The greatest majority of fossils found in Arizona are found in sedimentary rocks. Such rocks, usually shale, sandstone or limestone, were produced on sea bottoms where sand, mud or thick beds of shells have been compressed into solid stone
Several basic requirements must be met by nature before an animal or plant can become a fossil. First, the organism must be rapidly buried within sediments which are not soon disturbed. Sea animals are most common as fossils because, after death, they are rapidly covered with mud which will in time become hard shale or limestone. Second, the organism must have parts capable of preservation. This is a very ambiguous rule, for a plant or animal does not necessarily need hard parts to become a fossil. If the sediments are fine enough, such animals as worms or jellyfish can be perfectly preserved in the finest of detail as carbon films or impressions. Taphonomy, the science of death and decay makes detectives out of many fossil hunters.
Did he swim head first into a meat grinder? Taphonomy is a fascinating part of paleontololgy, the how and why of death and decay.

Even if plants or animals become fossilized, permanent preservation after being buried for eons is not necessarily guaranteed. Often, sedimentary rocks become metamorphosed, or altered due to the tremendous heat and pressure. The Volcanic activity that was regionally so prevalent in the Jurassic and Cretaceous cooked many fossils in sedimentary rocks, and the forces involved in mountain building often compressed fossils in to meaningless smudges in the rock, into nothing at all. The fossil record in these cases no longer exist in Arizona, but fortunately over the next hill conditions could have been quite different and an abundance of specimens might be found. In the Tucson Mountains, where I live, much of the rock is volcanic, having been formed as the great 10 mile-wide caldera here blew its stack in the Late Cretaceous. But among these beautiful, craggy volcanic formations are virtually untouched sedimentary rocks that contain dinosaur bones and tracks, and both invertebrates and plant fossils. No wonder geologists call this the "Tucson Mountain Chaos."

1. Molds and casts: Animals with shells or hard exoskeletons buried in sediments are often acted upon by acid-rich ground water which may dissolve away shells or other organic structures. The void created by this action is called a mold.

Geologist call it a "steinkern"

Cast (L) Cast (R) of a fossil snail

The mold's positive counterpart is called the cast. Both molds and casts should be collected as each often holds some detail which its counterpart has lost.
2. Distillation:
Most often referred to as carbonization
This process of preservation occurs when volatile elements in organic matter distill away, leaving a thin carbon film as the only fossil record Many fossils in Arizona are preserved in this way, especially carbon copies of leaves, the flesh of fish and certain soft bodied invertebrates such as worms and arthropods.
3.Petrification: Fossils in which the entire cellular structure of the organism is replaced by mineral matter are considered petrified.
"Rainbow Wood"

Carnelian replaced Coral

Agatized palm Wood

Fossil wood of the Petrified Forest and many dinosaur bones are preserved in this manner. The dinosaur bone shown on the right is the work of a skilled lapidary. Its not unusual for fossils to be replaced by colored agate or carnelian. Originally the lattice of cells were open and porous, and if buried in just the right conditions with lots of surrounding water that contains dissolved iron or other secondary minerals, the result can be spectacular. Does someone who wears such an item promote paleontology? I've seen it happen over and over; a comment on the adornment turns into a discussion on fossils then to paleontology. I've seen outcrops littered with broken and battered bone fragments, most of which are useless to science. I guess the purist would disagree.
Dinosaur bone replaced by red agate
Petrified wood (wood turned to stone) falls into two categories: casts and permineralizations. The first step in formation of both of these types of fossils involves quick burial in sediment so that fungal and bacterial decay is retarded due to low oxygen levels. A cast is formed as minerals or other sediments fill and harden within the sedimentary cavity formed as the original wood deteriorates. Casts show the external form of the fossil but do not preserve internal cell structure, and consequently cannot be identified to genus or species. Permineralized woods are formed when minerals dissolved in groundwaters infiltrate the wood, filling the spaces within and between cells, gradually embedding and preserving the entire tissue. Permineralized woods retain the original cellular structure and therefore can be identified by anatomical study.
In some cases the minerals which replace organic tissue is colored with one or more elements which impart pleasing colors to the fossil. Iron and manganese will color a fossil red, yellow or orange; manganese blues, and the color green in Arizona usually means copper or chromium had something to do with the fossilization. For the fossil collector, who may also be a lapidary, the discovery of such well preserved fossils poses an obvious dilemma. Clearly, a well preserved fossil should be spared. For the weathered fragments or items out of geological context, I have no answers as the subject can make some rather hot under the collar.
When Fossils are "pretty" is it ethical to slice and dice a fossil that has been replace by a pleasingly colored mineral such as jasper? In some areas of Arizona, dinosaur bone can be found in which cells are filled with bright red agate or carnelian. When does a paleontological specimen become fair game to the lapidary. For me, not often, but there is often badly weathered bone showing no exterior structure, and not found in situ, that is in place. Its a judgement call, and I'm not the judge.

