Friday Headlines, July 12, 2013
THE LATEST IN THE GEOSCIENCES
Today’s round-up:
Impact crater in Iowa
Origin of the turtle’s shell
Geophysics is a fascinating thing. It turns out that there are subtle differences in the force of gravity in different areas, related to the type of rocks that lie below. Denser rocks have a stronger pull.
Recently, scientists were doing an aerial geophysical survey that included gravity analysis, in search of valuable mineral resources near Decorah Iowa. What they found was an unexpectedly circular shale layer.
Scientists had thought there was something interesting going on here before. Earlier well studies had discovered the shale layer, which had never been discovered before. Below the shale was found deposits of breccia, rock composed of sharp angular pieces which usually forms near where the rock originally broke up. Within the breccia was found shocked quartz, which is quartz crystals that have had their crystalline matrix disturbed due to some powerful force. Shocked quartz is characteristic of impact craters.
The crater is about 5.5 km across (nearly 3.5 miles), nearly five times the size of the famous Barringer (Meteor) Crater in Arizona. The Iowa crater is thought to be about 470 million years old, and is buried under much younger rocks, which is why it is not obvious on the surface.
THE ORIGIN OF THE TURTLE SHELL: MYSTERY SOLVED
OK, don’t get your hopes up. There’s embryology here.
You see, bones in vertebrates have two origins. Some started out as exoskeletons. Think of body armor, on the outside of the body. We, as humans, don’t have a lot of this (although it turns out many parts of our skull do have an exoskeletal origin). The huge armored fish of the Devonian Period exemplify the exoskeleton.
So, one might guess that the shell of a turtle is part of the exoskeleton. Maybe the genes for exoskeletal plates have lain dormant in the genome for millions of years and became re-expressed in turtles.
The other kinds of bones in vertebrates are endoskeletal. These are internal bones, and the ones we are most familiar with. The femur, ribs, and vertebrae etc. are endoskeletal in origin.
It’s thought that the bony exoskeleton in vertebrates preceded the development of the bony endoskeleton.
In turtles, the shell is clearly connected to the ribs, and thus it could be hypothesized that the shell is endoskeletal in origin. But the exterior parts of the shell look very much like exoskeletal plates in crocodiles and fish.
So maybe the shell has a two-part origin. Until now, no one has been able to prove or disprove this idea.
A new study using embryonic soft-shelled turtles, chickens, and alligators showed that the turtle’s carapace, while looking similar to the plates in crocodilians, is in fact solely derived from the endoskeleton. No exoskeleton involved.
And for our next trick, let’s figure out how turtles got their limbs inside their rib cages….
Penny Higgins - Storyteller • Artist • Scientist 