A half-billion-year-old fossil compound eye, showing exquisite detail of the visual surface (the individual lenses can be seen as darker spots).

Credit: J. Paterson/Univ. of New England

The compound eyes of a living insect - a predatory robberfly - showing the individual lenses.

Credit: Peter Hudson/South Australian Museum

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DUBLIN: Highly complex eyes have been found in half-a-billion-year-old rocks from South Australia, suggesting that vision evolved rapidly with the arrival of early modern animals.

A new study, by a team led by palaeontologist Michael Lee from the South Australian Museum and University of Adelaide and published in the current issue of Nature, documents the oldest complex eyes found in the fossil record. The study reveals that some of the earliest arthropods - the group consisting of crustaceans, insects, spiders and centipedes - had advanced compound eyes.

The fossils hail from the Early Cambrian (515 million years ago), and are so exquisitely preserved that the details of their optical design can be observed.

"The discovery tells us that whilst the beginning of the Cambrian saw the arrival of most modern animal groups and therefore brand new body plans, there was also rapid innovation at finer anatomical scales," said co-author John Paterson, palaeontologist at the University of New England, New South Wales.

Seeing is believing

Previously, knowledge of the earliest Cambrian eyes was limited to those of trilobites, which had relatively simple yet unusual compound eyes made of calcite, and tiny ill-defined fossils which lack detail.

The newly discovered eyes have a dense and regular hexagonal array of over 3,000 large lenses. The design turns out to be just as advanced as the compound eyes of modern arthropods, Paterson said.

Given the advantages conferred by sharp vision for avoiding predators and locating food and shelter, there must have been great evolutionary pressure to refine visual organs. The new fossils are consistent with the idea that the development of advanced vision helped drive the great evolutionary event known as the Cambrian explosion.

"They bear witness to the fact that the development of very refined visual systems was present and rapid in early Cambrian times," commented Derek Siveter, professor of earth sciences at the University of Oxford. He said the complex, organic eyes are much more highly developed than the biomineralised eyes present in Cambrian trilobites.

The scientists found seven specimens of the eye over four years of excavation on a farm on the northern coast of Kangaroo Island near Emu Bay. The fossils were taken from dark grey shales that were once mud at the bottom of an ancient sea floor. "They are extremely rare, given that we collected over 5,000 fossil specimens from this site," explained Paterson.

Mystery predator

The fossil eyes, which are 7-9 mm long, were found isolated from their owner, so it's uncertain what the creature looked like. However, the scientists are confident that they belong to an arthropod. A free-swimming, shrimp-like animal, is Paterson's best guess as to its identity.

"The optical design of these eyes points to an active, highly mobile predator capable of seeing in low light conditions, suggesting that complex predator-prey relationships were already in place during the early Cambrian," said Paterson. The discovery shows arthropods achieved sophisticated vision in the earliest stages of their evolution.