Fossils||
Portsdown||
Echinoids||
Ammonites||
Belemnites||
Bivalves||
Brachiopods||
Ammonites
The Ammonites are a family of the extinct subclass Ammonoidea, of the class Cephalopoda, phylum Mullusca.
The following discussion covers:
Introduction to Ammonoidea
The Ammonoidea constitute the largest Cephalopoda subclass, with 163 families including the ammonites (in 1990), They range in age from Devonian to Upper Cretaceous, but are not known after the Cretaceous, being a victim of the KT impact. They are one of the most important groups of fossils for dating Mesozoic rocks, as they changed rapidly with time, and had wide geographical distributions.
The ammonite, although extinct, is related to the nautilus, a member of the subclass Nautiloidea.
The living nautilus was discovered in 19xx, and is the single surviving genus of this subclass.
From it, much can be deduced of the living habits, and soft parts, of the ammonites.
The most [obvious? noticeable? ] fact that became apparent on discovery of the swimming nautilus was its orientation. Prior to then, every drawing and reconstruction of the ammonite had been upside down.
However, from comparison with its 'nearest living relative' the nautilus, we now know that the ammonite in its living position was with the head at the bottom, and the empty bouyancy chambers at the top:
 Traditional orientation of ammonites |
 Correct orientation of ammonites |
Reconstructing the life habit and anatomy
The internal structure of the extinct ammonite was probably the same, or at least very similar, to that of the nautilus. Palaeontologists have thus been able to reconstruct the probable anatomy and life habits of the ammonites, using all of:
- Homology and analogy - similarities, due to either sharing a common ancestor (homology), or having evolved the same features (albeit separately) to fulfil the same function (analogy)
- Soft parts and gut contents that have very rarely been preserved in ammonite fossils
- Information form other fossils and sediments, in which ammonite fossils have been found and are now associated.
Morphology
Ammonites generally have flat ('planispiral'), tightly-coiled shells - although loosely coiled, and spiral, varieties are known. These shells are of [calcite or aragonite?]
This shell is external, and is multi-chambered, and possess a 'siphuncle' (or tube) running through all chambers.
The animal lives in the last-formed chamber, known as the body chamber.
Siphuncle
The siphuncle is a tube running thorough each chamber of the shell. In the nautilus (so probably in the ammonite too) it allows the injection and extraction of water and gas into the body chambers. This allows the water content (and thus weight) of the hollow shell compartments to be adjusted, allowing the animal to control its buoyancy (and weight). Most ammonites was thus pelagic free-swimming animals.
Sutures
As mentioned, the body cavity was divided into a number of chambers. The dividing membranes are known as septa, and where they join the external shell, the resulting mark is a suture. The sutures may be visible as a narrow wavy line, or a depression on the face of the shell, and is very important for identification. In some ammonites, it ban be very convoluted and [highly??] decorative.
Head
Ammonites probably had well-developed heads and eyes, with mouths surrounded by a ring of tentacles.
The mouth contains jaws and a radula. (The radula is a tooth-studded ribbon of tissue which runs over a protrusive, cartilaginous bar in the mouth. When applied to a food-bearing surface, food is brushed off and into the mouth.)
Behind the head would have been the visceral mass, containing the body, gills, anus and various other organs. A funnel beneath the head gave propulsion by expelling water from the body cavity
Living Habits
The range of shell forms in ammonoids implies widely differing swimming abilities, and consequently varied life styles.
Some species had flattened shells, so were apparently 'stream-lined' for rapid movement. These may have been predators, expelling water to give themselves jet-propelled rapid movement while chasing their prey.
Others, with fatter shells, were more sedentary.
Speculation has suggested that the wide variety of feeding habits of ammonites included predation, nektonic suspicions feeders, scavengers and detritus-collectors, who swam near the sea floor.
For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk