Cleithrum
Bone in fish below the pectoral fin
The cleithrum (pl.: cleithra) is a paired membrane bone which first appears as part of the skeleton in primitive bony fish, where it runs vertically along the scapula. Its name is derived from Greek κλειθρον = "key (lock)", by analogy with "clavicle" from Latin clavicula = "little key".
In modern fishes, the cleithrum is a large bone that extends upwards from the base of the pectoral fin and anchors to the cranium above the gills, forming the posterior edge of the gill chamber. The bone has scientific use as a means to determine the age of fishes.
The lobe-finned fishes share this arrangement. In the earliest tetrapods, however, the cleithrum/clavicle complex came free of the skull roof, allowing for a movable neck. A small cleithrum is present in Triadobatrachus, the earliest known lissamphibian, where it formed a dorsal portion of the shoulder girdle. The cleithrum disappeared early in the evolution of reptiles, and in amniotes is very small or absent.
It has been argued based on position, muscle connectivity, and developmental origin that the nuchal element of the turtle carapace is formed from fused cleithra.
Sources and credits
This article is adapted from the Wikipedia article “Cleithrum”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.
Images, from Wikimedia Commons:
- Gessetal2023 Hyneria udlezinye skull.png by Robert W. Gess, Per E. Ahlberg, CC BY 4.0
- Pu carapace.jpg by Faendalimas, CC BY-SA 3.0
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