Hammerhead sharks are native to theCoco Islandsnear Costa Rica. They are named for the strange but fascinating shape of their heads. Their heads are flat and horizontally extended (水平延伸) into a “hammer” shape. Hammerheads can grow up to 20 feet long and weigh up to 1,000 pounds. 

The hammerhead’s unique head shape is different from those of other species of sharks, whose are usually long and narrow. Originally, researchers thought that the shape of their heads made them move and turn easily, but it was later discovered that their ability to make sharp turns without losing stability comes more from the placement of their vertebrae, the small hollow (中空的) bones down the center of their back. The positioning of their eyes gives them very great ability to see and 360 degree sight in the vertical level, meaning they can see what is below and above them continuously.

Even though they have great ability to see, they are rather clumsy when compared to other sharks. They have the least ability to float among all species of sharks and have very small mouths. Because of the small extent of their mouths, hammerheads often feed from the ocean floor and in large groups. Very few species of sharks feed like this. Hammerhead sharks eat a variety of foods including squid (乌贼), fish, and other sharks.

Besides having their special heads, hammerhead sharks are one of the few species of animals known to have skin that tans. If a person or an animal’s skin tans, it goes darker as a result of lengthy sun exposure. It is believed that sharks’ increased melanin (黑色素) production leads to a shark’s lack of hair which in most animals would be enough to protect them from the sun.

Not much is known about hammerhead sharks before the Tertiary Period (from 65 million to 1.8 million years ago). As their bones are not mineralized, they very rarely fossilize (成为化石). Researchers believe they may have developed from carcharhinid sharks which evolved in the mid-Tertiary Period. This link was made because of the similarities in the teeth of the two types of sharks.

(1) Compared to other types of sharks, hammerheads _______.
A. are good at floating B. have bigger mouths C. are quite awkward D. have worse sight
(2) What is special about hammerheads’ skin?
A. It is short of melanin. B. It becomes darker in the sun. C. It is covered with enough hair. D. It can change into different colors.
(3) Why do scientists think hammerheads evolved from carcharhinid sharks?
A. The teeth of the two species look alike. B. Both types of sharks transform into minerals. C. Hammerhead sharks’ fossils support the theory. D. Carcharhinid sharks existed before the Tertiary Period.