Sponges can use many materials to reinforce their mesohyl and in some cases to recreate skeletons, and this forms the main basis for classifying sponges. Sponges produce spicules made of calcium carbonate. Demosponges reinforce the mesohyl with fibers of a special form of collagen called spongin, most also produce spicules of silica, and a few secrete massive external frameworks of calcium carbonate. Although glass sponges also produce spicules made of silica, their bodies mainly consist of syncytia that in some ways behave like many cells sharing a single external membrane, and in others like individual cells with multiple nuclei. Freshwater ones, and have the widest range of habitats. Calcareous sponges are restricted to relatively shallow marine waters where production of calcium carbonate is easiest. The fragile glass sponges are restricted to polar regions and the ocean depths where predators are rare, and their feeding systems very efficiently harvest what little food is available. Fossils of all of these types have been found in rocks dated from 580 to 523 million years ago. In addition Archaeocyathids, whose fossils are common in rocks from 530 million years ago but not after 490 million years ago, are now regarded as a type of sponge.