Araucarioxylon arizonicum

Did You Know?

• During the Triassic, Arizona was a flat tropical expanse in the southwest corner of Pangaea.
• Fossils of A. arizonicum frequently show boreholes from insect larvae, possibly beetles similar to modern Ptinidae.
• The tree could reach heights of 60 m and diameters over 60 cm.

Description

The petrified wood of this tree is often called 'Rainbow wood' due to its varied colors. Red and yellow hues come from large particles of iron oxide—hematite for red and limonite for yellow—while purple results from fine hematite spherules in the quartz matrix, not from manganese. The trunks were large and slender, tapering slightly upward. The largest known trunk, 'Old Faithful' from the Rainbow Forest sandstone beds of the Sonsela Member, had a base diameter of 2.9 meters and an estimated height of 59 meters. Branches, usually broken off in fossils, left scars showing they were slender and angled upward at 30–40 degrees above horizontal, spaced regularly around the trunk. Upper trunk fragments sometimes bear thin branches in small clusters. Preserved bark is rare but shows thin, rippled texture with low ridges and longitudinal furrows like some modern pines. Roots differ from modern conifers: a massive central taproot up to 5 meters long is surrounded by four to six thick lateral roots. No foliage is directly associated, but common Chinle Formation leaves include Pagiophyllum, Brachyphyllum, and Podozamites. In life, it may have resembled a medium-sized giant sequoia.

Taxonomy

Araucarioxylon arizonicum has also been called Agathoxylon arizonicum because Araucarioxylon is a junior synonym of Agathoxylon. The species was first described in 1889 by Frank Hall Knowlton. Its validity has been questioned; a 2007 study of Knowlton's syntypes found they belonged to three species, tentatively reclassified as Pullisilvaxylon arizonicum, Pullisilvaxylon daughertii, and Chinleoxylon knowltonii. The genus Araucarioxylon may therefore be superfluous and illegitimate, and the petrified logs may represent greater diversity than originally thought.