Classification
Ammonites are fossil molluscs and belong to Cephalopoda, the class at which we find in the present day nautilus, cuttlefish, squid, octopus...
Appearance and dying out
The first Ammonoidea appeared 300 millions ago, in the Devonian age. Their ancestors were an archaic form named Batridtide. After this, they contined to develop throughout the Paleozoic and Mesozoic eras ( except during some biological crises). There were three broad groups : the goniatites, living at the Devonian and Carboriferous periods, the ceratites of the permian and triassic periods and the ammonites flourishing during the Jurassic and Cretaceous ages. But the number of genera and species considerably descreased from the Cretaceous period onwards.
They finally died out 65 million years ago, at the Maastrichtian stage, at the same time as several other animals of the Cretaceous fauna.
Living conditions
Ammonites were pelagic animals, most living in fairly deep water (50-250m). But the genera Oxytropidoceras, for instance, prefered shallow water, 1-35m deep. Lytoceras and Phylloceras, by contrast, lived in very deep water, about 500 m.
Coiling & Forms
From the planospiral shell to uncoiled or spiral forms, ammonites were really polymorphic
Some criteria for indentification
The diameter and ornamentation of the
species, section & shape of the whorls, deepness of the umbilicus,
degree of coiling, shape of the aperture, suture lines (different for each
species)...
Sexual dimorphism
Two centuries after the
French paleontologists De Blainville, D'Orbigny and Munier-Chalmas
suggested the possibility of a sexual dimorphism between some ammonites,
the work of Makowski and Callomon (1963) finally confirmed this concept.
The terms macroconch(M) and microconch(m) are used for the two shells of a
pair. When complete, the microconch, probably the male, had lappets and
the macroconch (the female) a simple aperture.
Nautilus
is a true fossile vivant. Though the nautiloid shell is similar to that of
the Ammonite, there are several differerences between them : the position
of the siphuncle, the suture line.
The suture is the line of junction of the edge of a septum with the
external shell. Each species has its distinctive suture line
About aptychus and anaptychus
In the body-chamber of several species there may be found two calcareous plates, forming together the aptychus or one single, thin and horny plate, called anaptychus. The form and the ornamentation of such fossils vary with the ammonite species to which they are related. They are thought to be a part of jaw mechanism.
Their importance for the stratigrapher
Using ammonites, we can date the geological deposits, because they are the ideal zonal index fossils. Some species existed for only a short time and they were widespread. Therefore, they are very important for stratigraphers.
Information provided by: http://perso.wanadoo.fr