are an extinct marine taxon (subclass Ammonoidea) in the
Phylum Mollusca and Class Cephalopoda. Their closest living
relative is probably the modern nautilus that they closely
resemble. Ammonite fossil shells are of particularly beautiful
spiral forms, except for some more uncommon forms without
spirals that are called heteromorphs. The ammonite’s
shell contained a spiraling progression of ever larger chambers
divided by thin walls called septa. The animal only occupied
the last and largest chamber. A thin living tube called a
siphuncle passed through the septa, extending
from the ammonite's body into the empty shell chambers. The
gas into these shell chambers, enabling it to regulate the
buoyancy of the shell. As the ammonite grew, it added newer
and larger chambers toward the larger open end of the coil.
first appeared in the late Silurian to early Devonian Periods
(~400 million years ago). Through the remainder of the Paleozoic
and through the Mesozoic, ammonites underwent repeated and
large radiations only to decline in several extinction events.
Ammonites were especially abundant in the Mesozoic marine
environment due to rapid evolution and diversification, leading
to widespread distribution. Only some 10% of species survived
the Permian Extinction, and their ultimate demise coincided
with that of the dinosaurs in the K-T extinction event that
closed the Cretaceous Period.