Secrets of the Chinle Formation

log of petrified wood found in the Chinle Formation

NPS photo / David Tarailo

The sagebrush and cacti-dotted landscape at Zion does not easily render thoughts of lakes, streams, and swamps. During Chinle time, no flowering plants existed; instead, club mosses and ferns grew in freshwater marshes, while horsetails and cycads occupied floodplains. The Chinle Formation's 220 million-year old ecosystem featured conifers, related to modern araucarians like the ornamental Norfolk Pine, which grew to 150 feet with 9-foot widths.
small piece of petrified wood with penny for scale
Zion Museum Collection: ZION 612

NPS photo / Scott Ireland

When they aged and fell or were toppled by erosion and floods, some of the conifers were buried rapidly by sediments. Deprived of oxygen needed for fast decay, the living woody tissues were slowly replaced by minerals and hardened to stone to form petrified wood. Often, minerals replace the wood quickly enough to preserve the internal cellular structure of the tree, and features like tree rings, bark, and knots are visible.

Learn more about how fossils are made
pyrite in petrified wood
Zion Museum Collection: ZION 15700

NPS photo / Jenny McGuire

Quartz is the primary mineral replacing the organic compounds in the wood, but some of the petrified wood at Zion has been replaced with an iron sulfide mineral called pyrite, commonly known as fool's gold.
Phytosaur tooth
Phytosaur tooth

Plants aren't the only fossil finds from the Chinle Formation at Zion. Within the murky marshes of thick vegetation, a carnivorous creature lurked, heavily protected with armor and built to attack. In this tropical lowland, phytosaurs, with their razor sharp teeth and long snouts, were dominant predators.
Phytosaurs measured up to sixteen feet and, although unrelated, looked similar to a present-day crocodiles.
pieces of Phytosaur braincase
Phytosaur braincase; Zion Museum Collection: ZION 15683
Bone fragments and teeth belonging to phytosaurs and the early amphibian Metoposaurus have been recovered from the Chinle Formation in Zion. The phytosaur partial fossil shown here includes the occipital condyle, a part of the braincase which articulated with the upper vertebra.
The museum collection also houses armor plates, called scutes, from the primitive Aetosaurus. Other evidence of ancient life found in the Chinle includes burrows of invertebrates and coprolites (fossilized poop). And after years of uncertainty, the source of a small, mystery tooth has been solved.

Learn more about the Chinle Formation

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Last updated: July 6, 2015

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