A huge, shrimp-like marine creature that pursued its prey through the warm waters of the Earth’s Cambrian Period 515 million years ago had more acute vision than most of its diminutive modern-day relatives.
This discovery, published today in the international scientific journal Nature (vol. 480, pp. 237-240), gives a surprising new twist to the story of the evolution of complex eyes and the “family tree” of the creature itself – Anomalocaris.
An international team of palaeontologists led by Dr John Paterson from the University of New England in northern NSW discovered the fossilised eyes – the first Anomalocaris eyes with preserved lenses ever found – while excavating in the Emu Bay Shale on South Australia’s Kangaroo Island.
“Anomalocaris is the stuff of nightmares and science fiction films,” Dr Paterson said. “It is considered to have been at the top of the earliest food chains because of its metre-long body, the formidable grasping claws at the front of its head, and its circular mouth with teeth-like serrations. And this new discovery confirms that it had superb vision to support its predatory lifestyle.”
The eyes of Anomalocaris, until now known only by their outlines from other fossilised remains around the world, are oval-shaped, with the longer axis measuring 2-3 cm. This new discovery clearly shows the structure of the eye, which contained at least 16,000 hexagonal lenses arranged in a pattern of larger hexagons (“hexagonal packing”). “The number of lenses and other aspects of their optical design suggest that Anomalocaris would have seen its world with exceptional clarity while hunting in well-lit waters,” Dr Paterson said.
The compound eyes of Anomalocaris place it firmly on the “family tree” of the arthropods (along with modern-day spiders, scorpions, crabs, shrimps, lobsters and insects), an evolutionary relationship that was unclear before this discovery. Also, as the eyes date back to an early phase of the “Cambrian explosion” in animal evolution, and as there is no evidence of eyes of any kind in the pre-Cambrian fossil record, they provide evidence that complex eyes appeared unusually rapidly in evolutionary terms – “in the geological blink of an eye”, as Dr Paterson said. Among the arthropods, the visual acuity of Anomalocaris eyes is exceeded only in the dragonflies we share the world with today.
Dr Paterson and his team have published previous discoveries from the Emu Bay site of a different type of compound eye from an unknown arthropod. Their discovery published today (Paterson, J.R., García-Bellido, D.C., Lee, M.S.Y., Brock, G.A., Jago, J.B. & Edgecombe, G.D. “Acute vision in the giant Cambrian predator Anomalocaris and the origin of compound eyes”. Nature 480, 237-240) is the first time they have been able to positively identify the owner.
“The existence of highly visual hunters within Cambrian ecological communities would have influenced the ‘arms race’ that developed between predators and their prey during this important phase in early animal evolution more than 500 million years ago,” Dr Paterson said.
THE IMAGE displayed here is a reconstruction of Anomalocaris by Katrina Kenny, University of Adelaide. A larger version of this image is at: http://farm8.staticflickr.com/7020/6469806567_7a8180d604_o.jpg.
A PHOTOGRAPH, by John Paterson himself, of one of the fossilised eyes of Anomalocaris is at: http://farm8.staticflickr.com/7018/6469806293_50f726f89c_o.jpg
A PHOTOGRAPH of Dr Paterson with a fossilised frontal claw of Anomalocaris is at: http://farm8.staticflickr.com/7171/6469806489_d40303851e_o.jpg