A few days ago I wrote about the approach of Darwin Day and used turtles as an example of creatures with the visible “scars” of evolution (their lungs). Just today I read about a new turtle fossil that, perhaps, shows even my simple statements might be wrong.
This turtle looks like a classic transitional form. It has half a shell – the bottom half. The idea is that perhaps the bottom shell evolved first, in a water-living creature. The plastron might protect the bottom of the animal as it swam near the surface.
What this would mean is that turtles themselves evolved in the water. Now of course their ancestors are still certainly land-living creatures. That doesn’t change. Some creature lived on the land, laid eggs on land, breathed air. Then it returned to the water. While there, it and its descendants evolved the turtle’s shell, starting with the plastron (lower shell) and finishing, sometime later, with the carapace (upper shell).
I’m not sure I buy it. First of all, it just seems like such an unlikely creature. Why only one shell? It doesn’t seem like much protection, particularly from big-jawed creatures that might bite top and bottom at the same time. I wonder if we’re missing something?
More importantly, though, is the lesson that evolution is rarely a straight line. We know very few fossil turtles; there are probably lots of twists and turns in turtle evolution that we just don’t know yet – maybe we never will. Something else I read about today reminded me of that.
One of the most amazing developments (maybe the amazing development) of life on Earth was photosynthesis. The ability to make food from just sunlight and common materials would seem like a pretty fantastic development, not one that any organism would ever want to give up. And yet there are plants (including the mistletoe) that have at least partially given up photosynthesis. Others, like the Indian pipe or ghost plant,
have completely lost their chlorophyll and taken up a parasitic lifestyle. This sort of development shows, without any doubt, that evolution has no direction. If plants can lose the ability to make their own food, then we shouldn’t be surprised at the loss of anything, including a turtle’s shell.
Is this turtle really a transitional form, or is it simply an early turtle that has, for whatever reason, lost its top shell? Until we get more information, i think both ideas have to be possible. Evolution is not a straight line, but a complex bush. We still have much to learn about turtles.