Evidence of How Long Sharks Have Been Around

Modified scales found in the mouths of ancient sharks, known as dermal denticles, provide evidence of some of the oldest known shark species. In 2012, paleontologists working in central Australia found dermal denticles resulting in a new genus and species of shark being named -- evidence of the oldest named shark on record. Scales an estimated 455 million years old were found in Colorado, but paleontologists don't agree on whether these are scales of a shark species or a similar sharklike fish. Dermal denticles dating back 420 million years have also been found in Siberia and Mongolia.

Because shark bones are made up mostly of cartilage, they tend to decompose before they can become fossilized, so few intact skeletons of the oldest sharks remain. The most complete fossilized shark remains date back some 380 million years, including a whole brain case found in Australia. Full skeletons of sharks from 318 million years ago also exist in North American limestone beds. Fossilized vertebrae contain isotopes that scientists study to learn the diet of prehistoric sharks and better understand the evolution of various shark species.

Shark teeth are actually even more modified denticles. The most ancient shark didn't have teeth and may not have even had jaws. Fossilized shark teeth don't appear until the Devonian period, which began about 418 million years ago. While the rest of a shark's bones don't fossilize well, the shark's teeth do, and a shark grows thousands of teeth in its lifetime. Paleontologists found some of the oldest shark teeth in Europe, and these are believed to be around 400 million years old. By analyzing fossilized teeth, paleontologists discover details about shark species, size and behavior.

When archaeologists uncover fossilized remains, they take samples of the soil, volcanic ash and other bone fragments from the geological sediment layer where the fossil they want to date was found. Using geochemical processes to date things surrounding the fossil they arrive at an approximate age of the fossil itself. One method focuses on rare earth elements that occur naturally in ground water in different amounts at different times in the Earth's history. These elements leave chemical signatures in fossilized bones that help scientists determine when that creature lived.