During the final week of the September, a group led by Smithsonian curator Dr. Kay Behrensmeyer and professor Dr. Robin Whatley of Columbia College, Chicago, journeyed into the wilderness of Petrified Forest National Park, Arizona, with the goal of collecting fossils and stratigraphic measurements from the beginning of the age of dinosaurs.
Our interest was focused on one particular rock outcrop of the Owl Rock Member, the uppermost part of the Late Triassic Chinle Formation. This formation makes up the bright red beds that give the Petrified Forest’s Painted Desert its stunning color. The Owl Rock Member was deposited between 209-205 million years ago, and its fossils capture life at a time of transition. Dinosaurs had not yet taken over the landscape as towering giants, but were instead small and struggling to gain a foothold in a realm of larger reptiles. Phytosaurs – crocodile-like reptiles – were the dominant predators, and armored, flat-backed aetosaurs were the largest herbivores. Flying pterosaurs, fish, and turtles also shared the Late Triassic ecosystem. Drs. Behrensmeyer and Whatley have made repeated visits to Petrified Forest National Park to study the geology and paleontology of the Owl Rock Member, and the focus of this and previous trips has been a particular 8 by 10 meter stretch of outcrop at the top of an eroding ridge of sediment. This section of sediment represents the channel of an ancient riverbed, where water washed and jumbled together an unusually high concentration of bones, some large and robust, others small and delicate.
Our six member team spent most of its time at this outcrop during our expedition. The site first had to be carefully inspected for fossils that had eroded to the surface since the last visit. After those had been collected, excess weathered sediment and gravel was swept up and piled in a safe place. Then, measurements were taken to establish the boundaries of a rectangular quarry, in which all finds could be mapped on a grid system, and location information preserved for later study of spatial associations among the fossils and the shape of the riverbed. Fossils began to reveal themselves quickly. While the sediments of the riverbed alternated between pink and white in color, the fossils appear as bright white with an outside coating of black manganese. Many fossils were close to the surface, but unfortunately their exposure to weathering, combined with their small size, made them very fragile and already highly fractured when found.
(Left) Clusters of large bone fragments are occasionally seen weathering out on the surface. These are likely from a phytosaur, the largest predator known from the Owl Rock Member. (Right) Early in our work, the quarry was swept clean of loose sediment and our grid system for mapping the excavated fossils was laid out. Click to zoom.
Each new discovery was quickly coated with a hardening solution – a mixture of an archival plastic dissolved in acetone – that helped to keep the specimens together and strengthen them for excavation. Each specimen was numbered and mapped on the quarry’s grid system. Isolated specimens were excavated with a small surrounding ‘pancake’ of rock matrix, and bundled in toilet paper to protect them from breakage. More concentrated groups of fossils were excavated in blocks wrapped in plaster bandages, then carried out on our backs to camp. Over 250 lbs of matrix blocks and fossils were collected for later preparation in our museum’s FossiLab.
(Left) Two isolated fossil specimens are made ready for excavation by coating them and surrounding matrix with a hardening solution. This coating can easily be removed if needed when the specimens are prepared in the lab. (Right) Three neighboring blocks of fossils and their surrounding matrix are shown wrapped in plaster bandages and labeled, ready to be excavated from the quarry. Click to zoom.
(Left) A fossil freshly removed from the quarry in a surrounding patch of matrix, numbered and marked with an orientation arrow. This is likely the root of a phytosaur tooth. (Right) FossiLab volunteer Suzy McIntyre wraps one of the many a quarry specimens excavated individually in toilet paper to protect it during transport to the Museum. Click to zoom.
Field work in the park’s wilderness area means roughing it – bringing all supplies and materials into and out of the site on foot. Our camp was established on a grassy plateau within about half an hour’s walk from the quarry, at about 6,000 feet in elevation. Our daily ‘commute’ took us across the tops of thin ridges and down along winding washes, and our nightly routine included being awakened repeatedly by the singing of coyotes.
(Left) Our camp was on a grassy plateau at the edge of the desert. (Right) Thin, tall ridges emerge from the Painted Desert as curtains of sediment erode down either side. This one offered us a twisting path from camp to the quarry site. Click to zoom.
Our neighbors in camp included lizards, butterflies, meadowlarks, and one small Hopi rattlesnake. By far the greatest spectacle of the trip was witnessing a total lunar eclipse, with a cloudless night and the full starscape of the Milky Way over our heads as the moon darkened.
(Left) This tiny lizard was spotted near our quarry site several times a day for the duration of our trip. (Right) A rattlesnake found within our campsite was less desirable company. This visitor was safely relocated away from camp and sent on its way.
Because most of the fossils found during the trip were not fully exposed in the field, our discoveries won’t be fully understood until they are prepared in the National Museum of Natural History’s FossiLab. Based on previous finds from this quarry, we are hopeful that turtle, fish, and maybe even pterosaur remains are waiting to be discovered. In addition to the fossils themselves, a wealth of information was collected about the way the fossils were located and oriented in the quarry sediments. Stratigraphic layers around the quarry and the greater area were also examined, to connect the riverbed sediments to other exposures elsewhere in the park. This careful analysis will help will help us understand the physical environment in which these animals lived more than 200 million years ago.
Please note that fossil collecting within Petrified Forest National Park requires a permit.
Photos by Michelle Pinsdorf and Kay Behrensmeyer.