Arizona Fossils

From the Arizona Geological Survey:

Susan Celestian of Phoenix’s Earth Science Museum cobbled together a nice pictorial on common fossils of Arizona to round out Earth Science Week 2020. You can view or download the 12-page report here.

What is a fossil? Fossils are the prehistoric physical remains (or traces) of organic life. By definition, prehistoric means older than 6000 years, although some people define the minimum age of 10,000 years, before a specimen is called a fossil.

It is hard to become a fossil. While billions of organisms have lived and died through geologic time, very few of them have been preserved in the fossil record.

By using fossils, we can develop a history of lifeforms & increase our understanding of biological evolution.
Fossils assist geologists in establishing a chronological order to geological events and strata. Fossils can be used to establish a relative age date1 for a rock unit. This is best employed by using index fossils (fossils with short and distinct spans of existence, and wide geographic distribution) and unique assemblages of fossils (rather than individual fossils).

This report contains a further explanation of fossils and shows many photographs.

One fossil not mentioned in the report is that of a dinosaur.

Dinosaurs roamed the land, including Arizona’s Sonorasaurus thompsoni, a new species of brachiosaurid dinosaur whose remains were first discovered in the Whetstone mountains by UofA graduate geology student Richard Thompson in 1994. Sonorasaurus is estimated to have been about 50 feet long and 27 feet tall, about one third of the size of other brachiosaurus. It may have been a juvenile or just a small dinosaur species. Sonorasaurus was an herbivore. Tooth gouges on its bones suggest it was killed and eaten by a larger dinosaur. A single blade-like tooth of a huge meat eater called Acrocanthosaurus was found near the bones and suggests that this was the predator that killed Sonorasaurus. You can see an exhibit dedicated to Sonorasaurus at the Arizona-Sonora Desert Museum.

Arizona zeolites, some ancient camels, and maybe a saber-toothed cat

Zeolite minerals (34 to 48 species, depending who’s counting) are natural sieves. Violent volcanic eruptions produce airborne tuff which is composed mainly of vitric ash. When this volcanic glass happens to fall into saline-alkaline water, such as that in a shallow lake, the glass is altered into minerals with a particular structure. Zeolites are hydrated alumino-silicate minerals that contain exchangeable alkali or alkaine-earth cations (positively charged ions).

Zeolite minerals can act as physical sieves, but their greatest value is their ability to exchange sodium, calcium, potassium, and magnesium in the mineral for other elements.

According to St. Cloud Mining, the major producer of zeolites in the U.S., zeolites “consist of an open, three-dimensional cage-like structure and a vast network of open channels extending throughout. Loosely bound, positively charged atoms called cations, are attached at the junctures of the negatively charged aluminosilicate lattice structure. The aluminosilicate framework provides exceptional strength and stability to the lattice structure.”

“The channels, typically 0.3 to 0.7 nanometers in diameter (3 to 7 angstroms, slightly larger than a water molecule), selectively screen molecules according to size and exchangeable cations. Molecules too large to pass through the entry channel are excluded, thus giving rise to the term ‘molecular sieve’.”

zeolite_structure“The molecular structure, surface area, surface charge density, and cation exchange capacity of each particular zeolite will determine its loading, shrinking, swelling and stability under various conditions.”

“Zeolites have a rigid, three-dimensional crystalline structure (similar to honeycomb) consisting of a network of interconnected tunnels and cages. Zeolites in general have high specific surface areas and their rigid framework limits shrinking and swelling.”

Zeolites are used to help purify sewerage and other water, to purify sour natural gas, to treat animal wastes, to make plant food, and to remove cesium and strontium from liquid nuclear reactor waste. Zeolite is also used in the pressure shift devices called ‘oxygen concentrators.’ Someone on oxygen using a machine at home that makes ‘oxygen’ from the air, this is the stuff doing the work – the compressor sends air through this sieve material it scrubs the nitrogen out of the air and the trace gases with ‘oxygen’ pass though filters to the patient as ‘concentrated oxygen at about 94% purity. This process is used in some military jets to do the same thing – an F-22 has this type of oxygen system.

