Fulcra

Fulcra are peculiar bony flaps behind the dorsal fin of sturgeons that form the dorsal keel of the caudal fin.

For example, in Sterlet, behind the dorsal fin begins a series of arch-shaped bony flaps called fulcra, which form the dorsal keel of the caudal fin and gradually develop into bony rays, lepidotrichia, which support the dermal blade of the caudal fin.