by Dr. Andrew T. McDonald, Western Science Center Curator
Setting the Scene
The San Juan Basin is a place unlike any other. This expanse of badlands and steppe in northwestern New Mexico is remarkable in its natural beauty, uncompromising in its wind and sun, and harbors a rich story in its rocks.
Journeying northward through the Basin, from I-40 towards the Colorado state line, one travels up in time through a series of successively younger rock layers deposited during the Late Cretaceous Epoch. Spanning about 96 to 69 million years ago, the rocks chronicle rising and falling sea levels in the area.
During this interval, sea level was extremely high around the world – Europe was an archipelago, North Africa was a vast shallow sea, and in North America the interior of the continent was flooded by a shallow saltwater seaway that stretched from the Gulf of Mexico to the Arctic Ocean. Moving from my hometown of St. Louis, Missouri to California in the Cretaceous would have required taking a boat across Kansas and keeping a weather eye out for giant marine lizards.
Before it fully retreated from the continent about 70 million years ago, the seaway rose and fell many times during its long history, changes that are now reflected in the rock layers of the San Juan Basin. Layers of sandstone and shale containing the fossils of marine creatures represent beaches and deep water – times when sea level rose. Layers of mudstone, sandstone, and coal containing the fossils of land-based organisms represent freshwater rivers and swamps – times when sea level fell.
The Plot Thickens
Among the layers of rock in the San Juan Basin, we set out to explore the Allison Member of the Menefee Formation, which dates to around 80-81 million years ago. Staff and volunteers (and one plush mastodon – of course we didn’t leave Max behind) from the Western Science Center (WSC) were joined by our colleagues from the Zuni Dinosaur Institute for Geosciences in Springerville, Arizona; paleontology students from the University of Arizona – Tucson; and volunteers from the Southwest Paleontological Society (SPS) based in Mesa, Arizona. We operated under a permit issued by the U.S. Bureau of Land Management. This was the first expedition we undertook since I became Curator at WSC in 2017.
After weeks of telling the WSC team to prepare for extremely dry conditions, when we arrived in the field area on May 21 it was naturally raining buckets. However, the next day, the sun was blazing and we got to work. In addition to prospecting for new sites with fossil bone, our big goal was to finish excavating the partial skeleton of a ceratopsid, a large, plant-eating, horned dinosaur related to Triceratops, which was discovered in 2016. However, as so often happens in the unpredictable world of field work, a new discovery upset our best laid plans.
On May 25, we rediscovered a site that had been found by SPS volunteers in 2017. We could see pieces of two limb bones and some other fragments poking out of a mudstone layer. Nearby was another, nicely preserved limb bone with half its length exposed. The shape of this bone and its hollow interior revealed that it belongs to a group of dinosaurs called theropods, which include animals as diverse as Tyrannosaurus and other giant meat-eaters, Velociraptor and kin, and the only dinosaurs alive today – the birds. As we began to dig around the first two limb bones, we found additional bones in the same layer, all apparently from the same individual theropod. The bones were all so close to each other and jumbled one on top of the other that it was clear we could not separate them without breaking them. So, we excavated all these bones and the rock surrounding them in a single 800-pound block. After a week of digging and with a great deal of grunting and imaginative language, on June 2, ten of us hefted the block down a short slope and into the back of the WSC pickup truck.
And the Allison Member was not done yet. Even as the crew split up to dig at the ceratopsid and theropod sites simultaneously, two SPS volunteers found some armor plates and vertebrae from a huge crocodile. With every expedition we learn more about this amazing bygone ecosystem.
The Story Continues
In the end, I returned to WSC with over half a ton of fossils, including substantial portions of the ceratopsid skeleton (vertebrae, ribs, a large hip bone, and possible skull bones) and the theropod skeleton (vertebrae, a possible hip bone, and at least five limb bones); bones of other dinosaurs, crocodiles, turtles, and fish; freshwater invertebrates; and even some beautiful plant fossils. The fossils will now be freed from the remaining rock and pieced back together at WSC, a long and exacting process that will also ensure that our visitors and followers on social media will always have new discoveries to see.
An 18-day expedition is a mammoth task, and it could not have succeeded without the support of the Western Science Center and its staff and volunteers, our partners at Zuni Dinosaur Institute for Geosciences, students at University of Arizona – Tucson, and Southwest Paleontological Society volunteers. Everyone worked tirelessly together to achieve these discoveries. We will return to the Allison Member of the Menefee Formation in August, to continue exploring, finding, digging, learning, and sharing what some good geological luck has preserved of an ancient and vibrant world.