Sharks began evolving about 450 million years ago. Of the roughly 340 living species some have changed little in the past 100 million years.
Sharks were the first vertebrates to develop an immune system and may have a greater immunity to cancer than humans.
Shark teeth are made of hard enamel, which may explain why ancient shark teeth are the most commonly found vertebrate fossils today.
Some living shark species replace old and broken teeth as frequently as every ten days. There are 12,000 bull-shark fossil teeth on view in the Museum’s Hall of Vertebrate Origins—approximately the number of teeth a bull-shark will have during its lifetime.
Shark bones are made of light, tough cartilage, which is rarely fossilized.
Like other cartilaginous fishes, sharks do not have a gas bladder to keep them afloat so many species (but not all) must move constantly to keep from sinking. Many sharks have large, oil-filled livers that make them more buoyant.
Most sharks bear live young. Some species can remain pregnant for over two years, longer than any other vertebrate.
Sharks typically bear three to 12 pups and many do not reproduce until age 30, making it hard for them to recover when large numbers are killed by humans.