Rainbow Wood

Appearance

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.

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 at its base, and a maximum estimated height of 59 meters 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. The roots, when preserved, are significantly different from modern conifers: There is a massive central taproot, up to 5 meters 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".

Habitat

In the Triassic period, 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 and measured more than 60 centimetres in diameter. Fossils frequently show boreholes of insect larvae, possibly beetles similar to members of the modern family Ptinidae.