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The little ancestor of the Great 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 Jurassic Museum Eichstätt. Picture credits: © Jürgen Kriwet

Mackerel sharks (Lamniformes) are a group of the most famous sharks, including the mako-shark (the fastest shark in the world), the notorious Great White Shark and megalodon, the world's largest predatory shark ever roamed the oceans. An international research team led by Patrick L. Jambura from the University of Vienna discovered a peculiarity in the teeth of these apex predators, which allowed them to trace the origin of this group to a small benthic shark from the Middle Jurassic (165 mya). Their study was recently published in the journal Scientific Reports .

Similar to humans, shark teeth consist of two mineralized structures: a hard shell of hypermineralized tissue (in human enamel, in shark's enamel) and a dentin core. Depending on the structure of the dentin, we distinguish two different types: Orthodentin and Osteodentin.

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

Patrick L. Jambura and his colleagues used high-resolution CT scans to examine the tooth composition of the Great White Shark and his relatives found a particular condition of the teeth of members of this group: the osteodentin of the roots penetrates the crown and replaces the orthodontist there completely, making it the only type of dentin present. This condition is not known by any other shark, all of which possess orthodentine properties to some degree, and is therefore restricted to members of this group.

  The ancestor of the Great White Shark
High-resolution micro-CT images show the same Unique Histology of Great White Sharks and the 160 million year old shark Palaeocarcharias stromeri. Photo credits: © Patrick L. Jambura

Another species investigated was the fossil shark Palaeocarcharias stromeri which is well represented by complete skeletons of the famous 150 million year old Platten lime from Solnhofen in southern Germany. The oldest find of this species dates from the Middle Jurassic (165 million years ago) and had little to do with today's mackerel sharks. Palaeocarcharias was a small, sluggish benthoshai no more than a meter in length, apparently hunting small fish in shallow waters. To this day, his affiliation with scientists has been a mystery, as his body shape resembles a raccoon shark, while his tooth-like teeth resemble mackerel sharks. Examination of the tooth microstructure revealed the presence of the same unique tooth composition found only in white sharks and their relatives. Common tooth histology is a strong indicator that this small inconspicuous shark has spawned one of the most well-known shark lineages, including giants such as the extinct megalodon or the living white shark, including mammals, including all modern sharks, with the exception of mackerel sharks , The discovery of this unique tooth structure in the fossil shark Palaeocarcharias strongly indicates that we have found the oldest known ancestor of the Great White Shark, and shows this too. This charismatic basking shark started with a short jump, "says Patrick L. Jambura.

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Further information:
Patrick L. Jambura et al. Microcomputed tomography images show the development of a unique tooth mineralization pattern in mackerel sharks (Chondrichthyes; Lamniformes) in deep time, Scientific Reports (2019). DOI: 10.1038 / s41598-019-46081-3

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University of Vienna

Quote :
Tracking Evolution Through the Teeth: The Little Ancestor of the Great White Shark (2019, 5 July)
retrieved on 5 July 2019
from https://phys.org/news/2019-07-tracking-evolution-teeth-small-fry-ancestor.html

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