Why Don’t Sharks Have A Skeleton?

In today’s Osteology in Action, I’m going to let you all in on a secret that every 7-year-old kid instinctively knows. Sharks are cool. Not a day passes at the museum where a kid fails to inform me of this sage wisdom. If you were to ask a kid what makes sharks cool, they’d probably stare at you with a look of concern mixed with pity. They’d be flummoxed. What should they say? Isn’t it obvious?

Eventually, they’d come up with a list of boss shark traits. They’re strong, fast, scary, they’ve got creepy eyes, and their ancestors were the first vertebrate species. It’s pretty hard to come up with an animal that just exudes the level of awesome that sharks do. If you were to ask a kid, they’d say dinosaurs, because kids are the font of all wisdom. But, I mean, sharks survived the mass extinction that took out the dinosaurs, so there’s that. Before you run off to google sharks in order to pick a favorite species (mine is a Greenland shark), allow me to share with you one or two more cool shark facts.

As you are probably already aware, sharks and their relatives don’t have a skeleton in the traditional sense of the word. Instead, their skeleton is made of cartilage, the flexible connective tissue that lends shape to your nose and ears, as well as structural support throughout your body. While this is comfortably in the neat category, it’s not full-on cool. But one thing that is supremely cool about sharks is that their skin is made of teeth

Yes, you read that right. Their skin is teeth. These skin-teeth (the proper term is dermal denticles) share the appearance and structure of the teeth in the shark’s mouth, just incredibly small. They are sharp enough that divers have reported that simply being bumped by a shark can shred a wetsuit. If you’ve ever been lucky enough to pet a shark, you might recall that they are rough and have a sandpaper-like texture. If you were to pet the shark the wrong way (tail to head), there is a possibility of lacerations due to the denticles. 

But why do sharks have tooth-skin? Well, the most obvious answer is protection. However, the grooves caused by the denticles also make sharks more aquadynamic. They can move through the water without disturbing it and therefore can sneak up on prey. They are also arranged in an interlocking pattern that surrounds the body like a helix. This structure acts like a corset, supporting the internal structure of the shark. This is particularly important due to their cartilaginous skeleton which requires that the muscles for swimming be attached directly to this “corset”.  However, if you were to ask a shark this question, they might turn around and ask “why do you have mouth-teeth?” They wouldn’t do this to be rude, although they have been swimming the world’s oceans longer than trees have existed and have, therefore, earned the right to be rude. They’d ask this because dermal denticles are found earlier in the fossil record than oral teeth. The fact that ancient shark-like creatures and our modern sharks have very few differences, including their denticles shows that it’s a very successful system. Afterall, nature knows when it’s peaked.

Skeletons Museum of Osteology

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