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Thecleithrum (pl.:cleithra) is amembrane bone which first appears as part of the skeleton in primitivebony fish, where it runs vertically along thescapula.[1] Its name is derived from Greek κλειθρον = "key (lock)", by analogy with "clavicle" from Latinclavicula = "little key".
In modern fishes, the cleithrum is a large bone that extends upwards from the base of thepectoral fin and anchors to the cranium above thegills, forming the posterior edge of thegill chamber.[2] The bone has scientific use as a means to determine the age of fishes.
Thelobe-finned fishes share this arrangement. In theearliest amphibians however, the cleithrum/clavicle complex came free of theskull roof, allowing for a movable neck. The cleithrum disappeared early in the evolution ofreptiles, and inamniotes is very small or absent.
It has been argued based on position, muscle connectivity, and developmental origin that the nuchal element of theturtlecarapace is formed from fused cleithra.[3]
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