From Tree to Stone

Araucarioxylon, arizonicum (Petrified Forest National Park) Drawing Rendition

The drawing above is a rendition I drew with colored pencil of Araucarioxylon, arizonicum, an extinct conifer tree identified from the petrified wood of the infamous Petrified Forest National Park located in the U.S. State of Arizona. Originally, it was thought to be a distant relative of the Araucaria tree or the Norfolk Pine, which you often see for sale during the Christmas season as potted plants. Without detailed microscopic examination of the wood, its link to present day trees is only speculation at this point. At any rate, modern scientific discoveries indicates there were more species than originally thought lying on the dry Arizona plateau.

Araucarioxylon, arizonicum petrified wood from Arizona Petrified Wood National Park

Triassic Beginnings

During the Triassic Period, around 225 million years ago, the present day continents were melded together as one supercontinent called Pangea. By the end of the era, around 200 million years ago, widespread volcanic activity began to break the continents apart. The first dinosaurs and mammals had evolved and there had been a boom in cycads (palm like trees), giant tree ferns and conifers . . .  enter the Araucarioxylon of Arizona, a type of conifer or pine. It differs from modern day conifers with its sporadic branch growth pattern around the trunk rather than growing in level whorls. It grew up to 200 feet (60 meters) tall with a 9 foot (2.7 m) diameter. Compare that to the tallest current living conifers, the Sequoias, that grow up to 188 feet (57 meters). To help put that into perspective, maple trees reach up to about 50 feet (15 m) and oak trees top out at about 80 feet (24m) tall.

Triassic Landscape

From Tree To Stone

The high and dry tableland in northern Arizona where the National Petrified Forest Park rests, was a vast flood plain during the Triassic Period overflowing with streams and widespread ponds. Prehistoric, extinct plants and trees concentrated along the spilling bodies of water. Dying or blown over trees were washed down the streams into the flood plains where they gathered and decayed. If they got buried under mud, silt or volcanic ash deep enough to cut off oxygen, it slowed the decaying process. Silica laden groundwater gradually seeped through the logs replacing the original wood tissue with silica deposits. As the process continued over the ages, the wood was replaced, atom by atom, with silica crystallized into mineral rich quartz, turning it to rather attractive stone.


Petrified Wood Araucarioxylon, arizonicum from Petrified Forest National Park

Petrified Wood Colors

Various colors, often striking, are produced depending on the mineral contents in the stone:

PINK or RED – Hematite present – a form of oxidized iron

This interesting process is well explained from Iron dissolves in ground water when no oxygen is present. The ground water becomes re-oxygenated as it moves though the tree trunks causing oxygen to bond with the iron. The iron then precipitates to produce a solid form of iron called hematite. This hematite is incorporated into the log’s cell walls. The same process occurs when iron stains porcelain sinks. The soluble iron in ground water becomes oxidized into a solid form when it comes in contact with air, causing a reddish stain.

YELLOW BROWN or ORANGE – Goethite present- a weathered hydrated iron oxide that becomes crystallized

GREEN – Pure native iron present

WHITE  – Pure Silica present – Silicon, Si, and oxygen, O, are the two most abundant elements in the earth’s crust which together form silica dioxide quartz

BLACK – Carbon or Pyrite or Iron Sulfide (the most wide spread sulfide) present – The wood was affected as hydrogen sulfide from decaying organic matter interacted with iron forming pyrite.

PURPLE or BLUE  Manganese present – This is a secondary material formed when water leaches manganese from igneous rock and re-deposits it as a concentration of manganese dioxide.

TAN   Silica Dioxide present – naturally found in water, plants, animals, and the earth


How did the petrified wood of Arizona become uncovered? First, millions of years ago, the area sank to the point which completely flooded everything with freshwater sediments. It continued to sink deeper becoming completely buried. Millions of years later, the area was lifted far above sea level from westerly continental plate pressure. The uplift created stresses that cracked the giant logs. Over time, wind and water have worn away the layers of hardened sediments, exposing the fossilized wood.

Lake Michigan Petrified Driftwood

Petrified Driftwood Lake Michigan Beach

While the Petrified Forest of Arizona has the highest concentration of petrified wood in the world, it can be found in every US state and other countries around the world. I found this sample of petrified driftwood on a Southwest Michigan beach off Lake Michigan called Oval Beach. It’s predominantly gray with a slight bluish green cast and has a few streaks of rust. In certain light, it casts a very bright sheen or luster. It’s difficult to capture the sheen in a photograph, but you can get a better idea from its flip side in the photo below.

Petrified Driftwood Lake Michigan Beach

After researching the possible mineral contents of my sample, according to its dense property, color and luster, my best guess tells me that it’s hematite specularite. At any rate, it’s an exciting find. I collect driftwood on the beach all the time, but never before a piece that was petrified.

