GEOLOGY | FOSSILS | key fossil groups
Ammonoids are sub-classes of Cephalopoda. Cephalopods belong to the phylum Mollusca. Modern Cephalopods are squid, octopuses and cuttlefish. Cephalopods were extremely successful in the late Palaeozoic and throughout the Mesozoic. Ammonites were diverse and varied in the Mesozoic, so much so they are a useful zone fossil for this era.
The Jurassic Period began about 201 million years ago and the Cretaceous Period ended about 66 million years ago. Ammonites disappear from the fossil record at the end of the Mesozoic.
As marine organisms, with hard shells, their fossils are abundant and we are able to compare and contrast fossil ammonites with the modern relative Nautilus, to reconstruct their mode of life.
Image: Ammonite graveyard, Lyme Regis, England, UK.
INTERNal & External morphology of ammonites
The ammonite has a coiled, conical shell, which originates at the protoconch (the original body chamber of the juvenile animal - later chambers are added as the organism grows). Each complete coil is called a whorl. As the animal grows into a new body chamber, it seals off previous chambers behind it with a wall called a septum. A growing series of empty body chambers are interconnected with a delicate tube, called the siphuncle, which is used to regulate the proportions of gas and liquid present in the body chambers - this ability to control ballast, would more than likely aid the animal to position itself vertically, in the water column.
Evolute coiling is common in the shells of ammonoids, with external ornamentation such as radial ridges (ribs). Ammonite shells were constructed from the mineral aragonite, which in fossils has often dissolved away, or recrystallised as calcite. Some ammonites have a protruding keel above a sulcus, both running the length of the ventral margin, which probably yielded greater stability when moving through the water column.
Well-preserved fossilised soft tissues (eg tentacles) are yet to be discovered.
The body chamber is where the animal lived.
The umbilicus is the central cavity of the ammonite where the later whorls do not meet centrally. It is defined by the diameter between the innermost margins of the final whorl.
The septum is the successive back wall of chambers, it is constructed by the animal to seal off a redundant chamber.
Suture: the line where the septum and the shell fuse.
Septal Necks: are where the septum is breached by the siphuncle. Septal necks are extended to support the soft tissue of the tubular structure.
impression of a living
Planispiral means that the whorls are coiled in a single plane.
Evolute Shell: the coils and wide umbilicus can be seen.
Involute shell: the inner whorls are obscured by the latest. Nautilus has this kind of shell.