A large deposit of zeolite minerals occurs near Bowie, Arizona. This site produces the mineral chabazite and it is the only commercial producer of this mineral in the world. Bowie chabazite is pelletized and used for removal of carbon dioxide from low-BTU natural gases and the removal of radioactive elements from water. Bowie chabazite was used to mitigate the damage caused by the Three-mile Island nuclear accident. The Three Mile Island accident was a partial nuclear meltdown that occurred on March 28, 1979, in one of the two Three Mile Island nuclear reactors in Dauphin County, Pennsylvania. Zeolites are now being investigated to mitigate acid mine drainage problems.

Tucson geologist Ted Eyde (a friend of mine) was one of the original discoverers of the Bowie deposit. Ted’s son Dan is now president of St. Cloud Mining which operates the property as well as other zeolite properties in New Mexico, and Nevada.

Dan Eyde was interviewed recently on episode 35 of the Arizona Geological Survey’s weekly broadcast: Dan Eyde starts at the 10:38 point in AZGS broadcast.

Now for the camels and cat:

Bowie footprints

When the volcanoclastic tuff was deposited at the Bowie site, it covered and preserved animal tracks in the underlying clay. As careful mining progressed, they uncovered tracks of camels and an extinct llama. As Dan Eyde described it during the interview, they found tracks which suggested that a small herd of camels was crossing the playa. They suddenly changed direction and started to run. Dan speculated that a predator came on the scene. Dan implied that they also found tracks of a saber-toothed cat at the site.

Rocks have many stories to tell. We just have to learn how to read them.

There are many other zeolite occurrences in Arizona (see first reference), but none are currently commercial.


Eyde, T.H. and Irvin, G.W., Arizona Zeolites, Mineral Report No. 1, State of Arizona Department of Mineral Resources (Link)

Eyde Ted H., 1982: Zeolite deposits in the Gila and San Simon valleys of Arizona and New Mexico. Circular – New Mexico Bureau of Mines and Mineral Resources 182): 65-71

Eyde, T.H., and Holmes, D.A., 2006, Zeolites, in Kogel, J.E., Trivedi, N.C., Barker, J.M., and Krukowski, S.T., eds., Industrial minerals and rocks (7th ed.): Littleton, CO, Society for Mining, Metallurgy, and Exploration Inc., p. 1039–1064.

Sheppard, R. A., T. H. Eyde and C. S. V. Barclay (1987). Geology, mineralogy, and mining of the Bowie zeolite deposit, Graham and Cochise Counties, Arizona. Zeo-Trip ’87: an excursion to selected zeolite and clay deposits in southwestern New Mexico and eastern Arizona. Brockport, New York, International Committee on Natural Zeolites.27-46 (Link)

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Geology of kitty litter

Book Review – Paleozoic Fossil Plants by Bruce Stinchcomb

Paleozoic fossil plants coverThis book, illustrated with nearly 700 color photographs, traces the history of plant evolution from the tiniest marine algae to large trees that inhabited the Earth between 542 million years ago to 235 million years ago.  The book is written in an easy-to-understand, conversational manner.

The fossil record provides a picture of the first forests in the Devonian Period, some peculiar plants of the Mississippian Period, the coal swamps of the Pennsylvanian Period and the more sparse plants of the Permian, just before a great mass extinction.  The book also examines some peculiar fossils that were first thought to be plants, but are more likely the tracks and burrows of animals, and it also mentions some things that look like they should be fossils but are not.  Stinchcomb points out that plant fossils are rarer than animals fossils because plants don’t have shells or bones and are thus harder to preserve.

Much of the story in this book is told in the photo captions which are in eight-point type, a bit small for the eyes of this old fossil.  I suppose, however, to print the captions in ten-point type would have caused the book to be at least twice as long.

This book can be used as a scientific reference, a reference for collectors, or simply as an introduction to Nature’s art work.  Stinchcomb writes that “This book is for all who are curious about the ancient earth.” I was able to use it to identify a fern fossil I have as Aleothopteris from Pennsylvania.  Stinchcomb provides a value range for many of the fossils he shows, that is, what a collector would pay for a specimen.  He gives a value range for “A grade” specimens at $1,000-$2,000 down to “H grade” specimens at $1-$10 each.

I found it very interesting to browse through this book and see all the various forms fossil plants can take.  It’s not just leaves, seeds and stems are also fossilized. The story takes the reader to some collecting areas in the mid-west.

