Saturday, 18 July 2015

Redefining The Extent Of The Cambrian Explosion

Treptichnus pedum, the trace fossil which defines the base of the Cambrian
The Cambrian Explosion is so named for the eponymous time period in which it occurred. In turn the eponymous time period is defined by the biological products of the Cambrian Explosion. The appearence of a trace fossil known as Treptichnus pedum is the official marker for the base of the Cambrian but, in the minds of all but the most obstinate of stratigraphers, the period is characterised by the oddities produced by the evolutionary radiation - the marellomorphs and the anomalocarids just to name two salient groups. When we think of the Cambrian sea floor, we picture a scene which is alien in comparison to those inhabited by later biotas.

One of the remarkable fossils from Fezouata,
specifically of a marellomorph arthopod
It was once thought that the temporal range of the more unusual families produced by the Cambrian Explosion did not extend beyond the Cambrian period itself. More recent discoveries of anomalocarids from the early Ordovician showed that some of these families did have a greater geological longeivity. A study of fossils from the Ordovician Fezouata formation has shown that other families also lasted longer than their geological record previously suggested. 'The Fezouata is extraordinarily significant' said Professor Derek Briggs from Yale University. 'Animals typical of the Cambrian are still present in rocks 20 million years younger, which means there must be a cryptic record in between, which is not preserved.'

Over 160 genera have been recorded from the locality, including a large number of lobopodians - the group of arthropods which contained within it the majority of the Cambrian oddities, including Hallucigenia and Anomalocaris. They show that the biotic legacy of the Cambrian Explosion extended well into the Ordovician, which itself is thought to have overseen its own evolutionary radiation - the Great Ordovician Biodiversification Event. 'There is much more to learn from the Fezouata' said Professor Briggs. 'Why do we not see more assemblages like this in the Ordovician? What ecological changes happened at the Cambro-Ordovician interval? Are the Cambrian Explosion and the Great Ordovician Biodiversification Event separate, or phases of the same event?'

Aegirocassis benmoulae was an Ordovician
arthropod which was larger than a human
An added advantage of the specimens from Fezouata is their quality. Preserved in fine grained mudstone, soft tissues and highly articulated structures are well displayed. Some specimens are even metres in length - relative battleships compared to their more modest Cambrian forebears. It is intriguing to note that the anomalocarid specimens from the formation are not only larger than their Cambrian ancestors, but also show a greater degree of specialisation, suggesting that the evolutionary radiation recorded at Fezouata is an extension of the preceding Cambrian event.

In addition to Cambrian hanger-ons, other fossils have redefined the evolutionary histories of other organisms besides. 'Horseshoe crabs, for example, turn out to be at least 20 million years older than we thought. The formation demonstrates how important exceptionally preserved fossils are to our understanding of major evolutionary events in deep time' said Peter Van Roy, also of Yale, who first recognised the scientific importance of the Fezouata fauna. The Kootenay and Maotianshan sites have yielded remarkable finds from the Cambrian which have added to the wealth of knowledge originally collected at the Burgess Shale. Fezouata, however, demonstrates that we should also examine the Ordovician to gain a fuller picture of the true extent of the Cambrian Explosion.