This kingdom includes bacteria and blue-green algae. They are characterized by having poorly defined organelles such as the nucleus. Although individual organisms in this kingdom are for the most part microscopic, their colonies can reach great size. In the fossil record, this division (=phylum) is primarily represented by fossil colonies of algae called stromatolites. First appearing well over 3 billion years ago, these stromatolites were formed by the fossilization of layered mats of algal growth. Virtually identical mats still form today.
