Limestone is a hard sedimentary rock consisting of calcium carbonate, formed by the deposition of animal remains on the sea floor and is thus known as a calcareous rock. Limestones often contain the visible remains (fossils) of shells and corals. As limestone is a sedimentary rock, it is laid down in layers or ‘beds’ separated by ‘bedding planes’ which are caused by changes in deposition rates or content of material deposited. Limestone is an ideal rock to see fossils.
This particular slab of limestone is from Morocco and has been polished on one side. The whole slab is full of a type of fossil we call Orthoceras.
We are here to look for the fossil of an marine animal called Orthoceras, which was part of the family of shellfish known as Cephalopods. Orthoceras had long slender conical shells. These animals were hunters in a marine sea. The animal lived in the end chamber of the shell - the wide end, and as the animal grew, the shell developed more chambers as it got larger. The term for the line in between each chamber is known as a septum. Orthoceras fossils also have a tube type structure running along their length, this tube is known as a siphuncle. This was a tube that ran the entire length of the shell, and had two functions, once filled with water, the nautiloid could force the water out, propelling itself backward with a kind of jet propulsion. By releasing the water and leaving air space, the tube could serve as a bouyancy device allowing the animal to rise and lower itself to different depths. This was also used as a counter weight for the softer body tissue, allowing the animal to swim in a straight line.
In order to log a find against this EarthCache you will need to answer some questions and send your answers to me via the Messenger Centre facility or through my profile.
1) Please describe the fossils on the polished side of the limestone. Please measure the length of the
largest fossil you can find in centimeters.
2) In each fossil you can see the septa exposed. In the longest fossil you have just measured, please tell me exactly how many septa it has.
3) On the west side of the limestone boulder, you will get a three dimensional view of the fossils. Please describe what you see. Are there any fossils on this side of the rock which have their septa chambers exposed, i.e showing visible breaks in the shell?