In anatomy, a suture is a fairly rigid joint between two or more hard elements of an organism, with or without significant overlap of the elements. Sutures are found in the skeletons or exoskeletons of a wide range of animals, in both invertebrates and vertebrates. Sutures are found in animals with hard parts from the Cambrian period to the present day. Sutures were and are formed by several different methods, and they exist between hard parts that are made from several different materials. Vertebrate skeletons The skeletons of vertebrate animals (fish, amphibians, reptiles, birds, and mammals) are made of bone, in which the main rigid ingredient is calcium phosphate. Cranial sutures The skulls of most vertebrates consist of sets of bony plates held together by cranial sutures. These sutures are held together mainly by Sharpey's fibers which grow from each bone into the adjoining one. Sutures in the ankles of land vertebrates In the type of crurotarsal ankle which is found in crocodilians and some other archosaurs, the astragalus is fixed to the tibia by a suture, and the joint bends around a peg on the astragalus, which fits into a socket in the calcaneum. Invertebrate exoskeletons In molluscs The shells of most molluscs are made of calcium carbonate (the main constituent of limestone and chalk), and of conchiolin, a protein. For more information, see Mollusc shell. Sutures in the shells of cephalopods In cephalopod mollusks which have external shells (e.g. Nautilus, ammonites), the shell is divided into compartments by septa (partitions). The septa are joined to the external shell by sutures formed by repeated invagination (they interlock like pieces of a jigsaw puzzle). The sutures are visible from the outside and often form complex and elaborate patterns. The suture in the shells of gastropods Nearly all snail shells (except for the shells of limpets, abalone, sea hares, etc.) can be visualized as a tube of increasing diameter, closed at the small end, and spirally wrapped around a central axis. For more information, see Gastropod shell. Each complete rotation of this spirally-arranged tube is called a whorl. The whorls of a snail shell usually overlap one another, forming a spire. Where the whorls overlap, there is usually a clear (if narrow) indentation. This indentation forms a visible line, which is continuous and reaches from the apex of the shell to the aperture; this line is the suture. Details of the