The First Animal Reefs

Sandwiched between two deserts, the dry landscapes of Namibia see less rainfall than any other country in sub-Saharan Africa. But things looked very different 550 million years ago. These desolate expanses were submerged under an ancient sea, where they witnessed — and took part in —the Cambrian explosion, an incredible diversification of multi-cellular animal life when most modern phyla made their first appearance. Researchers have now shown that the first animal reefs also appeared around the same time, tracking down fossils of reef-building creatures on the ancient sea floors of the Great Namibian Escarpment.

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Before the Cambrian explosion, most of the animals on Earth were soft-bodied creatures which don't fossilize well. The oldest known skeletons belong to Cloudina, a filter-feeder that left behind a distinctive exoskeleton consisting of cones stacked atop each other. Cloudina may have anchored themselves to stromatolites or thrombolites, ancient reef-like structures which bacteria had been building for billions of years before we animals got into the game.

According to the team's findings, Cloudina was also the first animal to build reefs. They found evidence that these creatures attached themselves to each other (and to other surfaces) using calcium carbonate, the same cement used by modern corals. Forming a reef might have helped these animals protect themselves from predators and also given them better access to nutrient-rich currents. Given the immense competition for food and space in these ancient seas, both benefits are likely to have offered a selective advantage.

"Modern reefs are major centres of biodiversity with sophisticated ecosystems. Animals like corals build reefs to defend against predators and competitors," said Rachel Wood, a professor at the University of Edinburgh who took part in the study. "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."

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Ref
A. M. Penny et al. Ediacaran metazoan reefs from the Nama Group, Namibia. Science 344, 1504 (2014); doi: 10.1126/science.1253393

Image credits
The escarpment image is by Scott A. Christy on Wikimedia Commons and is distributed under a CC-BY license. The Cloudina image is by Smith609 on Wikimedia Commons and is distributed under a CC-BY-SA license.