The Grand Canyon: A Paleozoic Textbook
There can be no better place to start the story of Arizona's fossils than the Grand Canyon of the Colorado River. Nowhere on earth does nature reveal its history so majestically, with such uniform simplicity that even the casual viewer with little or no geologic training can appreciate the vastness of geologic time and the magnitude of events taken which formed the rocks he views there from the canyon's rim.
Fossils from the Grand Canyon (and the Grand Canyon's Paleozoic strata exposed elsewhere in northern Arizona), are abundant and are often seen weathering from the rocks along the many trails which snake down to the muddy Colorado River. A hiker making his way from the canyon rim would view these fossils in reverse chronological order, the most recent remains comprising the Kaibab Plateau where he begins his descent and the older fossils occurring in progressively lower strata. For the purposes of this book, our hypothetical hiker will begin his paleontological exploration after a thrilling raft trip on the river and begins making entries in his notebook on the base of the high cliffs at Shin Creek:
The Grand Canyon is of relatively recent origin; apparently the river began its work of erosion about six million years ago. Coupled with the downward cutting of the river has been a general rising or upwarping of the Colorado Plateau, which has added its effect to the action of the river.
Although the canyon itself is of comparatively recent origin, the rocks exposed in its walls are not. Most of the strata were originally deposited as marine sediment, indicating that for long periods of time the canyon area was the floor of a shallow sea.
Seen in the wall of the Grand Canyon, if you're standing at the eastern end, you can clearly see nine separate rock layers piled vertically like a pile of books. And books they are. At the very top is a layer of Kaibab limestone. Right below this is the Coconino ,a thick deposit of sandstone. Below that a layer of soft, shaly rock known as the Hermit shale, then the shales and sandstones interbedded with each other called the Supai Formation.
Fossils found in the Supai tell us that these rocks were deposited in the Permian Period, at the end of the Paleozoic Era, from 225 million to 280 million years ago. Further down, light gray-blue limestone is the Redwall limestone, the color of which may deceive you because it is sometimes stained by the red iron from Supai rocks above.
The Redwall is 500 ft thick and is easily identified because of the majestic cliffs that it forms in the canyon walls. This layer is Mississippian Period rock (360 million to 330 million years old). A relatively thin layer of sandstone, the Temple Butte, lies beneath the Redwall, and these sediments were laid down in the Devonian Period (410 million to 360 million years ago). The next three rock layers, consisting of the brown Muav limestone, the green Bright Angel shale, and the Tapeats sandstone are Cambrian, (570 million to 500 million years old), and is seabottom that accumulated at the very beginning of Paleozoic Era. Beneath the Tapeats Sandstone, at the bottom of the canyon where the Colorado river continues cutting deeper and deeper, are the most ancient rocks of all, Precambrian schists and gneisses, from half a billion to a billion years old.
This is what made up the rocks below the lower-most strata in the Grand Canyon, the earth's proto-oceans. It was a Co2-dominated atmosphere keeping the earth hot. The earth was growing by the impact of meteorites and comets some 4.6 billion years ago. Really,when looking at it, it's a lot like my sister-in -law Lloy's "Horny Toad Chili"...No life could possibly exist in it.

Precambrian Era

From 4.6 billion years to 570 million years ago was called the Precambrian (Simply meaning that really long time before the Cambrian Period. Where it is exposed in northern Arizona (and elsewhere) these early rocks can show some of the very first, and most primitive, kinds of fossils that existed on earth.

Slice a fossil stromatolite in half and like rings of a tree its growth history is revealed.

Here's what Arizona looked like in the Precambrian. Lots of stromatolites
The Cambrian boundary with the succeeding Cambrian period marks the time when animals first developed hard outer parts (exoskeletons) and so left abundant fossil remain, but before that, it was all slime. Following this slime thing, it (life) gets interesting. In Arizona we've got mostly Stromatolites as early Precambrian fossils and in some later rocks that aren't too messed up by geological forces of metamorphosis, record of life on earth is limited to stromatolites and bacterial microfossils until development of the metazoans in the late Precambrian.
These layered deposits, which have a distinctive "signature" are called laminar stromatolites, showing alternating layers of the algal mats and sediments, showing that these algal mats made up of filaments (like that stuff that clogs up your swimming pool filter) were regularly being washed over by tides and fine sediments. Most often, stromatolites appear as variously-sized arches, spheres, or domes. Learn to recognize the characteristic, alternating layers and you'll begin to see stromatolites everywhere in marine sedimentary rocks. (note here that stromatolites can be found in other, much more recent rocks, like the Cretaceous rocks that surround Vesey Elementary school on Tucson's west side.)
The Hakatai Shale and the Bass Limestone are both good examples of Precambrian rocks in Arizona. These show an early environment of both shallow marine deposition (cross bedding, ripple marks and fossil stromatolites) to lowland above water mudflats which include mud cracks, rain droops (yes it rained in the Precambrian!) and crossbedded sand dunes. Stromatolites are fossils which show the life processes of cyanobacteria (formerly called blue-green algae). The primitive cells (Prokaryotic type), lived in huge masses that could form floating mats or extensive reefs. Masses of cyanobacteria on the sea floor deposited calcium carbonate in layers or domes.
Near the bottom of the Grand Canyon you can raft past Precambrian stromatolites 1,000,000,000+ years old.
Paleozoic Strata
The Tonto Group (Cambrian)
The Tonto Group consists of three Cambrian-period formations: the Tapeats Sandstone, the Bright Angel Shale, and the Muav Limestone.

