|Petrified Araucarioxylon arizonicum|
|Agate House Pueblo, constructed with petrified wood|
Araucarioxylon arizonicum is an extinct species of conifer that is the state fossil of Arizona. The species is known from massive tree trunks that weather out of the Chinle Formation in desert badlands of northern Arizona and adjacent New Mexico, most notably in the 378.51 square kilometres (93,530 acres) Petrified Forest National Park. There, these trunks are locally so abundant that they have been used as building materials.
The petrified wood of this tree is frequently referred to as "Rainbow wood" because of the large variety of colors some specimens 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.
In the Triassic 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 60 metres (200 ft) and measured more than 60 centimetres (2.0 ft) in diameter. Fossils frequently show boreholes of insect larvae, possibly beetles similar to members of the modern family Anobiidae.
The validity of the name Araucarioxylon arizonicum has been questioned. A. arizonicum may actually be composed of several different genera and species. 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.
- List of extinct plants
- Araucaria mirabilis
- Araucarites sanctaecrucis
- Sidney R. Ash & Rodney A. Savidge (2004). "The bark of the Late Triassic Araucarioxylon arizonicum tree from Petrified Forest National Park, Arizona". IAWA Journal, 25 (3): 349–368.
- Adele Conover (June 1997). "The Object at Hand". Smithsonian.com. Retrieved November 19, 2011.
- Frank H. Knowlton (1889). "New species of fossil wood (Araucarioxylon arizonicum) from Arizona and New Mexico". Proceedings of the United States National Museum 11: 1–5.
- Wayne P. Armstrong (December 2008). "A Taxonomic Problem With Araucarioxylon arizonicum". Wayne's Word, Palomar College. Retrieved November 19, 2011.
- Rodney A. Savidge (2007). "Wood anatomy of Late Triassic trees in Petrified Forest National Park, Arizona, USA, in relation to Araucarioxylon arizonicum Knowlton, 1889". Bulletin of Geosciences 82 (4): 301–328.
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