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| Where does the name Ammonite come from ? | ||||||||||||||||||||||
| Pronounched AM-uh-nites. From Ammon's Stone, named after the Egyptian god Ammon, it is the name used for an extinct group of Cephalopods (which means 'head-foot'). | ||||||||||||||||||||||
| When were they alive on Earth ? | ||||||||||||||||||||||
| The Ammonites were present on the Earth for a
period spanning over 140 million years. They first appeared on Earth some 208 million
years ago, during the Carboniferous period. They evolved during the Mesozoic
era into a variety of forms and were very abundant and widespread. The rapid evolution of
the Ammonoids and their widespread distribution, makes them a valuable tool when studying
both the Paleozoic and Mesozoic periods. Large specimens were commonly up to one metre
across and weighed up to 100 kg. It is currently believed that they evolved from the same
ancestors as the Belemnites. The Ammonites became extinct about the same time
that the majority of the dinosaurs disappeared. This is believed to be at the end of the Cretaceous
Period, some 65 million years ago, known in paleontology as the time of the K-T
mass-extinction. They were a form of fast moving marine invertebrate, closely related to the modern-day squid, octopus and nautilus. |
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| What did they look like in real life ? | ||||||||||||||||||||||
| Most ammonoids had a coiled shell, which
internally, was partitioned into small chambers. They lived in a shelled chamber,
occupying the largest and most recently formed chamber, all combining to form a
spiral.
Once the ammonite grew too large for its living chamber, it formed a new one next door and then moved into it, in the process creating the characteristic spiral shape of the shell. The unused chambers were capable of being filled with gas to provide buoyancy at different depths in the oceans. It is these unused chambers which tend to be preserved in different colour calcite. Unlike the Nautiloids the suture line between the chamber wall and the outer shell was folded. The folds which point towards the front of the shell are called saddles (or peaks), while the folds pointing towards the back are called lobes (or valleys).
These folds of the suture are important, as they are used in classification of the ammonoids:
The suture lines became progressively more complicated through the evolutionary history of the ammonoids. The complicated folding of the chamber walls is believed to have increased the mechanical strength of the shell, making it possible to live in deeper waters. The largest ammonites had a shell with a diameter of 2.2 metres. The ammonoids evolved from the nautiloids during the Devonian. |
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What do the fossils look like ? |
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This is a fossil of an ammonite 'as found' on the foreshore under Stonebarrow Hill. The 95mm diameter iron pyrite fossil is attached to a piece of limestone, and shows minor signs of water erosion from sea action.
This fossil is known as being 'in matrix'. Even so, it shows the coil shape of the ammonite quite well. As above, the ammonite lived in only the last part of the outer coil of the spiral shell. |
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Why are ammonites important ? |
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Ammonites are globally recognised as 'index
fossils'. This means that specific genera are found only in specific time spans, enabling
them to be used as dating tools. The they Lower Lias of Charmouth, they are used to name
specific zones and sub-zones within several stages of the stratigraphic map:
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Where can you find Ammonites (and other things) in the Charmouth area ? |
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The photo above shows where you should go to collect the iron pyrite ammonites. You need to time your arrival for about 1 hour after a high tide, and then follow the tide line as the tide goes out. The photo below shows the range of fossils that was found by walking along the beach from the River Char, along the foreshore under Stonebarrow Hill, to the waterfall at Westhay Water. Taking a leisurely three hours, all the fossils were found lying on the sand and shingle between the low tide and high tide lines.
The collection includes several types of ammonites, both free and attached to a matrix of pyrite/limestone. The fossil in the top left hand corner is a piece of Pentacrinite Bed. |
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How are different types of Ammonite classified ? |
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A cephalopod mollusc belonging to the subclass Ammonoidea (over 600 genera) forming part of the following taxonomy: Kingdom - The highest taxonomic classification into which organisms are grouped, based on fundamental similarities and common ancestry. One widely accepted taxonomic system designates five such classifications: animals, plants, fungi, prokaryotes, and protoctists. Phylum - A primary division of a kingdom, as of the animal kingdom, ranking next above a class in size. Class - A unit of scientific classification that comes after order but before genus/genera in terms of biological relationships Order - A taxonomic category of organisms ranking above a family and below a class. Family - A taxonomic category of related organisms ranking below an order and above a genus. A family usually consists of several genera. Genus - A taxonomic category ranking below a family and above a species and generally consisting of a group of species exhibiting similar characteristics. In taxonomic nomenclature the genus name is used, either alone or followed by a Latin adjective or epithet, to form the name of a species. Species -
A fundamental category of taxonomic classification, ranking below a genus or subgenus and
consisting of related organisms capable of interbreeding. |
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Help with identification of Ammonites |
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Not the easiest thing for the amateur collector to do, I have found several website that have photographs of ammonites along with their correct names:
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My Ammonite collection photographs |
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Unfortunately there is a limited amount of information as to the easy identification of different genera of Ammonite. If you can help with any identifications, please let me know, via e-mail or my guest book. As I photograph more of the collection, I will upload the images. The dimensions quoted below are the maximum measured on the specimen. |
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