Wednesday, February 4, 2015

Shellementary, My Dear Watson!


The turtle is certainly a bizarre creature. Along with longevity that rivals our own, they can grow up to 6.6 feet and weigh upwards of a ton. They are ectotherms (well, mostly,) and have fascinating reproductive rituals, which include migrating thousands of miles each year to nest at the same beach at which they were born. Turtles have been around for 220 million years, ranking them among some of the oldest species on earth. Turtles have no shortage of interesting evolutionary caveats, but perhaps the most fascinating aspect of the Testudine (the fancy, cladistic name for both Turtles and Tortoises) is their shell. We tend not to think much about the Turtle shell (or Carapace, as it is formally known), but it really is an evolutionary marvel. Not convinced? Keep reading.

Look Ma! No ribs!

Eunotosaurus
If you were take a look inside of a modern turtle, what you see may surprise you. In place of a rib cage (which serves to protect our inner organs from external abuse), turtles have developed a shell. Where on earth did this come from? Thanks to a recent study at Yale, we think that the turtles eldest ancestor was Eunotosaurus, a protoreptile (which is a subclass of Reptilla, the class to which turtles belong). This crazy creature looked like a lizard that swallowed a frisbee. Eunotosaurus possessed a broadened rib cage, which is thought to be the earliest precursor of the modern turtle shell. The story of the turtle shell continues with the now extinct Odontochelys, which had the lower shell (or plastron) of modern turtles, but it still lacked the distinctive carapace. In its place, it had groupings of bones in its skin called osteoderms. The presence of dermal bone in Odontochelys suggests that the turtle carapace is not exclusively rib material, but instead a concoction of dermal and skeletal bone. As the genus continued to evolve, greater investments of dermal bone resulted in the creatures we have today.

The other crazy aspect of a turtle's shell is their scapula. Turtles walk on all four legs - their back legs are attached to their pelvis, and their front legs are attached to their scapula. Most creatures have their scapula on the outside of their rib cage; however, this isn't true in the case of Turtles. A turtle's scapula are actually found *inside* their shells. How did this happen? There are many theories about what exactly caused this inversion, but the current theory is that it is due to a mutated expression of the Sonic Hedgehog (SHH) protein, which plays a role in notochord development. This change in expression caused the scapula to form inside the rib cage, which allowed turtles to maintain a solid carapace while still retaining the ability to walk. The incredible ingenuity of this system is marvelous to look at in the context of evolution. There are still things we don't know about the turtle shell, but what we do know shows that evolution truly is a wonderfully complicated, yet elegant system that produces results that reach further than our wildest imagination.


If you'd like to know more about how the turtle shell was formed, here's a cool video illustrating the process!

Posted by David Almanzar (1)

12 comments:

  1. I really enjoyed this post, not just because reptiles interest me but because it was very clear and well-written as well. It introduced me to an aspect of turtle evolution that I hadn't really thought about before, and I really liked the inclusion of the links for further information as well as the video at the end. Good job!
    - Ian Mallor

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    1. Thanks Ian! I'm glad I was able to pique your interest.
      ~David

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  2. Great post! I was very intrigued with the information presented on the ancestry of Testudines. The video was also really helpful for visualizing the evolution and changes in shell structure.

    -Mitch Logan

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    1. Thanks Mitch! I found the video really helpful as well. There is another one (made by the same fellow) with a little more detail. You should be able to find it by going to his Youtube channel.

      ~David

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  3. Very interesting and well narrated post. It is fascinating to see how the carapace evolved, and I am interested in learning about it's advantages throughout time, especially in the turtles' earliest ancestors. Also, it is interesting how the scapula is located inside the shell, I've heard that those who study that kind of thing are baffled by that! Very bizarre and very cool.

    Erika Nevins

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    1. Thanks Erika! When I took Herpetology last year, Al made special mention of this phenomena. Even today, people aren't sure if SHH is the true culprit, but it's the most likely conjecture.
      ~David

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  4. Very interesting and educational post! I had never really though about how complex the origins of natural phenomena like the turtle’s shell can be. I found it interesting to note that the shell is not just modified rib bone, but rather a mixture dermal and skeletal bone. Adding the video at the end gave a great and concise review of what was discussed in the article, nice added touch!

    - Hilary Mello

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    1. Thanks Hilary, I'm glad you liked it. For a while, people actually thought that Eunotosaurus was a (because of Odontochelys having dermal bone as well as a thicker rib cage). This recent study has been the first effort for a while in putting Eunotosaurus back into the running for Testudine ancestry.
      ~David

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  5. Great post! You seem to have a knack for writing blogs!

    Turtles are awesome. I didn't realize they can grow to be over six feet long! I've always wondered how in the world they evolved their rigid shells, and your post helped me to understand the evolutionary origins of this peculiar physiological trait. The pictures and video within your post added an aesthetic feel, helping the viewer to better visualize the science you described in writing. Great job!

    - Michael Salhany

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    1. Thanks Michael - I try! I'm not totally satisfied with the picture placement though, we'll see if I can make it better next time I do this.
      ~David

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  6. Very interesting post. Your writing style fits the blogging medium very well. The clever title and little jokes that you dropped serve to liven up your post without distracting from the great scientific information present.

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    1. Thanks John! I'm glad you liked the jokes, and I'm relieved they didn't distract from the material.
      ~David

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