Jeepers, creepers, where did the Cambrian ocean's top predator get those peepers?
Each of the shrimp-like animal's two pear-shaped eyes sported at least 16,000 hexagonal lenses. This would have given the 3- to 6-foot-long creatures vision as keen as the sharpest-eye insect today, the dragon fly.
A formal report of the discovery appears in the Dec. 8 issue of the journal Nature.
Anomalocaris's newfound eyeballs establish it as an ancient relative of today's arthropods – a broad classification of organisms that includes creatures ranging from lobsters and shrimp to spiders and damsel flies, the researchers say.
But what a relative. "Anomalocaris is the stuff of nightmares," said team leader John Paterson, a paleontologist with the University of New England in Australia, in a prepared statement.
In addition to its length, Anomalocaris sported a pair of barbed arms that protruded like pincers in front of its head, and a circular mouth with rasp-like serrations.
Based on fossil specimens of the creature uncovered in the US, China, and Canada, scientists have inferred Anomalocaris's position at the top of the food chain not only from its outward structure, but from the circular mouth marks found on hard-shelled trilobites, which could grow to lengths comparable to a large umbrella, as well as trilobite remains in fossilized fecal matter associated with Anomalocaris. Some researchers hold that these creatures would have been more partial to soft-bodied marine animals.
Whatever the mix of prey, the discovery and analysis of Anomalocaris eyes "confirms that it had superb vision to support its predatory lifestyle," Dr. Paterson said.
The team found the remains in shales from a quarry on Kangaroo Island, just off the southern coast of Australia. The shale formation, known as Emu Bay shales, is noted for preserving soft tissue, as well as skeletal remains.
The Emu Bay shales have yielded two species of Anomalocaris. The pair of eyes appeared in a section of shale that also yielded other specimens of Anomalocaris's arms and body flaps.
The researchers add that the presence of these eyes at this stage in the evolution of life on Earth indicates how quickly, in geologic time, novel features can emerge.
Anomalocaris lived during a period known as the Cambrian explosion, a period that lasted roughly 55 million years and saw an exponential growth in the number of new species compared with the rates in prior periods. Evolutionary biologists say that most of the broad body plans seen in organisms today first emerged during the Cambrian explosion.
No creature whose fossil dates to before the Cambrian period has eyes, the researchers say. Indeed, the discovery pushes the evolution of the compound eye much deeper into past than previous evidence suggested, and this form of vision probably emerged before the appearance of hard external skeletons – such as the one armoring Anomalocaris, the team posits.