Suture (anatomical)

In anatomy, a suture is a fairly rigid joint between two or more hard elements of an animal, without significant overlap. Sutures are found in a wide range of animals, from the Cambrian period to the present day. Hence they are formed by several methods and between hard parts made of various materials.

Vertebrate skeletons

These are made of bone, in which the main rigid ingredient is calcium phosphate.

Cranial sutures

Side view of the skull. The wavy lines are cranial sutures.

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.

Sutures in ankles of land vertebrates

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Crocodilian form of crurotarsal ankle. The astragalus (pink) is fixed to the tibia (green) by a suture. Adapted with permission from Palaeos

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.

Sutures in the shells of cephalopods

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A variety of ammonite forms, from Ernst Haeckel's 1904 Kunstformen der Natur (Artforms of Nature). The ridges on the shells are sutures.

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).

Sutures in the carapaces of trilobites

The trilobite body is divided into three major sections: a cephalon with eyes, mouthparts and sensory organs such as antennae; a thorax of multiple segments which are similar to each other; and a pygidium, or tail section. In many species the cephalon had sutures running from back to front round the outside edges of the eyes.

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 of chitin (a complex sugar).