Researchers said on Thursday they found fossils of the oldest-known animal-made reef in Namibia, built by a small, filter-feeding seabed creature called Cloudina 548 million years ago.
The discovery indicates that important evolutionary developments were unfolding millions of years before the so-called Cambrian explosion when many of the major animal groups first appeared. It also showed that reef building by marine invertebrates, akin to today's coral reefs, began 18 million years earlier than previously known.
Cloudina, one of Earth's earliest-known animals, was the first one with a hard skeleton, in this case an outer shell.
Its fossils have been found in Europe, North and South America, Asia and Africa but it had not been known that it built reefs - a collective activity that helps gain protection from predators and improves food gathering.
The reefs - now on dry land in southern Namibia - were small, about three to six feet across (1 to 2 meters), and stood alongside larger ones made by microbes. Cloudina, perhaps related to jellyfish, corals and sea anemones, was up to six inches long (15 cm) with a diameter of about three-tenths of an inch (8 mm).
"Cloudina's key innovation was the skeleton – it is the first animal known to have produced any kind of biomineralised skeleton. Skeletons have been especially important in the history of animal evolution, providing support, protection and mineral storage," University of Edinburgh geoscientist Amelia Penny said.
"The skeleton is made up of a series of long, nested conical structures which fit one inside another, a bit like a stack of ice cream cones."
Scientists think the animal itself occupied only the top cone of the stack so that - like some modern corals - a small living animal was supported by a larger, unoccupied skeleton that grew over time.
It lived during the Ediacaran Period, a remote time in Earth's history that preceded the torrent of animal evolution seen in the Cambrian Period that followed. The oldest animal fossils date from the Ediacaran.