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The small ancestor of the great white shark.

The ancestor of the great white shark.

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

Mackerel sharks (Lamniformes) are a group of some of the most iconic sharks we know, including the mako shark (the fastest shark in the world), the great infamous white shark and Megalodon, the largest predatory shark that has ever roamed for the oceans of the world. An international team of researchers around Patrick L. Jambura of the University of Vienna found 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 population). ). His study was recently published in the magazine. Scientific reports.

Similar to humans, shark teeth are composed of two mineralized structures: a hard cover of hypermineralized tissue (in human enamel, in shark enamel) and a dentin core. Depending on the structure of the dentin, we distinguish between two different types: orthodentin and osteodentin.

Orthodentin has a very compact appearance and is similar to dentin that we can find in human teeth. In shark teeth, orthodentin is limited to the crown of the tooth. In contrast, the other type of dentin is spongy in appearance and resembles real bone and is therefore called osteodentin. It can be found in the root, anchoring the tooth to the jaw and in some species also in the crown of the tooth where the orthodentin supports.

Using high-resolution computed tomography, Patrick L. Jambura and his colleagues examined the dental composition of the great white shark and its relatives and found a peculiar condition of the teeth of the members of this group: the osteodentine of the roots intrudes into the crown and it replaces Orthodentin is completely there, which makes it the only type of dentin present. This condition is not known from any other shark, all of which possess orthodentin to a certain extent and, therefore, is limited to the members of this group.

The ancestor of the great white shark.

The high resolution micro-CT images reveal the same dental histology in the great white sharks and in the 160 million year old Palaeocarcharias stromeri shark. Credit: © Patrick L. Jambura

Another species that was examined was the fossil shark. Palaeocarcharias stromeri, which is well represented by complete skeletons of the famous Solnhofen Plattenkalks of 150 million years in southern Germany. The oldest find of this species is from the Middle Jurassic (165 million years ago) and it did not have much in common with the sharks of today's mackerel. Paleocarcharias It was a small and slow benthic shark, not exceeding lengths of more than one meter, and apparently hunting small fish in shallow water. To this day, their affiliation has been an enigma to scientists, since the shape of their body resembles a carpet shark, while their fang-like teeth are similar to those of mackerel sharks. The examination of the dental microstructure produced the presence of the same unique dental composition found only in the great white sharks and their relatives. The shared dental histology is a strong indicator that this small and discreet shark gave rise to one of the most iconic shark lineages that includes giants such as the extinct Megalodon or the great living white shark.

"Orthodentin is known by almost all vertebrates, from fish to mammals, including all modern sharks, except mackerel sharks, and the discovery of this unique tooth structure in the fossil shark. Paleocarcharias It clearly indicates that we found the oldest known ancestor of the great white shark and shows that even this charismatic giant shark began with few resources, "says Patrick L. Jambura.

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More information:
Patrick L. Jambura et al. The images of microcomputed tomography reveal the development of a unique pattern of dental mineralization in mackerel sharks (Chondrichthyes, Lamniformes) in the deep time. Scientific reports (2019). DOI: 10.1038 / s41598-019-46081-3

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University of Vienna

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

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