Hammerhead shark, (family Sphyrnidae), any of 10 shark species belonging to the genera Sphyrna (9 species)
and Eusphyrna (1 species), which are characterized by a flattened hammer- or shovel-shaped head, or cephalofoil.
Hammerhead sharks, or sphyrnids, are perhaps the most distinctive and unique of all sharks.
These cartilaginous fishes vary in size; the small scalloped bonnethead (S. corona) measures only 90 cm (35 inches) long,
whereas the great hammerhead (S. mokarran) grows to over 6.1 metres (20 feet) in length.
Although they are considered one of the most recently evolved groups of sharks, sphyrnids are known
to date back in the fossil record to at least the early Miocene Epoch (about 23 million to 5.3 million years ago).
Some authorities believe that this group emerged as early as the Eocene Epoch (roughly 56 million to 33.9 million years ago).