Tuesday, October 5, 2010

How did turtles get their shells?

I was flipping through the channels last night and found an awesome documentary on Turtles and Tortoises. I wanted to know about where these guys split off on the tree and some common ancestors. What I found was Odontochelys semitestacea a 220 million year old ancestor to the turtle. The ancestor swam in China's costal waters long ago, and are now being studied by Chun Li of the Chinese Academy of Sciences in Beijing and his colleagues.
Before this the oldest know turtle ancestor was Proganochelys found in Germany. However, this new species sheds much more light on turtle evolution. These two differ in "in presence of teeth on premaxilla, maxilla and dentary; relatively long preorbital skull; distinct transverse process on pterygoid; absence of fully formed carapace; no acromial process on scapula; dorsal ribs articulating at midline of centrum; free sacral ribs; free caudal transverse processes; presence of long tail; four (rather than three) phalanges in digits III and IV of manus and pes; absence of osteoderms and tail-club." (Nature 456, 497-501 ) The most surprising feature here the absence of the carapace or top shell.
Since Odontochelys semitestacea spent its time swimming in aquatic environments, it is more likely that the bottom shell (Plastron) developed first, to protect from predators attacking from beneath. It has only a partially formed shell on top. Li believes this proves that the plastron developed before the carapace. The image of the skeleton above shows the lack of a top shell. More than likely, when turtles started walking on land their carapaces developed better because the ground was also protecting their undersides. However, some researchers believe that Odontochelys semitestacea lost its top shell and that an even older ancestor had the presence of both the carapace and plastron.

References:
Article from Science Daily

6 comments:

  1. I see the reasoning behind how the plastron developed. I wonder what kind of hard material comprises the carapace and plastron.

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  2. This is really interesting! Usually when I think of an animal with body armor, I picture bone / scales down the back. But it makes sense that while you are living in the water, you would be more worried about death from below rather than death from above. This must follow the same logic as countershading.

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  3. Turtles are AWESOME! No, but really, turtles are very different from their other reptilian counterparts, so it is very interesting to see an example of one of the ancestral forms that had a more general reptilian body. As stated above, the reasoning behind the modification of the turtle's skeleton to allow for the formation of the plastron makes sense to help protect from unseen predators from below and the large, strong carapace above once on land because most of the turtle's predators would have to attack from above rather than below.

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  4. Here's a good question:
    Since the top shells on current turtles are obviously much larger and stronger than the bottom shells, does this mean that they have been living on land longer than in water, or is this due to stronger predation pressure causing an inducible defense?

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  5. What is the evidence that the lack of the carapace is a loss in this fossil? And we will see that turtles do not have teeth. Are there other fossil turtles that have teeth like this one?

    You generated a nice discussion on this post.

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