Petrified Forest National Park 1
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Petrified Forest National Park
Images © W.P. Armstrong November 2008
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Late Triassic Petrified Logs
Petrified Forest National Park in northeastern Arizona contains hundreds of acres of perfectly preserved logs from an ancient conifer forest that dates back to the late Triassic Period (approximately 225 million years ago). The trees of this forest coexisted with dinosaurs. Most of the petrified logs were previously assigned to an extinct species Araucarioxylon arizonicum, a presumed distant relative of Araucaria; however, new evidence indicates that these fascinating deposits of petrified logs represent a broad diversity of conifer species. Streams carried dead logs into this once swampy lowland region where they were buried in sediments rich in volcanic ash. Over countless centuries, the woody tissue has been replaced by minerals and gradually turned into stone. These ancient trees flourished during a time when all of the continents were united into the vast supercontinent Pangea (see following diagrams). The area of Petrified Forest National Park was located near the equator, at approximately the latitude of present-day Central America. Although most of the logs do not show cellular detail, there are some permineralized specimens in which minerals permeated the porous cell walls and filled the cell cavities (lumens). Thin sections of these samples viewed under a microscope show remarkable cellular detail.

Although the binomial Araucarioxylon arizonicum has been used in the literature for more than a century, Rodney A. Savidge of the University of New Brunswick (Bulletin of Geosciences Vol. 82 No. 4: 301-328, 2007) states that it is superfluous and therefore an illegitimate name (nomen superfluum). He examined thin sections of the original three specimens (syntypes) housed at the Smithsonian Institute upon which the species was first described by F.H. Knowlton in 1889. He concluded that they represented different species within two new genera of extinct trees. According to the International Code of Botanical Nomenclature, a valid species can have only one type specimen or holotype. Consequently, only one of the three specimens was retained as the new type or lectotype Pullisilvaxylon arizonicum. Savidge examined several other logs previously identified as A. arizonicum and concluded that they also represented additional new genera and species. His extensive anatomical studies indicate that the majority of logs at Petrified Forest National Park do not belong to a single species. It appears that the superfluous name "A. arizonicum" actually refers to a complex of extinct conifers. 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 araucaria family. 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 most logs in Petrified Forest National Park without detailed microscopic examination of the wood.

Trees of "Araucarioxylon arizonicum" 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 (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. These ancient conifers grew in a tropical rain forest with marshes and river lakes, in an environment very different from today's Arizona landscape.

In 1915 Alfred Wegener, a German climatologist and geophysicist, published an expanded version of his 1912 book The Origin of Continents and Oceans. His novel hypothesis, which has now been confirmed by other scientists and elevated to the status of a scientific theory, states that continents are not fixed; instead, their relative positions and the positions of the oceans have changed over time. Wegener's original hypothesis led to the science of plate tectonics, where scientists study the gradual movement of large plates of the earth's crust along major fault zones. About 200 million years ago, the continents were joined into one supercontinent that Wegener called Pangea (Figure A). By 135 million years ago, Pangea had divided into two large subcontinents called Gondwanaland and Laurasia (Figure B). In the great southern subcontinent Gondwanaland, South America and Africa were connected with each other and with Antarctica, India, and Australia. This is roughly the time when the first flowering plants began to appear on earth. By 65 million years ago, about the time when dinosaurs became extinct, the continents had divided into positions resembling the present-day configuration (Figure C). As of 2 December 2008, the continents resembled the configuration shown in Figure D.

Diorama of araucariad forest from 200 million years ago, when all the continents were united into the vast supercontinent Pangea. Whether any logs at Petrified Forest National Park came from trees such as these is unknown at this time. From all the thousands of petrified logs, one can only imagine the extent and diversity of this ancient forest of giant trees. Diorama on display at the Rainbow Forest Museum, Petrified Forest National Park.

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