MIDDLE DEVONIAN to
million - 65
million years ago
are invertebrates and extinct members of the subclass Ammonoidea, class Cephalopoda.
Modern members include the nautilus, squid, cuttlefish and octopus.
Ammonites first appeared during the Middle Devonian Period around
400 million years ago. They became especially abundant and widespread in the seas of
the Jurassic and Cretaceous periods, 175 million to 65 million years
ago. Ammonites were so plentiful throughout various stages of
geologic history that scientists use them as INDEX FOSSILS whereby the
presence of certain types in a rock layer can denote a specific time
Ammonites were mollusks with
shells that were predominantly tightly coiled on a single plane like a
wheel. These are called HOMOMORPHS. There are also other
varieties called HETEROMORPHS which include members that have open
helical-coiled and irregular-coiled shells, as well as straight shell
examples. The soft-bodied animal living in this shell most-likely
resembled an octopus but with shorter arms. Fossils are few and
rare showing any soft body tissue so the concept of what the soft parts
of these creatures looked like is speculative.
Only the last and largest chamber was occupied by the living animal. As
it matured and grew, larger chambers were added at the opening.
Ammonites varied greatly in size.
known ammonites have been found in Europe and grew to massive sizes
exceeding 2 meters (6.5 feet) across. This is the exception, of
course. Average diameters throughout the ages showed that most
ammonites grew to sizes not much larger than a man's hand. Some
ammonites were only as large
as 2 cm (0.75 inches) in diameter.
The shells of ammonites
had hollow chambers separated by walls called septa. A tube called the siphuncle, connected the body with the
chambers allowing the animal to fill them with water or air, changing
its buoyancy in order to rise or descend in
the ocean water column.
Shell designs varied from globular which
were probably ammonite types that lived in slow-moving water or stayed
close to the bottom of the sea bed, to very laterally compressed making
them very hydrodynamic and thereby allowing them to swim swiftly.
Earlier varieties had smooth-walled, simple shells with more rudimentary
internal chamber geometry.
some cases, ammonite fossils are found in association with a small hard
plate-like structure that was part of the anatomy of the ammonite.
This is called an aptychus and was initially believed to be a type of
"door" that the ammonite could shut over the opening of its shell.
Current scientific theory now believes this structure was part of the
jaws of the former living creature. In rare instances, this
structure is found fossilized in place at the opening of the fossil
During the Jurassic Period, outer ornamentation dramatically increased
with outward ribs, ridges and spines. Internal chamber detail and
suture patterns became much more intricate, as well. By the
Cretaceous Period, outer shell ornamentation developed into even more
diverse and downright exotic anatomy. Several ammonite types in
the Cretaceous developed ornate arrays of spines and horns. It is
not known if this was for defense, sexual display or both.
probably fed on plankton (tiny free-floating organisms), sea lilies,
and smaller orthoceras. Although some slow-moving varieties likely
fed off the ocean floor, most were believed to be open-water swimmers
and probably caught plankton while floating or swimming via jet
propulsion, expelling water through a funnel-like opening to propel
themselves in the opposite direction.
Because ammonites lived exclusively in marine environments, their
presence also indicates the location of prehistoric seas.
Although, the fossil record shows that ammonites were in a steady
decline before their final extinction, they died off completely at the
end of the Cretaceous Period, 65 million years ago.
fossils are mostly casts or flattened compressed impressions of the
former outer shell which left a cavity that served as the mold when
first buried in prehistory. If three-dimensional, these fossils
may also contain remnants of the internal chamber anatomy sometimes seen
if sliced open. In most cases, the actual outer shell is gone.
Some rare instances in the fossil record show preserved ammonites that
have retained the actual outer shell of the animal. These
ammonites may still retain original coloring which may display pearl
essence or iridescent phenomena due to the presence of the original
mother-of-pearl layer in the fossilized shell.
Misspellings: Ammanites, Amonites, Amanites, Aminites,
- copyright Paleo Direct, Inc.
All images and text on this
site are protected by copyright and may not be used in any way.