They had a thin aragonite shell, flat-spiraled and with a very marked ornamentation, consisting of well-defined ribs. This shell is divided into two zones: the “fragmocone”, which is the partitioned part of the shell where the gases that control the flotation of the animal are stored; and the “inner chamber”, which is where the soft parts were housed.
Inside the “fragmocone” are the “septa”, which partition this part of the shell, their concavity directed towards the anterior region. Ammonites were thus opistocelic organisms. The septa have an orifice, called a “siphon neck”, oriented towards the posterior region that allows communication with the rest of the septa through the siphon. The intersection of the septa with the external wall of the fragmocone draws in it the so-called “suture”.
There are several types of sutures:
Orthoceratitic: simple, with a small inflection
Goniatitic: with more marked and rounded inflections.
Agoniatitic: the inflections are pointed.
Ceratitic: horseshoe-shaped and crenulate inflections alternately.
Ammonitic: the inflections are sharply cut and with multiple subdivisions.
Section of an ammonoid showing the chambers with different sedimentary fillings and mineral growths.
The direct advantage of the complication of the suture is related to the increased strength of the shell. The more contact there is between the tabique and the wall of the fragmocone, the more resistant it will be.
This advantage was not taken advantage of by the ammonoids to be able to descend to greater depths, but was accompanied by a decrease in the thickness of the shell, thus losing weight but not resistance and gaining swimming effectiveness. The fact that species that could go down to greater depths did not appear is related to the internal pressure of the siphon, since it was less than that of the fluid outside, so this siphon would explode if they went deeper.
PTo define the morphology of the shell we must pay attention to the section of the spiral, which can be lanceolate, semicircular or circular, and the degree of embracing of the turns, which can be oxycone, serpentine or platicone.
Measurements: 4.5 cm.
