Monday was a goniopholid crocodilian. Tuesday was mosasaurs, the largest of the marine predators. For day 3, we’re going to the other end of the scale.
What is a picture of a rock outcrop doing here? This is a picture of the Saratoga Chalk, courtesy of the Arkansas Geological Survey. Look at it this way, and it is tons of chalk, the same that they used to make for blackboards when they used the real thing.
But look at it under a microscope and you open up an entirely new world. For chalk is not just a rock. It is a rock made of trillions of shells of microscopic organisms that live in the oceans.
Two kinds of microorganisms make up most of the chalk. The Saratoga is primarily noted for its abundance of foraminifera, (forams for short) one-celled organisms that form shells, or tests, out of minerals dissolved in the sea water. The ones that make up chalk and limestone form theirs out of calcium carbonate. It is unclear what they are related to, but one thing is clear. They have developed a huge diversity in their over 500 million years of existence.
I find forams fascinating because of their wonderful diversity. Here is another picture posted on the blog “Letters from Gondwana.” The article is a nice description of forams if you want more information on them.
The other group that is commonly found in chalk is called the coccolithophores. These are very tiny, once-celled plants found in the ocean and make up one of the largest groups of phytoplankton. They also make shells of calcium carbonate, but instead of a shell like the forams, they create their home with a few dozen intricately formed scales.
The coccolithophore Gephyrocapsa oceanica. Wikipedia.
When the cells die, the scales scatter and become tiny grains of calcium carbonate which, when piled up on the ocean floor with all the other debris from the oceans, can form those particles of chalk that you used to clean off the blackboard.
Both of these groups prefer shallow, warm seas. Go to the Bahamas or the Persian Gulf and you will get an idea of the environments that are admirably suited to making modern day chalk, as well as getting a good idea of what Arkansas was like 100 million years ago.