By visiting the listed coordinates, you should be able to locate some fairly obvious fossils in a large lightly coloured wall. Your tasks are to find two particular fossils - ammonites - and answer a few questions. Send me the answers before logging a find.
Questions
Q1) Have a look at the ammonite on the right (inbetween the belemnite fossils). Discuss:
- The septas in terms of length and pattern. Are they straight, wavy or jagged, or a mixture of all three?
- The size and shape of the body chamber.
- The outer ridge of the shell. What makes this similar to a ram horn?
Q2) Now look at the ammonite on the left. This looks very different to the ammonite on the right. Explain
- The texture and visual difference of the siphuncle
- The colourings of some of the inner chambers and why these are different.
- Which part of the ammonite is clearly missing from this fossil.
Q3) Explain which minerals can be found in each of the two ammonites (i.e. which minerals are in the left, and which minerals are in the right).
Q4) Please add a photo of yourself, a piece of paper with your caching name or something personal to you, at GZ, to your log. This is an optional task.
**To help you find the ammonites in question, find the small rectanglar slab at the bottom middle section of the wall, identified by the red rectangle in the photo. The ammonites can be found by the blue rectangles on each side of the red rectangle by counting up/left/right accordingly.**
Sedimentary Rocks
Some common sedimentary rocks include limestone, chalk, clay, sandstone and shale, which covers approximately 75% of the earth's surface. These rocks are predominantly made when sediment, including dead plants and animals build up at the bottom of rivers, lakes and oceans. When buried, they lose water and become cemented to become rock. Limestone is prone to holding fossils, such as ammotites and belemnite fossils, which are categorised as a ceplalopod.
Cephalopods
A cephalopod is any member of the molluscan class Cephalopodan species. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head and a set of arms or tentacles, modified from the primitive molluscan foot.
Cephalopods became dominant during the Ordovician period, represented by two primitive yet distantly related subclasses:
- Coledgidea, which includes octopuses, squid and cuttlefish. In this category, the molluscan shell has been internalised or is absent.
- Naurilodea are represented by Nautilus and Allonautilus, where the external shell remains. Two extinct species are ammonites and belemnites.
Ammonites
Ammonites are common and conspicuous fossils in Mesozoic marine sedimentary rocks - they're basically squids in coiled shells. The living chambered nautilus also has a squid-in-a-coiled-shell body plan, but ammonites are a different group. The nautilus and the ammonite are similar organisms, where both are aquatic molluscs with spiral shells. Ammonites, however, have been extinct since the K-T event that killed the dinosaurs 65 million years ago while the nautilus still roams the seas. Ammonites get their name from the coiled shell shape being reminiscent of a ram's horn. The ancient Egyptian god Amun ("Ammon" in Greek) was often depicted with a ram's head and horns.
The Shell
Ammonite shells are made up of the calcium carbonate mineral argonite. Over long time periods, the argonite can change into a more stable mineral form of calcium carbonate, called calcite.
The ammonite’s shell was divided into chambers separated by walls known as septa (singular: septum). These strengthened the shell and stopped it from being crushed by the external water pressure. Ammonites could probably not withstand depths of more than 100 m. The septa had frilled edges. Intricate lines of varying complexity known as sutures mark where the septa joined the shell wall. The ammonite itself lived in only the last chamber, the body-chamber; earlier chambers were filled with gas or fluid, which the ammonite was able to regulate in order to control its buoyancy and movement, much like a submarine.
A tube-like structure, called the siphuncle, linked the chambers by passing through the upper part (venter) of the coiled shell. A recently sealed chamber would contain seawater, but this was gradually replaced by gases (mainly nitrogen, oxygen and carbon dioxide), which diffused into the chamber through osmosis. Once filled with gas, a chamber generally stayed that way – although small amounts of water could re-enter through the siphuncle for fine tuning of buoyancy at various depths.
Minerals in the Shell
Over time, the chambers of the buried ammonite shell can fill with mineral deposits. This produces detailed fossils and some shells can remain intact. In other cases, the shell dissolves and mineral deposits fill the cavity to produce a cast.
In silification. silica-rich solutions replace calcium carbonate with silica. Pyritisation occurs in sediment saturated with iron sulphides. Pyritised fossils can oxidise and slowly disintegrate in humid conditions.