See a sample of petrified wood of extinct scale trees, from one of my previous posts.

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Earth’s Original Land Tree Plant

Was the “Calamite” the first tree-like plant to grow on land? Many scientists believe so. It grew as high as 100 feet, towering above its counterparts in the earlier periods of its lengthy lifespan, which began some whopping 400 million years ago during the Devonian Period.

Annularia Leaf Imprint Trace Fossil of Prehistoric Calamite Tree

The trunk was a woody hollow tube, lacking true bark. The leaves were primitive and needle like, arranged in whorls around a stem.

Trace Fossil Imprint of Prehistoric Calamite Tree Stem

The Calamite thrived in the hot swamp tropics of the past, particularly during the Pennsylvanian Period around 300 mya. Many of their fossils have been found worldwide including, USA, China, Canada, South America and Europe.

Internal and External Imprint Fossil of Calamite Tree Stem

These amazing fossils were found in Sebastian County, Arkansas in an old coal strip mine in 1993 by Michael A. Whitkanack, who donated them to my classroom. They are actually the imprints of the Calamite’s leaves and stems which scientists refer to as trace fossils.

Prehistoric Calamite (Earth’s First Tree-Like Land Plant) Rendition Drawing


Scientific Name: Calamite Common Name: Horsetail / Wiskfern

Kingdom: Plantae

Division: Pteridophyta (Ferns, reproduce by spores)

Class: Sphenopsida or Equisetopsida (means ribbed, vertical jointed stem; bamboo like in appearance)

Order: Equisetales

Family: Calamitaceae

Genus:  (STEM) Calamite (LEAF) Annularia

Special Note: The Calamite may look familiar to some, as their modern descendants are the “horsetails” of today, growing in open fields and edges of woodlands, but only reaching a few feet tall. See photo below


The “horsetail” or Equisetum is an amazing living fossil related to the Calamites, being the only surviving genus from the entire class of Equisetopsida. For over one hundred million years, the Equisetopsidas were much more diverse and dominated the understory of the late Paleozoic forests. Through the millenniums, they decomposed layer by layer, transforming into the sunken coal deposits of today.

Comparing Neuropteris and Pecopteris Fossil Leaves and Their Trees, Medullosa and Psaronius

Neuropteris Fossil Leaf Imprint

Extinct Neuopteris leaves are associated with the seed fern tree called Medullosa, an ancestor of the flowering plant group. They flourished during the hot swamps of the Carboniferous through the Permian time slots about 360 to 250 million years ago. When Earth’s climate turned colder, it contributed to their final disappearance.

One way to tell the difference between Neuropteris and Pecopteris leaf fossil imprints is by examining the mid-vein of their leaflets. In Neuropteris, the vein stops midway up the leaflet and splits into several fine veins, whereas the mid-vein in Pecopteris extends up to the tip. Neuropteris leaflets are more blunt tipped and attach by a single stem as opposed by the entire base, as with Pecopteris. Also, Neuropteris has an overall heart shape.


Kingdom: Plantae

Division: Tracheophyta (vascular plants with system of transporting nutrients and liquids)

Class: Gymnosperm (means bare seeds – today’s examples i.e. conifers, cyads, ginkgo)

Order: Pteridospermales (extinct group of seed ferns which bore seeds on leaves)

Family: Medullosales (plants with complex pollen organs and large fronds)

Genera: Neuropteris (given name of foliage)

Prehistoric Medullosa Seed Fern Tree Rendition Drawing

Medullosa Seed Fern tree associated with Neuropteris was a medium sized, seed fern tree reaching about 33 feet (10 meters) tall. It really was not a true fern because it produced seeds, instead of spores. It only resembled fern trees and grew during the same era as the true fern trees, for example, Psaronius associated with the Pecopteris leaves. The leaves of Medullosa had many leaflets attached to a stem and could grow quite large, as much as 10 feet (3 meters) long.


Kingdom: Plantae

Clade: Traceophytes (Large group of vascular plants with transport system for nutrients and fluids)

Division: Pteridosperm(aphyta) (Extinct group of seed bearing plants)

Order: Medullosales ((plants with complex pollen organs and large fronds)

Family: Neurodontopteridaceae (Neuropteris Leaf)

Genus: Medullosa

Explanation of Pecopteris on following page . . . 

Pecopteris Leaf of Fern Tree

Fern leaves called Pecopteris grew abundantly in the coal swamps of the Carboniferous and Permian time periods, approximately 359-251 million years ago. These leaves dropped off a medium sized, 35 foot (10 meters) tree called, Psaronius, one of the most common Paleozoic varieties. With its expansive leaves and branches, it resembled modern day palm trees. Impressively, it produced as many as 7000 tiny spores on the underside of its leaves. These fossil samples are preserved in gray coal shale from Pennsylvania as with many Carboniferous leaf fossils.

