AMMONITES
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Ammonites MIDDLE DEVONIAN to LATE CRETACEOUS PERIOD: 400 million - 65 million years ago
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.
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.
Ammonites 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.
Ammonite 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, Ammonnites - copyright Paleo Direct, Inc. All images and text on this site are protected by copyright and may not be used in any way. |
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Ammonites varied greatly in size.
The largest
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. 
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.
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.