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''Araucarioxylon arizonicum'' is an extinct species of conifer that is the
state fossil Most American states have made a state fossil designation, in many cases during the 1980s. It is common to designate one species in which fossilization has occurred, rather than a single specimen, or a category of fossils not limited to a single ...
of
Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...
. The species is known from massive
tree trunks In botany, the trunk (or bole) is the stem and main wooden axis of a tree, which is an important feature in tree identification, and which often differs markedly from the bottom of the trunk to the top, depending on the species. The trunk is the ...
that weather out of the
Chinle Formation The Chinle Formation is an Upper Triassic continental geological formation of fluvial, lacustrine, and palustrine to eolian deposits spread across the U.S. states of Nevada, Utah, northern Arizona, western New Mexico, and western Colorado. In N ...
in desert badlands of northern Arizona and adjacent New Mexico and Chemnitz petrified forest in Chemnitz,
Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwee ...
, most notably in the Petrified Forest National Park. There, these trunks are locally so abundant that they have been used as building materials.


Description

The petrified wood of this tree is frequently referred to as "Rainbow wood" because of the large variety of colors some
specimen Specimen may refer to: Science and technology * Sample (material), a limited quantity of something which is intended to be similar to and represent a larger amount * Biological specimen or biospecimen, an organic specimen held by a biorepository f ...
s exhibit. The red and yellow are produced by large particulate forms of iron oxide, the yellow being limonite and the red being hematite. The purple hue comes from extremely fine spherules of hematite distributed throughout the quartz matrix, and not from manganese, as has sometimes been suggested. The trunks were large and slender, tapering slightly towards their apex. The largest known trunk in Petrified Forest was 'Old Faithful', from the Rainbow Forest sandstone beds of the Sonsela Member. This trunk had a diameter of 2.9 meters (9.5 feet) at its base, and a maximum estimated height of 59 meters (194 feet) based on modern conifer proportions. Branches are broken off in most fossils, but their bases leave scars indicating that the branches were slender and bent upwards at around 30-40 degrees above the horizontal. The branches are regularly and widely spaced around nearly the entire trunk. Some fragments from the upper part of the trunk have thin branches arranged in small clusters. Preserved bark is rare, but known bark fossils show that it is thin and "rippled", with low ridges and longitudinal furrows similar to some modern
pines A pine is any conifer tree or shrub in the genus ''Pinus'' () of the family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. The World Flora Online created by the Royal Botanic Gardens, Kew and Missouri Botanical Garde ...
. The roots, when preserved, are significantly different from modern conifers: There is a massive central taproot, up to 5 meters (16.4 feet) long, ringed by four to six thick lateral roots. Modern conifers, on the other hand, typically bear a circular cluster of relatively small, narrow roots. No known conifer foliage is directly associated with the trunks, but common conifer leaves such as '' Pagiophyllum'', '' Brachyphyllum'', and '' Podozamites'' are also known from the Chinle Formation. In life, ''Araucarioxylon arizonicum'' may have looked similar to a medium-sized '' Sequoiadendron giganteum'' (giant sequoia).


Paleoecology

In the
Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...
period (around ), Arizona was a flat tropical expanse in the northwest corner of the supercontinent Pangaea. There, a forest grew in which ''A. arizonicum'' towered as high as and measured more than in diameter. Fossils frequently show boreholes of
insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pa ...
larvae, possibly
beetle Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...
s similar to members of the modern family Ptinidae.


Taxonomy

''Araucarioxylon arizonicum'' is classified under family Araucariaceae. They were first described in 1889 by the
American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the "United States" or "America" ** Americans, citizens and nationals of the United States of America ** American ancestry, pe ...
paleobotanist
Frank Hall Knowlton Frank Hall Knowlton (September 2, 1860 - November 22, 1926) was an American botanist, ornithologist and naturalist. Born in Vermont, he joined the Geological Survey and took an interest in fossil plants in the local lignite, later becoming a spec ...
. The validity of the name ''Araucarioxylon arizonicum'' has been questioned. ''A. arizonicum'' may actually be composed of several different genera and
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
. A 2007 study on the syntypes used by Knowlton in describing the species has revealed that they belonged to three species. They were tentatively reclassified as '' Pullisilvaxylon arizonicum'', '' Pullisilvaxylon daughertii'', and '' Chinleoxylon knowltonii''. The genus ''Araucarioxylon'' may thus be superfluous and illegitimate; and the petrified logs of Petrified Forest National Park may be composed of a greater diversity than initially believed.


See also

* Paleobotany *'' Araucaria'' *''
Araucaria mirabilis ''Araucaria mirabilis'' is an extinct species of coniferous tree from Patagonia, Argentina. It belongs to the genus ''Araucaria''. ''A. mirabilis'' are known from large amounts of very well preserved silicified wood and cones from the Cerro Cua ...
'' *'' Araucarites sanctaecrucis'' *'' Nothofagus'' *'' Paleobiota of the Chinle Formation''


References

{{Taxonbar, from=Q135480 Triassic plants Flora of Arizona Flora of New Mexico Fossils of the United States Natural history of Arizona Triassic Arizona Triassic life of North America Araucariaceae Mesozoic trees Prehistoric plants of North America