Australia: The Land Where Time Began

A biography of the Australian continent 

Fossil Eyes - a key factor in the Cambrian Explosion

Fossil eyes have been found in the Emu Bay Shale that suggest vision developed rapidly once early modern animals evolved. The eyes are the earliest known complex eyes in the fossil record (Michael Lee et al., Nature, 474: 631-634 (2011). They are believed to be from an early arthropod, possibly from an animal such as Anomalocaris, though a specimen that was not fully grown. They are from the Early Cambrian, about 515 Ma, and are preserved so well that the details of their optical design can be observed.

Prior to the find of these eyes the earliest known eyes were from trilobites that had unusual compound eyes composed of calcite, that were relatively simple, and some very small eyes that were not preserved with enough detail to allow much of their structure to be determined.

In the new eyes there are more than 3,000 lenses, the design being as complex as those of living arthropods, according to palaeontologist John Paterson. Once eyes had evolved there would have been strong evolutionary pressure on them to improve as rapidly as possible in both predators and prey. These eyes are believed to support the proposal that the development of advanced vision was a key driving force in what has been termed the Cambrian Explosion. According to Professor Derek Siveter of the University of Oxford, these complex organic eyes are much more highly developed than the eyes of the trilobites from the Early Cambrian, that were biomineralised.

The excavators found 7 specimens of the eye, 7-9 mm long, on a farm near Emu Bay on the north coast of Kangaroo Island. They were found in dark grey shale that was deposited on the sea floor. Given that the site has produced more than 5000 fossils to the time of writing, they appear to be extremely rare. As the eyes were found isolated from any animal fossil, it is uncertain which animal they came from, Peterson suggesting they came from a shrimp-like animal that was free-swimming, the optical design suggesting an active predator that was highly mobile that was probably able to see in low light. Peterson believes these eyes indicate that complex predator-relations were already present in the Early Cambrian. These eyes also indicate that the arthropods acquired visual sophistication in the earliest stages in their evolution.

According to the 'light switch theory', the development of vision was a major factors leading to the Cambrian Explosion, the time when most of the animal groups of the present first appeared. As there is no evidence that is considered to be convincing that arthropods with eyes were present before the Cambrian, the finding of these eyes suggests that eyes evolved rapidly, on a geological timescale.

Derek Briggs of Yale University said "the arrangement of the lenses indicate that the eyes are those of a predator, prompting speculation about whether they might be those of the iconic Cambrian animal Anomalocaris which is also known from the Emu Bay Shale." It has been suggested by the authors that the eyes are too small to be those of a fully grown Anomalocaris, a large shrimp-like animal reaching up 2 m in length, believed to be a top predator in the Cambrian, though they could possibly from younger individuals.

According to Patrick Orr of University College Dublin, the ecosystems of the Cambrian had a much greater emphasis on predator-prey strategies that is seen in younger ecosystems, in the Cambrian sediment digestion being less important that it became later, with the result that acute vision was even more important. He also said the eyes were probably not globular structures that had been flattened as there are no wrinkles in the fossils, suggesting that in life they probably looked like "wrap-around shades."

Links

1. South Australian Geologist Newsletter September 2008
2. Trilobites of the Emu Bay Shale, Australia
3. Fossils reveal rapid evolution in ancient eyes
4. Modern optics in exceptionally preserved eyes of Early Cambrian arthropods from Australia

1. The Emu Bay Shale biota, Kangaroo Island: Australia's unique window into the Cambrian world
2. Life before time, Cosmos Magazine
3. Fossils reveal rapid evolution in ancient eyes