You probably recall that sharks have been around since the Silurian, about 420 million years ago, or possibly even longer ago. They became abundant in the Devonian and expanded in diversity during the Carboniferous. It’s not true that sharks have remained unchanged in all that time, but the basic plan and structure of sharks has been generally about the same. The second major radiation of shark varieties, including stingrays and their kin, began during the Jurassic and continued in the Cretaceous, and most of the modern families of sharks date to the Cretaceous.
|Photo of Cretaceous shark’s tooth from Morocco by|
Parent Géry, used under Creative Commons license.
Marine reptiles such as ichthyosaurs and plesiosaurs undoubtedly competed with sharks for the role of top predator in the Jurassic and Cretaceous oceans, but the marine reptiles went extinct at the end of the Cretaceous, while sharks survived.
Because sharks have cartilaginous skeletons rather than the hard calcium phosphate of bones, body fossils are quite rare. Shark’s teeth are by far the most numerous fossils representing sharks, because they are harder and more resistant – the calcium phosphate mineral, apatite, like human bones and teeth. Some individual sharks grew hundreds or even thousands or tens of thousands of replaceable teeth over their lifetimes, so they are abundant in the fossil record in many places. In the United States, the deposits of the Cretaceous Interior Seaway in the west and Great Plains area, as well as the Cretaceous rocks of the Atlantic and Gulf Coastal Plains, contain many sharks’ teeth. A nearly complete, articulated specimen of a shark has been found in the Cretaceous of Kansas.
—Richard I. Gibson
Rise of modern sharks
Hammerheads at the optometrist
Photo of Cretaceous shark’s tooth from Morocco by Parent Géry, used under Creative Commons license.