The Oldest Reefs On The Planet

Reefs are typically formed from biomineralised corals

Reefs are amongst the most biodiverse ecosystems on the planet. Containing tens of thousands of species which form a vast ecological network, reefs have existed at the centre of marine life for hundreds of million years.

Typically, corals form the substrate of reef networks, thanks to their ability to mineralise their bodies, making them remarkably resilient structures, especially when formed en mass. As a result they are common in the fossil record. The Wren's Nest at Dudley for example is composed of limestone chock full of coral specimens interspersed with trilobite shells and bivalves. Yet they are not the only reef-forming organisms. Certain sponges, like the siliceous glass sponges can contribute as well, owing to their ability to mineralise their tissues.

Reef systems made their first appearance during the Ordovician period when the rugose and tabulate coral families diversified following the end Cambrian extinction event. Prior to this corals and sponges existed as small, simple communities, rather than forming extensive, mineralised networks. Now fossil evidence has shown that the story of reefs extends further back then previously thought. The first animals, namely the members of the Ediacara biota, were all soft bodied, but towards the end of their evolutionary dominance some species began to develop hard exoskeletons composed of calcium carbonate. It is now known that these evolutionary pioneers built the first ever reefs on the planet.

A reconstruction of Cloudina in a gregarious state

In the 1970s fossils of a bizarre creature known as Cloudina were discovered in 548 million year old rocks in South Africa. Up to ten centimetres in length and eight millimetres in diameter, it consists of a series of funnel shaped sections stacked one on top of the other. Most likely a filter feeder, it lived anchored to the sea bed in small colonies. now researchers from the University of Edinburgh have shown that these communities were extensive in some circumstances, allowing Cloudina to create a biological superstructure - a reef - from their calcium carbonate enriched bodies.

Excavations in Namibia revealed the presence of a Cloudina reef spanning 4.3 miles. It is possible that this was simply the result of a breeding strategy where the offspring existed close to the parents, but the researchers suggest that it may actually be a response to predation. Cloudina fossils have been found with small boreholes in their cones which have been interpreted as marks made by a predator seeking out the soft tissue inside. The Ediacara biota were the first complex, multi-cellular organisms on the planet. While most were filter feeders it is likely that some evolved to feed on other organisms, tapping into a rich source of protein. Subsequently some Ediacarans evolved hard exoskeletons. To counter this predators developed the ability to bore through calcium carbonate shells.

Part of the Cloudina reef from Namibia

It is possible that Cloudina's response in this evolutionary race was to form gregarious reef communities. Only the top cone, which was the youngest, housed the living creature. The rest would have remained inert, but mineralised, keeping the creature high off the sea floor and out of the reach of predators. Multiple densely packed stems would have provided group protection, further increasing the chance of survival and reproduction, as well as creating turbulence in the water, causing the deposition of nutritious particles onto the reef.

'We have found that animals were building reefs even before the evolution of complex animal life, suggesting that there must have been selective pressures in the Precambrian period that we have yet to understand,' said Professor Rachel Wood from the University of Edinburgh.

Cloudina may have been a primitive form of coral, although it is hard to tell from its morphology. In either case its reef-forming abilities may help us understand the roots of the Cambrian Explosion itself. Numerous ecological niches and selective pressure exist within a reef community and it is possible that the diversity of life which developed in the first reefs laid the foundations for the greatest biological revolution in evolutionary history. While Cloudina has been extinct for over 500 million years, its impact lives on.