North America, Upper Chinle Formation.
Several small Coelophysis, a primitive theropod dinosaur, pick their way along a drying streambed, at the margins of a dense coniferous forest.
200 million years ago, in the Late Triassic period, the upper Chinle formation, in the Painted Desert portion of Petrified Forest National Park, was a very different place from today. It was an extensive floodplain of swamps, forests, lakes and braided streams, where heavy rains and periodic flooding would swell rivers and undermine the trees, washing their broken trunks into vast log jams, burying them beneath silts and mud, rich in iron oxide, limonite and manganese whose brightly coloured compounds today give the Painted Desert its name, while silicon dioxide, dissolved from fine volcanic ash, mineralised and preserved these fallen trees.
Araucarioxylon is the generic name given to these extinct conifers. It means ‘wood of the Araucaria’ but this is misleading, as they were not closely related to the Araucariaceae. In fact Araucarioxylon probably resembled the giant coastal redwoods of California, with crowns of lacy, umbrella-like foliage.
It is still uncertain whether Araucarioxylon grew in large forests or smaller stands of trees, but it is certain that these early conifers reached great heights of 60 metres or more, forming a dense canopy above the ginkgoes, cycads, ferns, horsetails and other coniferous trees that grew along the meandering streams beneath them.
In 2007 microscopic studies revealed the fossil wood designated as Araucarioxylon came from three separate genera, and with at least nine different species of tree now identified as having lived within the Chinle ecosystem, Araucarioxylon may become superfluous as a genus.
Coelophysis bauri was one of several early dinosaur species inhabiting the Chinle forests. Averaging a metre in height and three metres in length, its remains have been discovered in large concentrations, leading palaeontologists to speculate it may have been a gregarious animal, at least some of the time.
Perhaps, like birds today, Coelophysis formed flocks, or possibly environmental stresses like drought caused them to gather in larger concentrations, where evaporating streams offered them a source of both food and water. There, surprised by flash flooding, whole groups of these animals were washed away to be fossilised in a mass grave.
The excellent preservation of plant and animal material from the Chinle Formation, and the abundance of fossil remains, make it one of the best ‘windows’ into the Late Triassic world yet discovered in the Western Hemisphere.
Acrylic on board.
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