Pecopteris Imprint Fossil Leaf from Fern Tree

If you’ll recall the differences from, Neuropteris leaflets shown on the previous page, the mid-vein stops midway up the leaflet and splits into several fine veins, whereas the mid-vein in Pecopteris extends up to the tip. Neuropteris leaflets are usually more blunt tipped and are attached by a single stem as opposed by the entire base, such as with Pecopteris. Another way to identify Pecopteris is hinted in its name, derived from the Greek word meaning, to comb. Observably, the leaflets along the stems feature an arrangement resembling that of a comb.

Pecopteris Imprint Fossil Leaflets from Fern Tree

What makes fern trees so special? The large fronds produced by Pecopteris leaves, which grew upon the ancient Psaronius fern trees, cloaked the forest floor in deep shade together with the Medullosa seed fern trees. Consequently, they protected the ancient creatures below from the strong ultra violet rays of the sun as it was closer to earth and more powerful during the Paleozoic Era. Also, the shedding and decomposing of leaves created more layers of soil for roots to extend deeper and deeper, alleviating the need for trees to grow near water pools. Trees were then able to spread further inland. Yet another benefit was that the leaves fed inland water sources cultivating more fresh water fish varieties. But this fantastic fossil is most special to me because I inherited from my late father-in-law, Joseph Mirto II. It was found in Johnstown, Pennsylvania.


Kingdom: Plantae

Division: Pteridophtya (meaning vascular plant with transport system for nutrients and fluids)

Class: Filicopsida (Ferns which reproduce with spores)

Order: Marattiales (primitive ferns)

Family: Marattiaceae

Genus: Pecopteris

Psaronius Fern Tree Rendition Drawing


Botanical Name: Psaronius Common Name: Fern Tree

Kingdom: Plantae

Division: Tracheophyta (meaning vascular plant with transport system for nutrients and fluids)

Class: Marattiopsida (distinguished by massive roots and largest fronds of all seed fern trees)

Order: Marattiales (primitive ferns)

Family: Psaronlaceae

Genus: Psaronius Species: magnificus

Lepidodendron & Sigillaria Scale Trees

Lepidodendron Petrified Tree Root Fossil

The fossil above is a section from the root of a large sized, 100 foot (30 meter) Lycopod tree which originated over 400 million years ago. It contains deeply pitted circular patterns, but its trunk differed having deeply grooved diamond patterns. It’s a very dense, heavy fossil of petrified wood. Petrified wood materializes when plant matter is buried by sediment and protected from decay caused by oxygen and organisms. Then, groundwater rich in dissolved solids flows through the sediments, replacing the original plant material with silica, calcite, pyrite, iron or another inorganic material such as opal. This was a common occurrence in the coal swamp forests of the Carboniferous Period from about 360 to 300 million years ago during the late Paleozoic Era.

Sigillaria & Lepidodendron Tree Leave Fossils

The fossil above are leaf imprints of these giant Lycopod trees such as Sigillaria and Lepidodendron cast in coal shale. The trunks of Lycopods were topped with plumes of these long thin, grass-like leaves which were often arranged like that of a bottle brush. The trees had relatively short life-cycles growing rapidly reaching heights sometimes up to 130 feet (40 meters). Lycopod forests of plenty generated tremendous amounts of decaying peat. After millions of years, it became coal buried deeply in the ground, later, fueling the Industrial Revolution. More importantly, their decaying matter helped revolutionize Earth’s emerging forests by creating soil for trees to develop deeper root systems. This enabled new tree varieties to spread further inland without relying solely on wet swampy habitats.

Lepidodendron Scale Tree Rendition Drawing
Sigillaria Scale Trees Rendition Drawing


Botanical Names:  Sigillaria and Lepidodendron

Common Name: Scale Tree 

Kingdom: Plantae

Division: Lycopod-iophyta (oldest vascular plant group, reproduced by releasing spores)

Class: Isoetopsida (plants with hollow quill-like leaves spirally arranged on a single, unbranched vein) ie quillworts, scale trees, spike moss)

Order: Lepidondrales (primitive vascular tree-like plants related to lycopods which are loosely grouped with ferns)

Family: Lepidodencraceae (has arrangement of spores on cones born on the shoots)

Genera:  Sigillaria (possess deep lace pattern on trunk with bottle brush crown of leaves)

Genera: Lepidodendron (possess deep diamond pattern on trunk with plume of grassy leaves on crown. Roots lack diamond pattern.