Some of the earliest animals with skeletons helped build reefs

An electron micrograph of the concentric cones built by one species of Cloudina.

NASA

Today, stromatolites—large communities of microbes living in watery environments—are limited to a few environments where local conditions make the water extremely salty. But for billions of years before the first animal life, they were the only game in town. Tiny microbes built giant communities. One reef, described in a new paper, was seven kilometers long and over 300 meters thick. "Here microbial reefs individually form elliptical mounds that reach up to 20m in diameter, 5 to 10m in width, and 5m in height," the authors say.

But that reef was built as times were changing. The first animals were already present, and a few species had already evolved the ability to construct skeletons. And according to a new study, these first animals moved into the existing reefs, finding niches to occupy within the microbial structure and even starting to build their own, independent features.

Prior to this study, the earliest known reefs were built by sponges and date to the Cambrian, 530 million years ago. But the reef described above, located in Namibia, was buried by volcanic ash that has been dated to 547.3 million years ago, which places it in the Ediacaran period, the time when the first remains of animals appear in the fossil record. Most of these animals look nothing like any of the species alive today, and very few appear to have had any skeletal elements.