July 5 (UPI) -- By surveying the composition of great white shark teeth, researchers were able to trace the evolutionary origins of the mackerel shark family, Lamniformes, to a small benthic shark from the Middle Jurassic.
In addition to the great white, the Lamniformes family features the biggest shark in history, Megalodon, as well as the fastest modern shark, the mako shark. But according to a new study -- published this week in the journal Scientific Reports -- the group's oldest ancestor was a small bottom feeder that lived 165 million years ago.
Scientists were able to track the family's evolutionary origins after discovering a unique characteristic of great white shark teeth. Shark teeth feature a hard shell composed of a hypermineralized tissue called enameloid, like enamel in human teeth. Beneath the shell is a dentine core.
There are two types of dentine: orthodentine and osteodentine. In shark teeth, orthodentine is found in the tooth crown. It is compact and similar to the dentine found in human teeth. Osteodentine is porous and more bone-like. In sharks, it's found in the roots, connecting the crown to the jaw.
CT scans of the teeth of great white sharks, major sharks and their relatives revealed a surprising absence. In the teeth of modern mackerel shark species, osteodentine encompasses the entirety of the root and crown. The teeth of all other sharks have both types of dentine.
When scientists surveyed the fossil record for evidence of teeth similar to mackerel sharks in their composition, they identified Palaeocarcharias stromeri, a slow shark measuring no more than a couple feet that hugged the sandy bottom of ancient coastal seas.
The Middle Jurassic species didn't look much like a mackerel shark, but an examination of the microstructures inside its teeth reveled only osteodentine.
"Orthodentine is known for almost all vertebrates -- from fish to mammals, including all modern sharks, except for the mackerel sharks," Patrick L. Jambura, researcher at the University of Vienna, said in a news release. "The discovery of this unique tooth structure in the fossil shark Palaeocarcharias strongly indicates that we found the oldest-known ancestor of the great white shark and shows that even this charismatic giant shark started on a shoestring."