The vertebrae of billfishes can be recognised by the expanded flanges. These flanges are modified neural (dorsal) and haemal (ventral) spines. The flanges of each vertebra are overlapped by bony projections (the anterior zygapophyses) from the vertebra immediately behind. This results in a stiffening of the vertebral column, which improves swimming efficiency.
The top image shows a billfish skeleton found at Garden Bay, 12 km south of Batemans Bay, New South Wales. The skeleton is most likely from a marlin. The "skull-like" bone at the left of the image is the cleithrum. In life, this bone supports the pectoral fin. It has been moved into its current position for the photograph.
The lower image show vertebrae from a Black Marlin. The skeleton is currently on display in the Australian Museum's Skeleton Gallery.
1. Nakamura, I. 1985. Billfishes of the World. An Annotated and Illustrated Catalogue of Marlins, Sailfishes, Spearfishes and Swordfishes known to Date. FAO Species Catalogue. Vol. 5. FAO. Rome. Pp. 65.