The author, Bruce L. Stinchcomb, is a retired professor of geology and has collected fossils since he was a child, hence his enthusiasm for the subject.  He has written nine other books on fossils which cover the Mesozoic and Cenozoic Eras.

Paleozoic Fossil Plants

is published by Schiffer Publishing, Ltd. of Atglen, PA.,   It is also available at Amazon and at Barnes&Noble.

Where the Glyptodonts Roamed

Imagine a Volkswagen-sized armadillo. Such an animal roamed Arizona during the last glacial epoch and into the beginning of our current interglacial period. Its South American cousin sported a tail with a spiked mace which helped it fend off predators such as saber-toothed cats.


The Arizona Geological Survey has a special report on Glyptodonts by David Gillette.

Gillette writes, “Glyptodonts were 4-legged tanks, pseudo-tortoises with fur, protected by a rigid shell composed of tightly interlocking plates an inch thick and more. The largest glyptodonts weighed a ton, ate plants, and probably spent a lot of time in water, along shores of lakes and streams. They resembled their distant relatives, the armadillos, but the fossil record of these two groups spans tens of millions of years indicating they diverged early in the history of their branch of placental mammals.”

“Glyptodonts and ground sloths should be in the vocabulary of every native Arizonan, because these strange animals were among the Arizonans that lived beside mastodons, mammoths, saber-tooth cats, lions, extinct horses, camels, llamas and more in the North American Ice Age, right here in the desert Southwest. It’s desert here now, but 2 million years ago, these animals lived in well-watered savannas and riparian forests that later dwindled …and left little but these fossils as testament to wet times gone by. This was the ‘glyptodont fauna,’ so named because these armored tanks were at times the most abundant large animal in this bizarre landscape.”

“The story of these pilgrims from South America involves two supercontinents, three continents, two oceans, an island chain, an isthmus, glaciers, and sea level. It begins 200 million years ago, early in the Mesozoic Era, during the time of dinosaurs. Earth’s midsection was mostly ocean, and two huge landmasses occupied the northern hemisphere (Laurasia) and the southern hemisphere (Gondwana). The breakup of Gondwana began then, a process that produced the Gondwana continents: Antarctica, Australia, New Zealand, Africa, South America, India, and Madagascar. South America, the homeland of our armored immigrants, separated from Africa, Australia separated from one side of Antarctica and South America from the other side, and eventually Antarctica took its present, familiar polar position. The other land masses slipped away, in the process creating new ocean landscapes: the Atlantic, Pacific, and Indian Oceans, and with the final isolation of Antarctica, the Antarctic Sea.”

Eventually, North and South America were connected by the Isthmus of Panama which allowed the fauna of each continent to mix. Glyptodonts eventually reached Arizona. “The earliest population, known mainly from southeastern Arizona, became dominant members of the ecosystem. They were small as glyptodonts go, weighing perhaps a quarter to half a ton. These were the progenitors of at least two more species in the United States, and two other species known from single specimens in Mexico. But they all belong in the genus Glyptotherium (groove-toothed beast).” The descendant species, Glyptotherium arizonae, had adults nearly twice as large with weights exceeding a ton. They extended from Arizona to Florida.


One remarkable skull from southeastern Arizona has two elliptical puncture holes, interpreted as the consequence of a fatal attack by a large predatory cat. This skull is on display at the American Museum of Natural History in New York City. Arrows point to the two puncture holes.

Fossil hunters, be on the lookout for scutes, the interlocking pieces of the bony shell. The arrows point to follicles that housed bristles which helped the animal sense its surroundings.


Read the full story:

Arizona Geological History: Chapter 3: Devonian to Permian Time

Arizona warms from ice age, becomes tropical again, gets flooded by the ocean, suffers another ice age, warms up, makes coal, and suffers a major extinction of life.

In this chapter we will complete the Paleozoic Era with four periods: Devonian (416- to 359 million years ago), Mississippian (359-318 mya), Pennsylvanian (318- 299 mya), and the Permian (299-251 mya). In the European classification, the Mississippian and Pennsylvanian are, together, called the Carboniferous period because it was during this time that most coal deposits were formed.

Paleomap 390After recovery from the Ordovician ice age (about 440 mya), Arizona was apparently a highland on the southwest edge of a continental mass, about 30 degrees south of the equator. I say apparently, because there is no record from the Silurian period (444- to 416 mya ), so Arizona may have been dry land that was subject to erosion.

