Suture (anatomy)

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.{{cite journal

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 [[gastropod]]s{{anchor|Gastropod}}

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 suture are often useful in discriminating one species from another, for example, sometimes the suture is channeled.

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.

When an angulation of the whorls occurs, the space between it and the suture above it (i.e. the abaxial edge of the sutural ramp) constitutes the area known as the "shoulder" of the shell. The shoulder angle may be simple or keeled, and may sometimes have nodes or spines

In arthropods

Sutures in the carapaces of trilobites

Main article: Facial suture (trilobite)

A trilobite's carapace consisted of calcite and calcium phosphate deposited on a lattice (framework) of chitin (a polysaccharide).

The trilobite body is divided into three major sections: a cephalon (head section) 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. These sutures divided the cephalon into three 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.

References

References

1. (2013). "Anatomy & physiology". OpenStax.
2. [http://www.palaeos.com/Vertebrates/Units/270Archosauromorpha/270.500.html Archosauromorpha: Archosauria - Palaeos] {{Webarchive. link. (2005-04-05)
3. Ruppert, E.E., Fox, R.S., and Barnes, R.D.. (2004). "Invertebrate Zoology". Brooks / Cole.
4. [http://www.palaeos.com/Invertebrates/Molluscs/Cyrtosoma/Cephalopoda/Cephalopoda.Glossary.html#suture Cephalopoda Glossary - Palaeos]
5. "Palaeos Metazoa: Mollusca: Cyrtosoma: Cephalopoda: Glossary". Palaeos.
6. Fortey, Richard A. (2000). "Trilobite!: eyewitness to evolution". Alfred Knopf.