Fossils discovered in a 548-million-year-old reef in Namibia reveal how the oldest known animals to have hard shells were building reefs nearly 10 million years earlier than we thought.
Until now, the oldest known reefs made of metazoans (multicellular animals) have been dated to about 530 million years. The animals would attach themselves to fixed surfaces, like rocks or the seafloor, and to each other: Working together provides more shelter and protection, plus individuals get to eat more thanks to better harvesting from nutrient-rich currents. Scientists thought that many defensive strategies and ecological innovation -- like forming hard skeletons and building reefs -- emerged as a response to the Cambrian explosion when many major animal groups rapidly started appearing on Earth. Suddenly the oceans became a dangerous place, filled with competition for food and living space.
But according to University of Edinburgh’s Amelia Penny and colleagues, reef building happened before that great speciation event. This newly discovered, older reef in Namibia (now located on dry land) is comprised of tiny, filter-feeding animals, called Cloudina, that were attached to each other and cemented together. These widely studied organisms are the oldest known skeletal animals; up until the emergence of Cloudina in the Ediacaran right before the Cambrian, animals had soft bodies. "We found them in life position, forming a reef," Penny tells the Los Angeles Times. Cloudina formed these rigid structures by producing a natural cement made of calcium carbonate.
Reefs made of microbes date back at least three billion years, but as Penny explains to Nature: “As far as we know, these are the earliest animal building reefs.” The work was published in Science this week.
"Modern reefs are major centers of biodiversity with sophisticated ecosystems. Animals like corals build reefs to defend against predators and competitors,” study coauthor Rachel Wood of the University of Edinburgh explains in a news release. “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."
She adds: "It is the very beginning hint of the modern world." The team is now constructing a 3D model of the reef using rock slices brought back from their field site in the Nama Group of southern Namibia (pictured). They’re also hoping to figure out how the oxygen content of ancient oceans changed over time, Nature reports, and if that led to new selection pressures on animals.
Images: Fred Bowyer (top), Rachel Wood (middle)