hypural

noun

The bony structure (often fan-shaped) that chiefly is composed of the expanded, flattened and partially fused hemal spines (ventral extensions of the vertebrae), and that supports the caudal-fin rays in most bony fishes; when counting vertebrae, this composite element is treated as a single vertebra and included in the count. Amia has 10 hypurals, Salmo 7 but in most teleosts the number is reduced or fused into a single structure, the urostyle.