Why Sharks Are Covered In Teeth

From MinuteEarth.

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Sharks wouldn’t be known for their fierce teeth today if it weren’t for their ancient scales.

LEARN MORE
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To learn more about this topic, start your googling with these keywords:
Dermal denticles: also called placoid scales, they’re the most primitive form of fish scales that are structurally similar to teeth and are today found only in sharks, rays, and chimaeras.
Outside-in theory: The strongest theory for the evolution of teeth, which are thought to have evolved from dermal denticle scales.
Ostracoderms: Extinct armored jawless fish that lived in the Paleozoic Era
Thelodonts: Extinct jawless fish that had individual scales as opposed to plates of armor
Placoderms: Now extinct, they were some of the first jawed fish and lived from the Silurian through the Devonian era.

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REFERENCES
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Donoghue, Philip CJ, and Martin Rücklin. "The ins and outs of the evolutionary origin of teeth." Evolution & development 18.1 (2016): 19-30. https://onlinelibrary.wiley.com/doi/pdf/10.1111/ede.12099

Fraser, Gareth J., et al. "The odontode explosion: the origin of tooth‐like structures in vertebrates." Bioessays 32.9 (2010): 808-817. https://authors.library.caltech.edu/20188/3/nihms266189.pdf

Huysseune, Ann, Jean‐Yves Sire, and P. Eckhard Witten. "Evolutionary and developmental origins of the vertebrate dentition." Journal of Anatomy 214.4 (2009): 465-476. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2736119/

Pennisi, Elizabeth. "Eating Was Tough For Early Tetrapods." (2013): 390-391. https://science.sciencemag.org/content/339/6118/390.full