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August 15, 2011
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Greg Edgecombe in the Museum's Palaeontology Department has collaborated with Australian colleagues on the investigation of fossil eyes from the early Cambrian period (515 million years ago).  They published their findings in the prestigious journal Nature at the end of June.


The fossils were found in the Emu Bay shale - a very finely grained rock formed from mud - from Kangaroo Island in South Australia.  Shales can preserve fossil organisms in incredible detail, and those from the Cambrian have yielded an amazing diversity of invertebrate animals that lived in marine environments - those of the Burgess Shale in western Canada are probably the best known.  The Cambrian is of particular interest in evolutionary terms because it was a period at which many new groups of organisms are first seen - the term "Cambrian Explosion" is often used to describe the fantastic and rapid diversification of life in a relatively short geological period.


A fair amount is already known about the eyes of trilobites from this period - their eyes were biomineralized (containing minerals) which meant that the fossils are very well preserved.  However, Greg and his colleagues found fossil eyes that do not seem to have been mineralized - much more delicate structures that have only been preserved because of the exceptional nature of the shale fossils.


The eyes appear to be relatively similar to the compound eyes of modern arthropods - they are around 5-7 mm across and contain around 3,000 individual lenses (ommatidia).  The lenses in the centre of the eye are larger, with a falling gradient in size towards the edge of the eye - creating a bright zone for better sight in lower light.  This structure is characteristic of a modern mobile predator such as a robber fly - more advanced than those of trilobites and not seen in other fossils for a further 85 million years. This emphasises the apparent rapidity and complexity of evolution in this early period.

 

robber crop.jpg

The fossil eyes have similar characteristics to those of modern predatory arthropods such as this robber fly

 

However, a puzzle is that the eyes do not seem to be associated with any identifiable organism in the shale, such as Anomalocaris. It seems possible that the eyes may have been shed in moulting, but by which animal remains to be seen.


 

Lee, M. et al. 2011 Modern optics in exceptionally preserved eyes of Early Cambrian arthropods from Australia. Nature 30 June 2011 474 7353 631-634 http://dx.doi.org/10.1038/nature10097