Beachcombing for Fossils
Craven, John, Hogan, Tracy, Science Scope
Byline: John Craven and Tracy Hogan
Photograph courtesy of the authors
If you have ever taken a field trip to a natural history museum or an aquarium, you may have noticed in the gift shop a display of fossil shark teeth for sale. Often, these teeth can be as old as 65 million years (however, they typically range from 2 to 23 million years) Yet, it's likely that there is an entire bin of teeth available for purchase. Where did these teeth come from? How are the ages of these teeth known? How is it possible to have so many fossils available? Can I find such teeth on my own and, if so, where? These are questions that people of all ages often ask while marveling at these beautiful relics from eons past, and these are the questions we answer in this month's After the Bell.
Anatomy of the shark
Sharks, stingrays, and skates all belong to a class of fish called Chondrichthyes. All the fish in the class Chondrichthyes have cartilaginous skeletons; that is, they have no bones (although some species will have some vertebrae that are calcified). They do, however, have teeth composed of calcium, but these teeth are not fused to the jaws. Though sharks' teeth vary in size and shape, nearly all grow along rows and are continually replaced as old teeth break or fall out. Actually, shark teeth are a modified placoid scale. Analogous to scales on some boney fish, placoid scales cover the skin of sharks. Indeed, if you were to look at shark skin under the microscope, you would see what appears to be row after row of tiny scales shaped like teeth. And, like the placoid scales found on the skin, those modified scales in the mouth called shark teeth grow in abundance. Over the course of a lifetime, a single shark may grow and lose tens of thousands of teeth. With sharks swimming in Earth's oceans for nearly 400 million years, that's quite a lot of lost teeth!
Stingrays (order Myliobatiformes) and their relatives, including skates (order Rajiformes) and guitarfishes (family Rhinobatidae), like sharks, are also fish within the class Chondrichthyes. As such, these fish do not have bones. Similar to sharks, their placoid scales are modified for various functions, such as for service as teeth and dorsal spines, including the notorious venomous spine of some rays.
The fossilization process
A fossil is any trace or remains of a living species that is generally older than 10,000 years. Not all fossils are petrified remains of living organisms. There are cases in which fossils are preserved remains of soft tissue and skins. Woolly mammoths, for example, have been found preserved in ice, while insects have been preserved in amber. Nonetheless, in many instances, the processes associated with fossilization (a rapid burial in loose, dry, or moist sediment and protection from weathering conditions) result in the preservation of hard parts such as bones and teeth. In the absence of bones, shark fossils are most often found in the form of teeth. While some teeth can be permineralized (permineralization is a process in which organic material is replaced by inorganic minerals), in many cases the preserved specimens of these ancient marine animals are the actual teeth!
When choosing from the gift store bin of shark teeth, you may have noticed that not all teeth are the same hue. These differences in coloration, varying from light to dark to black, are a function of the color of the sediment in which the teeth were buried or the mineral content (chemistry) of the sediment. Fossil shark and ray teeth are commonly black with a shade of gray on the root due to the dark phosphates in the sediments.
The ancient environment
To begin a discussion of the places fossil shark teeth are found, we start with a brief overview of the geologic time scale. Paleontologists have separated Earth's history in which life was present into three major divisions (eras). …