During the Ediacaran period, two main groups of organisms are found in the fossil record: the "Ediacara biota" of soft-bodied organisms, preserved by microbial mats; and calcifying organisms such as Cloudina and Namacalathus, which had a carbonate skeleton.

There is good reason to think the slime is produced by the algae to control the way their carbonate skeletons are formed.

Some animals may already have had sclerites, thorns and plates in the Ediacaran (e.g. Kimberella had hard sclerites, probably of carbonate), but thin carbonate skeletons cannot be fossilized in siliciclastic deposits.

The process is caused by sulfate reduction which replaces carbonate skeletons (or shells) with pyrite (FeS).

The shallow, warm water is an ideal habitat for many small organisms that build carbonate skeletons.

Petroxestes is a shallow, elongate boring (a type of trace fossil) originally found excavated in carbonate skeletons and hardgrounds of the Upper Ordovician of North America (Wilson and Palmer, 1988, 2006).

They are large and widely spaced and are connected by a layer of translucent, jelly-like mesoglea tissue called coenenchyme which covers the surface of the carbonate skeleton.

When these organisms die their carbonate skeletons sink relatively quickly and form a major component of the carbon-rich deep sea precipitation known as marine snow.

In the 1970s sponges with massive calcium carbonate skeletons were assigned to a separate class, Sclerospongiae, otherwise known as "coralline sponges".

For the Great Dying, having a carbonate skeleton also turned out to be bad news, which suggests ocean acidification can be a problem.