Suture (anatomy)

The skeletons of vertebrate animals (fish, amphibians, reptiles, birds, and mammals) are made of bone, in which the main rigid ingredient is calcium phosphate.

The septa are joined to the external shell by sutures formed by repeated invagination (they interlock like pieces of a jigsaw puzzle).

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.

The shoulder angle may be simple or keeled, and may sometimes have nodes or spines A trilobite's carapace consisted of calcite and calcium phosphate deposited on a lattice (framework) of chitin (a polysaccharide).

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.

Crocodilian form of crurotarsal ankle. The astragalus (pink) is fixed to the tibia (green) by a suture. Adapted with permission from Palaeos
A variety of ammonite forms, from Ernst Haeckel 's 1904 Kunstformen der Natur (Artforms of Nature).
Two shells of Papillifera bidens , scale bar is in mm. These shells have 10 or 11 whorls and thus a very long suture, with an unusual sculpture of regularly placed papules along the suture itself.