Ya Just Gotta Love The Ammonites!

Ammonites are an extinct marine Molluscs in the Class Cephalopoda. Their closest living relative is the modern nautilus that they closely resemble. Most fossil ammonite shells are beautifully spiraled with a 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 ammonite secreted 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.

Ammonites 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 great extinction that defines the end the Cretaceous Period about 66 million years ago.
Cephalopods are the most advanced class of mollusks. They have a well developed head, tentacles, eyes, hearing organs and a beak or jaw. Cephalopods are good swimmers due to a sort of jet propulsion system that squirts water from a tube, propelling the animal along. Over ten thousand species of fossil cephalopods have been described, all were strictly marine. The soft body structures of the extinct ammonoids are not known, but because the shells of modem nautiloids are similar, it is assumed that the body parts were also similar.
The modern genus Nautilus lives in a coiled, chambered shell. The growth of cephalopod shells is fascinating. As the animal grows, it pulls itself forward in its shell and secretes a septum, which is a partition between chambers. The tubelike siphon runs the entire length of the shell and functions to regulate gas, for the empty chambers previously occupied are filled with gases which add buoyancy to the shell. Where each partition joins the outside portion of the shell a suture is formed. All ammonites of the same species have the same suture pattern.
BELEMNITES: Shaped a little like an ink pen, belemnites are the heavy, internal counter balance to the animals head. In some rocks in Arizona, belemnites are abundant, and usually the original structure is replace by the mineral aragonite.
Modem squids and octopods belong to the subclass Coleoidea. Besides two or three species of nautiloids, the rest of all living cephalopods belong to the subclass Coleoidea. Fossil squids and octopods are very rare fossils, probably due to their general lack of hard parts, but ammonites and nautaloids are abundant in some Arizona formations.
Gastropods are the largest group of molluscs comprise thousands of fossil species. As a Class, the gastropods have adapted to every lifestyle except flight. Marine to fresh water forms exist along with terrestrial types. The snails are all single-valved with some species having any opercula, a circular hatch plate which covers the shell opening like a hatch when the animal retreats into its shell. The shell is normally coiled into a spiral, but certain groups such as the limpets, are c shaped. Gastropods are bottom dwellers and crawl by means of a well developed foot.
Snails also have a head with mouth, eyes, and tentacles. The fleshy tissue occupies the inter of the shell. Most are scavengers eating plant and animal remains while some bore through shells other living shellfish to eat the flesh inside.. Gastropods range from early Cambrian to recent times.
Snail or Ammonite?

Sometimes when a collector finds the fossil shells of both gastropods and ammonites or nautaloids together in the same strata, it is at first a bit confusing as to which is which. Both are usually coiled into a spiral and with the lack of soft parts being preserved (tentacles in a cephalopod and a fleshy foot in a snail) there are two structures that will usually settle the argument.

Less complex shell in nautaloids.
Might be a snail were it not for the chamber-wall's sutures.
A dead give away!
Ammonite's shell usually a tight spiral
1. Cephalopod shells usually increasingly tighter and smaller towards the center, and
2. Cephalopod shells always exhibit a regular pattern of lines, called sutures, that are the chamber walls within the shell. In fossils, these sutures are seen as squiggly lines on the surface of the shell. On a snail, these lines are always absent.
And you thought you got of easy. Now, is your fossil an ammonite or a nautalioid? Again, it's easy. The sutures on ammonites are always complex, dendrite-like patterns. On the shell of the nautaloids, the chamber walls exhibit surface sutures that are smooth, simple curves.

Nautaloid's simple sutures

Ammonite's complex suture pattern
Fossil mollusks have, since the Cambrian period, made up a major portion of marine faunal assemblages. Consequently, they are often preserved in greater numbers than any other type of marine invertebrates. Like brachiopods, the shelled mollusks can be collected from weathered rocks that are softer than the fossils themselves, or they can be broken from matrix with the use of a hammer and chisel.
Although many mollusks can be prepared with the use of acid, care must be used with this method because shells are often incompletely preserved with silica and may dissolve away rapidly while hidden from view by a cloud of effervescence rising from the dissolving specimen.
There can be no set laws about how a collector should attack the mollusk-producing outcrop, and each collector must rely on his or her experiences in the field to aid in determining the best method to use. In some places, you can pick them up like so many peanuts. Most snails are small while some ammonites attained a diameter that sometimes can be measured feet not always inches, and I've come across some specimens that just had to be admired in the field, photographed and left for someone with a stout horse or helicopter.
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