said.