The now-extinct megalodon was the largest predatory shark to ever live, while the great white is currently the largest of the predatory sharks. It now turns out that both fish likely descended from a small, flat shark that skulked along the seabed.

Both megalodons and great whites (along with some other sharks, such as the mako) are members of the mackerel shark group, also known as Lamniformes. One of the things that sets these sharks apart from others are their teeth.

Shark teeth are made up of a hard mineralized enameloid shell surrounding a dentine core. In turn, the dentine usually consists of two types – orthodentine, which is confined to the tooth crown, and more bone-like osteodentine, which is found in the root and sometimes also in the crown. In instances where osteodentine is additionally present in the crown, it serves to support the orthodentine.

In a recent international study led by the University of Vienna's Patrick L. Jambura, however, CT scans of mackerel shark teeth revealed that in their case, the osteodentine extends up from the root and into the crown, completely replacing the orthodentine to become the only type of dentine present in the tooth. Looking back through the fossil record, the only other shark shown to have such teeth was one known as Palaeocarcharias stromeri (pictured below).

Measuring no more than about a meter long (3.3 feet) and likely hunting small fish in shallow waters up to 165 million years ago, it had a flat body resembling that of today's bottom-dwelling carpet shark. And while it otherwise didn't have much in common with the mackerel sharks, it is nonetheless now believed to be their distant ancestor.

"Orthodentine is known for almost all vertebrates – from fish to mammals, including all modern sharks, except for the mackerel sharks," says Jambura. "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."

A paper on the research was recently published in the journal Scientific Reports.

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