A study recently published in Scientific Reports shines a new light on the Lamniformes (Mackeral sharks) family tree, an order of shark that includes the infamous great white and the notorious (now extinct) megalodon (aka the Meg). The results suggest the humble origins of these apex predators can be traced to a meter or so long (3.3 foot) species more reminiscent of today's wobbegongs and nurse sharks.
A team of researchers led by Patrick L. Jambura from the University of Vienna in Austria identified a unique quality in various teeth fossils connecting extant mackerel sharks with an extinct species of benthic shark that existed as far back as the Middle Jurassic epoch 165 million years ago.
It all comes down to the structure of the dentine – the dense, bony tissue forming the bulk of the tooth below the enamel (in humans) or enameloid (in sharks). In sharks, there are two types of dentine. The first, the orthodentine, has a compact appearance and is usually confined to the tooth crown. The second, the osteodentine, has a spongy appearance that better resembles "real bone" and is most often found anchoring the tooth to the jaw in the root.
In mackerel sharks, the researchers discovered the usual pattern does not apply. CT scans of nine extant species' teeth show that unlike other sharks, Lamniformes possess just one type of dentine: osteodentine (found in the root and the crown of the shark's tooth). The result was replicated when the researchers took CT scans of seven extinct species of Lamniformes.
As far as the researchers are aware, mackerel sharks are the only order of shark known to possess this dental oddity. All other species display at least some orthodentine.
This brings us to the fossil shark Palaeocarcharias stromeri. The oldest complete skeleton we have available is from the Middle Jurassic epoch 165 million years ago.
Analysis of their teeth revealed the same dental pattern – a surprising lack of orthodentine. This, the researchers say, offers strong evidence suggesting the inconspicuous (and now extinct) benthic shark (eventually) gave birth to the more illustrious megalodon and great white. It predates the oldest confirmed lamniform shark, which is from the Early Cretaceous period.
"Orthodentine is known for almost all vertebrates – from fish to mammals, including all modern sharks, except for the mackerel sharks," Jambura said in a press 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."
But while the study may add to the argument that mackerel sharks descended from the Palaeocarcharias, it does not confirm it.
"Unfortunately, this phylogenetic hypothesis is inconclusive," wrote the study authors. "Consequently, it is most parsimonious to assume that the osteodont tooth histotype is a unique feature for Lamniformes and thus we conclude that the osteodont tooth histology of †P. stromeri adds very strong support of this shark being the oldest known lamniform shark."
Yet, a more robust comprehensive morphological analysis that includes cranial and postcranial skeletal characteristics, as well as dental, will be needed to resolve the mystery surrounding Palaeocarcharias' place on the shark family tree.