The crania (brain cases) 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.
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.
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 shells of mollusks are mainly made of calcium carbonate (which is also the main constituent of limestone and chalk).
Nearly all snails consist, in effect, of a tube of increasing diameter, closed at the small end, and spirally wrapped around a central axis.
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 indentation. This indentation forms a visible line which reaches from the apex to the body whorl; this line is the suture.
Details of the suture are often useful in discriminating one species from another. The suture also provides a sort of geographic marker from which one can refer to the positioning of patterning or sculpture where that is relevant: for example some species have a darker or lighter subsutural band on the shell.
Many trilobites had sutures which divided the cephalon (head section) into 3 pieces. The sutures in trilobites' cephalons were unusual because it seems their main function was to create weaknesses which made it easy for this part of the carapace ("armor") to split when the animal needed to molt. A trilobite's carapace consisted of calcite and calcium phosphate deposited on a lattice (framework) of chitin (a complex sugar).