Approximately 3 billion years ago, long before multicellular organisms roamed the earth, lush mounds of algae began to thrive along warm, shallow shorelines. These algal mats trapped sediment, which built up layer by layer into dome-shaped mounds. Such mounds are called stromatolites. Although they are not animals, stromatolites may have been a food source for some of the animal groups we describe. Stromatolites were abundant in the Precambrian and Cambrian, but their numbers decreased in later times. A few stromatolites are stilling forming today. Shark Bay, Australia, is well known for its abundant living stromatolites. In Wisconsin, stromatolites were most common in Cambrian and Ordovician seas, and therefore are most common in rock of those ages.

A slab of gray rock that has been cut to show its layers.
A cross-section of a stromatolite. Note the characteristic layered and domed structure. (Photo by Madeleine Haberman.)
Pencil drawing of stromatolite mounds as they likely appeared during their formation. The image resembles sand dunes punctuated by smaller domes at their crests.
Reconstruction of stromatolite mounds in the Cambrian Period (500 million years ago).
An intact stromatolite with a rounded, domed top and a layered ridge around its base.
An intact stromatolite. (Photo by Madeleine Haberman.)