Araucarioxylon Taxonomy Problem
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A Taxonomic Problem With Araucarioxylon arizonicum
© W.P. Armstrong   December 2008

During my recent trip to Petrified Forest National Monument, I was informed by an interpreter at the Rainbow Forest Museum that there was a problem with the name Araucarioxylon arizonicum, the state fossil of Arizona. This name has been used in the literature for more than a century. I contacted the Park Paleontologist Bill Parker who sent me a PDF of the recent definitive work on this taxon by Mn Center (manganese-protein complex), Here is a brief synopsis of Dr. Savidge's detailed study of petrified wood from this beautiful national park.

There are two major problems with the binomial Araucarioxylon arizonicum, originally named by F.H. Knowlton in 1889 ("New Species of Fossil Wood (Araucarioxylon arizonicum) From Arizona and New Mexico," U.S. National Museum Proceedings 1888 11: 1-4):

  1. There were three type specimens deposited in the Smithsonian Institute. According to the International Code of Botanical Nomenclature, a valid species can have only one type specimen or holotype. Consequently, one of the three specimens must be renamed and designated as the new type or lectotype.

  2. The original three type specimens represented three different species.

According to Dr. Savidge (2007), the 1889 name Araucarioxylon arizonicum is superfluous and therefore an illegitimate name (nomen superfluum). Therefore, it was necessary to rename the original three specimens. He selected one as the lectotype and named it Pullisilvaxylon arizonicum. The second specimen was named Pullisilvaxylon daughertii and the third was named Chinleoxylon knowltonii. Savidge examined several other logs previously identified as Araucarioxylon arizonicum and concluded that they also represented additional new genera and species which he named accordingly. It appears that the superfluous name "Araucarioxylon arizonicum" actually refers to a complex of extinct conifers. Savidge includes a key to these species in the appendix of his 2007 publication; however, it requires microscopic examination of thin sections of the xylem tissue. You simply cannot identify a species by examining a log or polished stem. You must have a properly prepared thin section showing cellular detail and a compound microscope. According to Dr. Savidge (personal communication, 2008), an immense amount of research into petrified woods is needed before the ancestries leading to modern trees will be clearly understood. At this time it would be purely conjectural to assign a scientific name to logs previously named Araucarioxylon arizonicum without detailed microscopic examination of the wood.

I planned to use some of my images from Petrified Forest National Park in my article for Pacific Horticulture entitled "The Araucaria Family: Past and Present." However, I am not sure which of the logs (if any) are related to the araucaria family. Based solely on the xylem structure of permineralized wood (including resin canals, rays and tracheid pitting), and without seed cones or DNA evidence, it is difficult to be certain which trees in the complex are ancestral relatives of the Araucariaceae. Trees of "Araucarioxylon arizonicum" supposedly grew to a height 200 feet (61 m) with a trunk diameter from 4 to 9 feet. According to Sidney R. Ash and Geoffrey T. Creber ("The Late Triassic Araucarioxylon arizonicum Trees of the Petrified Forest National Park, Arizona, USA," Paleontology Vol. 43 No. 1: 15-28, 2003), the living tree did not closely resemble any of the present-day Araucaria trees of the southern hemisphere as postulated in past reconstructions. The branches did not occur in whorls as they do in most conifers, instead they grew irregularly along the trunk. Sydney Ash and Rodney Savidge also studied the bark anatomy of Araucarioxylon arizonicum ("The Bark of Late Triassic Araucarioxylon arizonicum from Petrified Forest National Park, Arizona," IAWA Journal 25 No. 3: 349-368, 2004), and concluded that it was quite unlike the banded bark of extant Araucaria heterophylla.

Armstrong, W.P. 2010. "The Araucaria Family: Past & Present." Pacific Horticulture 71 (1): 4-11   See PDF File

There are some petrified logs from the Chinle Formation in Apache County that can apparently be identified from macroscopic identification, including Woodworthia and Schilderia. For anyone faced with the dilemma of what to call your petrified log previously named "Araucarioxylon arizonicum," about all you can say is "petrified conifer wood" from the late Triassic. There are undoubtedly petrified logs of other trees in this region that have yet to be properly named.

Polished petrified wood from the Chinle Formation of Arizona. The middle sample with large, warty bumps on the trunk is Woodworthia. The outer samples were originally, incorrectly named "Araucariozylon arizonicum," and probably represent two different extinct species.

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