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The ancestor of the little white shark

The ancestor of the great white shark

An entire skeleton of the fossil shark Palaeocarcharias stromeri (total length about 1 m) from the Jura Eichstätt Museum. Credit: © Jürgen Kriwet

Mackerel sharks (Lamniformes) form one of the most iconic sharks we know of, including the mako shark (the world's fastest shark), the infamous white shark and the megalodon, the largest predatory shark ever seen in the world. oceans of the world. An international team of researchers around Patrick L. Jambura of the University of Vienna discovered a unique feature in the teeth of these apex predators, which allowed them to trace the origin of this group to a small benthic shark of the Middle Jurassic ($ 165 million) ). Their study was recently published in the journal Scientific reports.

Similar to humans, shark teeth are composed of two mineralized structures: a hard shell of hypermineralized tissue (human enamel, shark enameloid) and a dentin nucleus. Depending on the structure of dentin, we distinguish two different types: orthodentine and osteodentine.

Orthodentine has a very compact appearance and resembles the dentin that is found in human teeth. In shark teeth, orthodentine is confined to the dental crown. In contrast, the other type of dentin has a spongy appearance and looks like real bone. It is therefore called osteodentin. It can be found in the root, anchoring the tooth to the jaw and, in some species, also in the dental crown, where it supports orthodentine.

Using high resolution CT scans, Patrick L. Jambura and his colleagues examined the dental composition of the great white shark and its parents and discovered a particular condition of the teeth of members of this group: osteodentin Roots introduced into the crown and replaces the orthodentin there completely, making it the only type of dentin present. This condition is not known to any other shark, which all has some degree of orthodentine and is therefore limited to members of this group.

The ancestor of the great white shark

High resolution micro-CT images reveal the same unique dental histology in large white sharks and Palaeocarcharias stromeri sharks, 160 million years old. Credit: © Patrick L. Jambura

Another species examined was the fossil shark Palaeocarcharias stromeri, which is well represented by complete skeletons of the famous Solnhofen Plattenkalks (150 million years old) of Southern Germany. The oldest discovery of this species dates back to the Middle Jurassic (165 million years ago) and did not have much to do with current mackerel sharks. Palaeocarcharias was a small, lazy benthic shark, no longer than one meter in length, which seemed to hunt small fish in shallow water. To date, his affiliation is an enigma for scientists because his body shape resembles that of a carpet shark, while his fang-shaped teeth resemble those of the mackerel shark. The examination of the dental microstructure revealed the presence of the same unique dental composition as in the great white sharks and their parents. The shared dental histology is a strong indicator that this small, unobtrusive shark has given birth to one of the most iconic shark lineages including giants like the extinct megalodon or the living white shark.

"Orthodentine is known from almost all vertebrates, from fish to mammals, to all modern sharks, with the exception of mackerel sharks." The discovery of this unique tooth structure in the fossil shark Palaeocarcharias makes it clear that we have found the oldest known ancestor of the great white shark and shows that even this giant charismatic shark has started at a derisory pace, "said Patrick L. Jambura.

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More information:
Patrick L. Jambura et al. Computed tomographic imaging reveals the development of a unique dental mineralization pattern in mackerel shark (Chondrichthyes; Lamniformes) during deep weather, Scientific reports (2019). DOI: 10.1038 / s41598-019-46081-3

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University of Vienna

Follow the evolution of the teeth: the ancestor of the great white shark (July 5, 2019)
recovered on July 6, 2019
at https://phys.org/news/2019-07-tracking-evolution-teeth-small-fry-ancestor.html

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