Fossil Ecosystems of North America: the Chinle Group

part 1 - BACKGROUND: PANGEA AND THE END-PERMIAN EXTINCTION

After the end-Permian extinction, trilobites, eurypterids, and graptolites are extinct; brachiopods released their dominant position as animals on the sea floor to bivalved mollusks; coral reefs are being built by new species; reptiles have diversified and grown significantly in size, and among plants, gymnosperms, conifers in particular, are now the dominant flora on land.

Earth also looked different geographically: most land masses were clustered together in a supercontinent called Pangea, and ocean waters surrounded Pangea in what is called the Panthalassa Ocean. Pangea stretched from pole to pole, but climate was much warmer then than today, and there were no ice caps. The poles had a monsoonal weather patterns (a dry season and a wet season, like the Indian subcontinent today) but most of the land was hot and dry. Oceans were warmer than today, with temperatures similar to the tropical seas of today.

On this page is a series of maps of Earth. Look under "Mesozoic Era" for maps at the end of the Triassic: they can be found in the last row. Once you find your map, click on it and it will be enlarged.
What is now North America is at the top left of the continental mass depicted in the images. Below "North America" you can easily identify the united masses of future South America and Africa.

Here is instead a series of maps of North America. Look under "Mesozoic Maps" and on the last row there is the Triassic. Again, once you find your map, click on it and it will be enlarged. You can easily recognize most of the familiar Canadian provinces and American states on this map.

Right after the beginning of the Triassic, the surviving mammal-like reptiles flourished again, and originated the first mammals by the end of the Triassic. Still, it was the amphibious reptiles that dominated Earth at first, to eventually leave that role to dinosaurs, which also appeared in the Late Triassic.

part 2 - HISTORY OF DISCOVERY OF THE CHINLE GROUP

The Petrified Forest National Park extends through spectacular badlands in northeastern Arizona, and has the world largest concentration of petrified wood from the Triassic Period.

Two different aspects of the Petrified Forest area landscape (Painted Desert)

The first fossils were found in 1849 in the Canyon De Chelly area, further north, and more materials were found in the 1850s south of the present-day National Park. The construction of the transcontinental railroad in the area and the arrival of tourists saw the removal of several of these fossils, which led President Theodore Roosevelt to declare this are a National Monument in 1906 Later on more land was added to the Monument but it was only in 1962 that the Petrified Forest became a national Park. This whole area, the park and the region around it, is known as the Painted Desert because of the beautiful colors found in the landscape (see the images above and the one below).

A National Park informative panel from the Petrified Forest

The Petrified Forest trees were first described as Araucaria, a kind of coniferous tree which is common today in the Chilean Andes. It is important to keep in mind that, despite the fact that the park is named "Petrified Forest", it is so named because it used to be a forest. Today it is in a desert (the Painted desert). A live Araucaria forest would of course look different:

A Chilean Araucaria forest today, from Jardineriaon.com.
These trees belong to the same family of those fossilized at Petrified Forest National park in the Painted Desert of Arizona at the end of the Triassic

The Chinle Formation was named for layers containing fossilized trees in the Chinle Valley of Arizona, but previously discovered time-equivalent layers from Ghost Ranch Quarry in New Mexico had yielded dinosaur bones instead. A much more varied and complete collection of macrovertebrates and microvertebrates was later discovered in the Placerias Quarry in Apache County, Arizona. At this location, among other taxa, dinosaurs, mammal-like herbivore reptiles, and crocodile-lied carnivorous reptiles were found.

part 3 - STRATIGRAPHIC SETTING AND TAPHONOMY OF THE CHINLE GROUP

Read the initial part where the nomenclature and lithology are discussed, but there will be no questions about it. Know the part that begins with "Taphonomically, the Chinle Group can be regarded as...".

This section tells you that:

the Chinle Group is a Concentration Lagerstätten
the Lagerstätten exists as a series of separated, different levels containing bones and others ones containing petrified wood
what happened to bones and wood after burial "shows some obvious differences"
the petrified trees were transported during huge floods: evidence for this comes from water wear on the logs and from their preferred orientation indicating direction of flow
the remains of some of the dinosaurs (Coelophysis) were also washed downstream; these probably died because of prolonged seasonal droughts
the remains of Placerias were also accumulating because of drought, but they are found in place, that is they were not washed away during a flood
in both the Coelophysis and the Placerias deposits, it seems likely that the dinosaurs congregated around shrinking water polls, and eventually died of dehydration
there is evidence of severe droughts in the American Triassic from sediments and sedimentary structures

These Araucarioxylon arizonicum petrified logs, found by the Petrified Forest National Park visitor center, show alignment in one direction only,
as it would happen if they were transported by a stream current

part 4 - DESCRIPTIONS OF THE CHINLE GROUP BIOTA

There would be no detailed questions from this section. Read it in its entirety but know what kind organisms have been found:

dinosaurs
mammal-like reptiles
crocodile-like reptiles
herbivorous reptiles
amphibians
fish (sharks, coelacanths, and others)
trace fossils
invertebrates (mollusks, crayfish, and insects)
Araucarioxylon arizonicum, a gymnosperm tree found in great abundance in the Chinle Group
other plants (including horsetails, ferns, cycads, ginkgo, and conifers)

A petrified trunk of Araucarioxylon arizonicum, showing growth rings on transverse section

A petrified trunk of Araucarioxylon arizonicum, showing growth rings on longitudinal section

part 5 - PALEOECOLOGY OF THE CHINLE GROUP

This section deals with the ancient sedimentary environments where animals and plants fossilized in the Chinle Group used to live.

The whole Chinle group represents a relatively flat floodplain, incised by both meandering and braided streams, with a few lacustrine environments. The streams carried gravel, sand, and mud from two nearby mountain chains, and were affected by strong monsoons, causing well-defined seasons, accompanied by periods of drought.

The lower unit of the Chinle is different in Arizona, where aquatic vertebrates are common, and in New Mexico, where the fauna is totally dominated by terrestrial vertebrates.
The upper unit of the Chinle still shows a freshwater environment in Arizona, but within a somehow different from the previous one; in New Mexico, large numbers of Coelophysis dinosaurs roamed floodplains crossed by streams, along whose margins a variety of horsetails, ferns, ginkgo and conifers thrived. On the higher mountain grounds around the floodplain, vast forests of Araucarioxylon arizonicum dominated the flora.

part 6 - COMPARISON OF THE CHINLE GROUP WITH OTHER PERMO-TRIAS TERRESTRIAL SITES

Read this whole paragraph. The only thing you need to know from this section is that the Dockum Group, from Texas and eastern New Mexico, is the lateral equivalent of the Chinle Group of Arizona and western New Mexico. As a consequence, even if correlation over such a vast area is difficult, it is not surprising that the biota are very similar. This is particularly true for the floras of the Dockum, which is more closely related to the flora of the Chinle than to any other.

A detail of the "chertified" structure of the trunk in an Araucarioxylon arizonicum petrified log, from the Petrified Forest National Park, Arizona

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