News in Science

Fossils push animal-made reefs back in time

Reef builders A 548-million-year-old reef in central Namibia is the earliest known ecosystem built by hard bodied animals, according to a new study.

The research led by Dr Amelia Penny and Professor Rachel Wood of the University of Edinburgh, and reported in the journal Science, sheds new light on how these ancient aquatic reefs were formed.

Until now, the oldest reefs on record made of hard-bodied animals had been dated to about 530 million years of age.

The reefs were built by tiny, filter-feeding animals called Cloudina that lived on shallow equatorial seabeds during the Ediacaran Period, which ended 541 million years ago.

The researchers found Cloudina.hartmannae was the dominant reef-building species in the deposit, however examples of a smaller species Cloudina.riemkeae and Namacalathus were also identified.

"Cloudina is an important fossil from the Ediacaran Period", says Penny.

"While Cloudina has been well-known for several decades now, it hasn't been clear how it made its living or what it was using this skeleton-building strategy for."

Fossil evidence suggests that all animal species had soft bodies until the emergence of Cloudina.

Cloudina are the earliest known animals to metabolise a calcium carbonate cement through biomineralisation to form rigid tube-like structures.

They anchored their tubes to each other, and to existing structures such as thrombolite and stromatolite microbial mats, in a reef more than seven kilometres long and over 300 metres thick.

The development of animals with hard structures around the Precambrian-Cambrian boundary sparked a dramatic increase in the biodiversity of marine ecosystems.

It is thought the animals may have developed the ability to build reefs to protect themselves.

"Skeleton building is enormously important for a range of animal groups today, allowing them to support and protect themselves," says Penny.

"Several animals groups independently evolved skeletons during the Cambrian Period, but it was the Ediacaran skeletal animals which pioneered this strategy."

Reefs also provided these animals with access to nutrient-rich currents at a time when there was growing competition for food and living space.

New ecosystem

While the fossils have been previously described, the study fills in some gaps about the structure of these ancient reef ecosystems, says Australian Ediacaran expert, Professor James Gehling of the University of Adelaide.

Cloudina, which formed these reefs, are found all over the world in geology from this epoch, says Gehling.

"They're usually found as random shells lying around muddy sediments, but these are the first to be found in arrangements that we would call reefs."

"[This paper] describes where these creatures lived and their orientation on these stromatolite reefs. This is the first really complex association of things that had mineral skeletons."

Stromatolites and thrombolites were some of the earliest oxygen producing microbial colonies on Earth.

"These worms built their tubes on top of the microbial mats put down by cyanobacteria like stromatolites and thrombolites, because oxygen levels during this time were rising," says Gehling.

"We think there was a critical level of oxygen where you could have multi-cell creatures, which need oxygen to metabolise their food. The best way to [get oxygen] is to live close to microbial mats that produce oxygen as a waste product."

The Ediacaran Period is named after the Ediacara Hills in the northern Flinders Ranges in outback South Australia where the world's first fossils from this period were found in 1946.