Finding Fossils in Petrified Forest National Park, Arizona
Arizona’s Petrified Forest National Park is known for its stunning views and beautifully preserved petrified wood. The trees here are Late Triassic in age (230-200 million years ago) and are preserved in agate, an often multicolored form of granular quartz. These fossil trees are what bring people from all over the world to the park each year but the petrified wood is not the only thing of significance to a paleontologist. The park is also an important source of vertebrate fossils from the Triassic.
The bones that are preserved here are weathering out of what’s left of local exposures of the Chinle formation. Most of the fossil-bearing rock here has long ago weathered away leaving the petrified trees scattered around the area. The bones tend to be less resilient than the wood so paleontologists rely primarily on the remaining outcrops of Triassic mudstone to find vertebrate fossils still in place in the mudstone.
As part of a month-long expedition across the Midwest, Virginia Tech’s paleobiology group visited PEFO (Petrified Forest) over the last week in order to collect fossils and/or help out as needed with work in the preparation labs on site. The weather has not been cooperating however (it is monsoon season after all) and this has limited our time in the field. We were only able to spend a few days out on the bone beds but we didn’t come out empty handed!
A day of prospecting in the canyons around our target site yielded lots of phytosaur (large extinct croc-like reptiles) bone fragments too damaged to keep and some teeth and osteoderms (bony armor plates) worth retrieving. Because of the weather during the days leading up to our arrival, we were cut off from the target locality but by the next day, the road out to the site had dried up enough for us to drive all the way in and actually start excavating there. The site, called “the green layer,” is a thin layer of greenish-grey mudstone that contains abundant vertebrate fossils including those of phytosaurs, aetosaurs (large armor plated reptiles), and dinosaurs.
We recovered several bone fragments and osteoderms that had to be jacketed as well as several hundred pounds of sediment from the layer. The extra sediment we collected can be screen washed for microfossils back at Tech. Altogether, it was a productive few days of collecting (despite the weather) and an incredible experience working in the park! Some of the specimens we collected could potentially help with reconstructing the fauna and ecosystems of the Triassic of what is now the American Midwest!
Written by Alex Bradley.
Follow VT’s Paleobiology & Geobiology Research Group as they hunt for fossils this summer.
Aetosaur cast in the on-site museum.
We managed to catch a lizard (Holbrookia maculata). The local wildlife is beautiful and quick!
Some of the crew looking at calcified root casts in the mudstone. These formed as a result of calcium carbonate precipitating inside the voids left by plant roots that, at one point in the distant past, infiltrated the sediment.