(Reuters) - A sea creature that looked like a stack of tiny ice cream cones is providing quite a treat for scientists studying the dawn of animal life on Earth.

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 biomineralized 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.

"Traditionally, the Ediacaran period has been viewed as a time when animal ecologies were quite simple," University of Edinburgh geoscientist Rachel Wood said.

"It's becoming clear that some of the evolutionary innovations of the Cambrian had precursors in the Precambrian. The discovery of reef-building animals in the Ediacaran adds to that picture of early animals dealing with a wider, more complex range of ecological pressures than we expect for the Ediacaran."

Reefs gave the creatures access to nutrient-rich water currents amid growing competition for food and living space. They attached to fixed surfaces and to one another with a natural cement of calcium carbonate to make rigid structures and aligned with a current to filter out passing food particles.

Life on Earth emerged in the primordial oceans. These Cloudina reefs were the earliest know animal-built structures. The earliest reefs of any kind were built by microbes more than 3 billion years ago.

The study was published in the journal Science.

(Reporting by Will Dunham; Editing by Tom Brown)