The Tonto Group is the lowest group in the Canyon which preserves complex fossils. The fauna is dominated by trilobites, 47 species of which have been reported. The most common genera include Olenellus, Antagmus, Zacanthoides, Albertella, Kootenia, Glossopleura, and Bolaspis . Olenellus is found only in Cambrian deposits. Also present are brachiopods (Lingulella, Paterina, Nisusia), primitive molluscs (Conchostraca), one species of 'primitive' echinoderm (Eocrinus), algal structures, two species of gastropod (Hyolithes, Scenella), and some sponge fragments (Chancelloria - common in Cambrian deposits). The specific assemblages of fossils found in the Tonto Group suggest an early to Middle Cambrian age for Tonto deposition.

Numerous bedding planes within the Tonto Group preserve trace fossils and burrows. These look very much like traces being made today in modern ocean sediments by bottom dwelling and burrowing organisms. The Temple Butte Formation rests atop the Muav Limestone.

Fossils of the Cambrian-age Tonto Group rocks below Devonian strata
Dolichometops productus from the Cambrian Bright Angel Shale
The Bright Angel Shale of the Grand Canyon contains a wealth of trilobite specimens. While collecting them in the canyon itself without a permit from the National Park Service is not permitted, shale of equivalent age and fossils are found in other areas in northern Arizona and adjacent Utah and Nevada. However, photo ops of trilobites along various trails are common.
Young men mining trilobites under the CCC program of 1930s
The Temple Butte Formation (Devonian)
Fossils found in the Temple Butte Formation include placoderm plate fragments assigned to Bothreolepsis, massive stromatoporoids, silicified rugose corals, and crinoid fragments. Also present are several species of conodonts, which which allow for correlation with well-dated localities elsewhere in western North America. Conodonts described from a well-studied outcrop in Matkatamiba Canyon include Polygnathus pennatus, P. xylus, and Icriodus subterminus at the base of the section, Pandorinella insita and Spathagnotus gradatus.
 The Redwall Limestone (Mississipian)
The Redwall Limestone is about 500-800ft thick, is visible in almost all areas of the Grand Canyon, and is easily recognizable because of its red stain. Fossils in the Redwall include some primitive cephalopods, Spiriferid brachiopods, corals (Syringopora is common), crinoids, gastropods, bivalves, blastoids, bryozoans, trilobites and trilobite fragments, conodonts, and some foraminifera and calcerous algae within the Redwall.
The Surprise Canyon Formation (Mississippian)
The Surprise Canyon Formation occurs as a series of discontinuous lens preserved within channel depressions in the underlying Redwall Limestone. The Surprise Canyon Formation preserved an abundance of marine fossils, including corals (Barytchisma, Michelinia, Palaeacis, Amplexus), brachiopods (Composita, and others, molluscs, bryozoans (Archimedes), foraminifera, conodonts, crinoids and other echinoderms such as the blastoid Pentremites.
The Supai Group and The Hermit Formation (Pennsylvanian to Permian)
Hermit Shale (Permian)
For 900 feet above the Redwall Limestone, the alternating slopes and ledges form the Supai Formation (Pennsylvanian) and above that lies the Hermit Formation (Permian). These deep, brick-red sandy shales are, in the canyon, barren of marine fossils, although in upper zones of the Hermit Formation the tracks of small reptiles or amphibians can be found along with raindrop prints and mud cracks (which indicate that these rocks were at least intermittently exposed to air). Also, preserved within the Hermit Shale are well-preserved plant fossils, most of which are either leaves of seedferns or branches of primitive cone-bearing trees (so far, nearly thirty land plants have been identified from the Hermit Sha
Coconino Sandstone (Permian)
The most distinctive cliff forming rocks in the Grand Canyon are formed by the Coconino Sandstone, which is about 300 feet thick. The Coconino, Permian in age, is composed of nearly pure quartz sand. Under a magnifying glass, this sand appears well-rounded and well-sorted, and is formed into heavy cross-bedded layers characteristic of sand dunes.
Many fossil footprints have been found in the Coconino Sandstone (some 22 varieties have been identified), and all are different from those of the Hermit Formation below. No fossil evidence of plant remains have been found in the Coconino Formation, and geologists believe these rocks to be the remnants of barren shifting sands similar to today's Saharan or Arabian deserts. (Specimen courtesy Tucson Mineral and Gem World)
For 500 feet above the Coconino, strata is divided into two formations, each about the same thickness. The lower, or Toroweap Formation, is a cream-colored limestone, and above is the Kaibab Formation, which is mostly sandy and resistant cherty limestone
Kaibab Limestone (Permian)
The Grand Canyon's rim is formed by Permian Kaibab limestone characterized by its massive cherty limestone grading into shale or gypsum, which forms conspicuous geographical features of the landscape where it outcrops. Exposed in many areas of northern Arizona, the Kaibab contains a wealth of fossil marine forms. A partial list includes: Brachiopods, Productus, Chonetes, Spirifer, Pseudocameratus, Pustula, Pugnoides, Pin guis, Hustedia, Punctospirifer, Composita arizonica, the abundant gastropod Bellerophon, Euphemus, pelecyopods Schizodus, Pseudomonotis, the trilobite Ditomopyge and many crinoids.
Some of the Kaibab Formation's invertebrate fossils are shown below
Sea urchin spines
sea urchin plates
Horn coral, Canina sp.
Brachiopod, Productus sp.
Brachiopod, Meekella sp.
Brachiopod, Chonetes sp.
A great many unanswered questions remain concerning the geology of northern Arizona and its fossil record. There is little doubt that continuing studies within the Grand Canyon will answer some of these some will never be answered. However wondrously documented in this deep scar on the earth's surface, the sequence of rocks in the canyon, pages, even chapters of the record are missing there. Fortunately some exist in other regions of the state and the book in Arizona has most of its chapter