Petrified palm-wood pendant

4. Permineralization: Porous organic structures, like bone, certain shells and woody tissue may not be turned to stone and may still be considered fossils. The bird's nest shown at right is at the Arizona-Sonora Desert Museum. While its age in indeterminate, percolating water from a hot spring infused into the twigs of the nest and the bird eggs themselves at some point and "permineralized" the entire structure. Is it a fossil? Quien Sabe? In New Mexico's Jemez Springs, I split shale and found newspapers that could still be read. Another hotspring kind of thing.
"Fossil" bird nest and eggs at
Arizona-Sonora Desert Museum
When an organism's pores are filled with mineral matter without removing the original organic substances, this process is called permineralization. Sometimes there is a gray area between these to types of preservation, as a small component of original organic matter may remain, even in the silica-replace woods of the Petrified Forest's Chinle Formation.
5. Footprints and Trails: Fossil track ways are an invaluable supplement to the study of ancient life forms. From fossil tracks much more can be learned about an animal's habits than by studying only the actual remains. Dinosaur footprints are widely known, but many prints and trackways of lower vertebrates such as lizards and amphibians are commonly found in Arizona rocks from early Paleozoic to the latest Pleistocene. Tracks of invertebrates such as trilobites and molluscs can be common in fine-grained shales and sandstones. On interesting site I dug was a cavalcade of mammal tracks in Late Pleistocene sediments in Brawley wash west of Tucson, and included lion, camel and mammoth.

Reptile tracks in Coconino Sandstone

6. Coprolites. Casts of excrement preserved as fossils. The name comes from the Greek kopros meaning feces. A great deal can be learned by studying fossil food material preserved in such fossils, and the shape alone of a coprolite can tell something about the internal anatomy of the long-extinct animal. Coprolites were produced by invertebrates, fish, amphibians, reptiles or mammals. Some of these are beautifully agatized also.
7: Mummification. Rarely, animals are preserved with no alteration of their soft parts.
Mummies usually are represented by animals preserved by natural desiccation in arid regions. In Arizona, mummies of Shasta ground sloths are found in dry caves and are famous throughout the world.
8. Amber: Amber is a natural tree resin that had hardened through various chemical changes. Sometimes this sap surrounds an insect (remember Jurassic Park?) and preserves it in perfect detail. Undocumented reports show that amber has been found in the Cretaceous Mancos Shale of Black Mesa and other Mesozoic rocks that represent offshore environments that were the ultimate source for runoff that carried debris from forested upland environments. The specimen at right is a classy piece of Baltic Oligocene amber trapping the best preserved spider in existence. I gave the specimen to my "Amber Maniac" friend Mark Collins in Dublin, Ireland. Keep splitting that soft lignite coal in Arizona; insects in amber will eventually be found.

Although it looks like stone, amber is actually ancient resin from conifers and some tropical broadleaved trees. Trees produce these resins as a defense against insect or disease, or as a protective cover for wounds from, say, a snapped branch. As the resin reaches the surface of the tree, it hardens. It takes specific environmental conditions -- and time- - for that resin to become amber. The resin must find an appropriate resting place -- in lake sediments, for example.

What makes that type of environment suitable is that it lacks oxygen, which would begin to decay the resin. Then, over several thousand years, the resin becomes a substance known as copal, and is considered "subfossilized"-- that is, it seems hard, but will melt when exposed to a hot flame. As compounds in the resin called terpenes become chemically linked over thousands or even millions of years, the copal hardens into amber, which softens and blackens when exposed to a flame, but won't liquify. This insect was trapped in amber about 180 million years ago.

9. Burrows: Burros are horses in Arizona (You've got to visit Oatman to see the wild ones!), but fossil burrows are evidence of bottom-living creatures that dig into the muck, on the bottom of the oceans or lakes, or in the sand if in streams, in the sand. In the specimen shown here, invertebrates, perhaps annelid worms, made a labyrinth of hollow tunnels that were eventually filled in by silt and preserved as this fossil. Burrows are very common, and can represent the work of worms, crustaceans, or even reptiles or mammals. Technically burrows are considered trace fossils as they rarely show much detail of the critter than produced them
Freezing: In Arizona? Dream on! The closest thing Arizona has to this kind of preservation is the package of carrots that has been frozen in the very back recesses of my refrigerator-freezer for 20 years. Some of the remarkable mummies that have been found in Arizona's bone-dry caves however do give us a glimpse as the soft tissues of animals. Is a mammoth clone in our future?
Mammoth frozen in Arizona?
I've got a bridge to sell you too!

10. Just to keep the Creation Science people happy, I've included this "indisputable proof" (right) that a piece of iron reinforcement bar ("rebar") was recently found in a deposit of Permian age limestone, 325 million years old.

Much of the creationist case is based upon intellectual dishonesty. Creationists depend heavily on quotations from evolutionary scientists and writers which they have pulled out of context and twisted to sound like something other than what the writer intended. They also depend heavily on half-truths, distortions, deliberate citation of data they know to be untrue, and outright fabrications.

Despite their arrogant claims to represent the "Christian point of view", the creationists and their fundamentalist friends constitute a very tiny minority in mainstream religion. Every mainstream Christian denomination in the United States rejects the paranoid and ultra-literalist world-view of the creationists, and sees no conflict at all between Christian faith and modern science.

If nothing else, we should remember from history that people have been controlled by appealing personalities using appealing messages, mostly by FEAR....False Evidence Appearing Real.

Fossil rebar in Paleozoic Limestone? Well, the crinoid stem and my piece of iron reinforcement bar might look "exactly" alike to someone who has no biological background, and this coincidence, just as do "human footprints" found along with dinosaur tracks in Texas, or "trilobites found squashed" inside a man's foot imprint in the Paleozoic of Arizona is the kind of ammunition for Creationist that a few would like to see taught in the public schools. Science trys to explain how, lets leave religion to answer the question "why?"