Paleomap 306During the last four periods of the Paleozoic, Arizona was mainly under water. The rocks deposited during this time represent deposition on a continental shelf environment. There were several episodes of transgression (encroaching) and regression of the sea from the west. Only what is now the northeastern corner of the state remained above sea level for most of the time. The rise and fall of the sea was due to both tectonic shifting of land and changes in water volume from the glacial epochs.

Limestone was the principal rock deposited during this time along with relatively minor shale and sandstones. All the formations contain fossils. These limestones currently make up most of the mountain ranges south of Tucson.

Mississippian rocks rest unconformably (not at the same angle or with evidence of erosion) on Devonian and older rocks. This means that there was some tectonic adjustment and erosion between the two Periods. (And by the way, the geologic Periods are usually defined by their distinct fossil assemblages). The principal formation of the Devonian is called the Martin Formation with type area in Bisbee. The principal Mississippian limestone is called the Redwall Limestone near the Grand Canyon and the Escabrosa Limestone in southern Arizona. Kartchner caverns are in the Escabrosa Limestone, but the caves formed recently.

Paleomap 255Pennsylvanian and Permian rocks represent complex cycles of transgression/regression by the sea, caused by changes in water volume due to glacial epochs, and by tectonic uplift and sinking of the continent. This tectonic shifting was the result of the collision of Gondwana on the south with Pangea on the north. Carbonate rocks dominate in the northwest and southeast, while sandstones and conglomerates dominate in central and northeast Arizona.

Most coal deposits  in the world were developed during the Carboniferous period.  Coal is mostly carbon accumulations from fossil plant material deposited in swamps so devoid of oxygen that bacteria and other critters couldn’t survive to feed on their remains. This implies that climate was warm and wet, and that the cyclic transgressions/regressions of the sea were relatively quick enough to bury the swamps before the luxuriant plant life could be destroyed.

Arizona coal was formed during the Cretaceous Period. It is mined in Navajo county, and, according to the Arizona Department of Mines and Mineral Resources, ranks second only to copper in economic importance.

Worldwide coal formation stripped the atmosphere of carbon dioxide. Beginning in mid- Devonian time, about 380 mya, through early Mississippian time, atmospheric carbon dioxide dropped from around 4,000 ppm to near current levels of 400 ppm by 340 million years ago. Temperature, however, remained high (about 68 F world average vs 57 F today). But near the Pennsylvania-Permian boundary time, about 270 million years ago, the planet was plunged into another ice age. Note the 70-million-year gap between lowered carbon dioxide and decreased temperature. By the end of the Permian, temperatures rose again to an average of about 63 F, soon followed by a rise in carbon dioxide to just under 3,000 ppm. (Rising temperature causes more carbon dioxide to be exsolved from the oceans.) Volcanism contributed to the rising carbon dioxide.

The first known land vertebrates, amphibians, appeared in late Paleozoic time. Devonian rocks contain fossils of amphibians called stegocephalians (roofed head) because of flat, broad heads. Most were one- to two inches long, but later forms became as large as a crocodile and most were probably carnivorous judging by the teeth.

Reptile fossils appear in Pennsylvanian rocks. The first were small like amphibians, but later Permian reptiles got up to eight feet long. One group, the Therapsids, had teeth differentiated into incisors, canines, and molars similar to present-day mammals.

The Permian ended with a mass extinction in which about 90% of species disappeared, including marine fauna, plants, and terrestrial animals. The reason for this extinction is unknown although there are many speculative theories. This extinction happened over a period of several million years and is coincident with the coalescing of continents and extensive volcanism.

When Pangea and Gondwana collided is reduced marine habitats and brought deep, oxygen-poor ocean water to near surface environments. Major volcanism, in what is now Siberia, lasted for about one million years and annually spewed billions of tons of sulfur dioxide and carbon dioxide into the atmosphere. These two events are probably contributory to the extinctions.

But, with the dawning of the new Mesozoic era, life rebounded and became more diverse and more robust.

OmphalotrochusPHOTO: Omphalotrochus (snail) from the Permian Colina formation, collected about 2 miles southeast of the Tombstone airport. Notice also the pits made by rain drops differentially eroding the limestone.