The first hammerhead shark was likely the result of a genetic deformity. A biologist explains how shark DNA reveals hammerheads’ history.
A great hammerhead shark's two eyes can be 3 feet apart on opposite sides of its skull. Ken Kiefer 2/Image Source via Getty Images
To make up for that trade-off, hammerhead sharks have special sense organs, called ampullae of Lorenzini, scattered on the underside of their hammer. These porelike organs can detect electricity. The hammerhead family treeIt would be nice if scientists like me could look at fossils and trace the development of hammerhead sharks over time. Unfortunately, fossils of hammerhead sharks are almost entirely of their teeth. That’s because the bodies of sharks do not have bones. Instead they’re made of cartilage, which is what your ears and nose are made of. Cartilage breaks down much more quickly than teeth or bones do, so it rarely gets fossilized.
Scientists long assumed the first hammerhead sharks did not have much of a hammer but, over time, some slowly evolved bigger hammers. We thought the different hammerhead sharks living today were snapshots from different periods in the evolutionary process — with the small hammerheads being the oldest species on the family tree and the huge hammerheads being the newest ones on the scene.
On the left is the expected evolution of hammerheads, assuming a gradual change of head shape. On the right is the observed pattern of evolutionary change based on DNA sequence data. Gavin Naylor, CC BY-ND
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