During millions of years, stromatoporoid sponges were building reefs on Earth. The earliest stromatoporoids built small mounds on the seafloor in shallow, warm seas. Later forms were larger and more abundant, and created massive reefs in deeper water. Fossil stromatoporoid reefs in Australia and Morocco are hundreds of meters tall. Their heyday was during Devonian (about 380 million years ago), before corals became the dominant reef-builders. What made stromatoporoids durable were their massive calcareous skeletons, composed of closely spaced layers and vertical pillars. At the beginning of the Carboniferous, stromatoporoids disappear from the fossil record, but it is not clear whether they went extinct or not. Some form of stromatoporoid (maybe not from the same lineage) began to participate in reef construction in the Mesozoic, especially during the Cretaceous (beginning about 145 million years ago). Unlike the giant reef builders of the Devonian, these new reef builders preferred shallow, rough seas.

Sponges are ancient. Scientists think they were the first multicellular (made of more than one cell) organisms on Earth. The earliest fossil sponges from China, Iran, and Australia have been dated to at least 555 million years ago. That is hundreds of times as long as humans have been around (about 2 million years). All that remains of a sponge in the fossil record are usually the mineralized rods (spicules) that make up its skeleton. Paleontologists use a microscope to analyze the shape of the spicules to determine the type of sponge. Two groups of sponges (Hexactinellida and Demospongiae) appear early in the fossil record, and scientists are still determining which one appeared first on Earth. The first sponges probably had simple, vase-like bodies.