Antoine Bercovici is a Peter Buck postdoctoral fellow in the Department of Paleobiology at the National Museum of Natural History. He spent most of July and August in the field in North Dakota and Montana. This is his first post about the trip.
This summer I continued working with a large multidisciplinary team on a major endeavor begun last year: mapping with great precision all known fossil sites in Southwestern North Dakota and Southeastern Montana relative to the Cretaceous-Tertiary boundary, the rock layer that holds residue from the asteroid impact that wiped out the land-dwelling dinosaurs and many other forms of life 66 million years ago. By assembling this map, we will be able to track the evolution of terrestrial ecosystems through the time of the last dinosaurs. Spearheading this project are four individuals, each providing a piece of the puzzle: Tyler Lyson, from the Denver Museum of Nature and Science, has found many dinosaur skeletons in the area; Dean Pearson from the Pioneer Trails Regional Museum (Bowman ND), has numerous sites with micro vertebrates (small fossils representing crocodiles, turtles, lizards, salamanders, snakes, birds, fishes, and mammals); Kirk Johnson, Director of the Smithsonian’s National Museum of Natural History and a paleobotanist, has numerous fossil leaf localities; I provide data on fossil pollen and stratigraphy which help us understand environmental conditions at each site and when the fossils formed. Tying all of our work together is Erica Evans, our GPS mapping wizard. Many students and volunteers are assisting us in the field.
Much of the rock in this area is part of the Hell Creek Formation, made up of sediments deposited during the end of the Cretaceous and earliest Paleogene. It is extremely fossiliferous, and as erosion and weathering carve the soft sediments, bones are left exposed on the ground. If you know what to look for, a walk in the badlands usually yields numerous fossil discoveries.
In the early 1980’s, a scientific breakthrough that would spark a new level of interest in the Hell Creek Formation was published in Science by geophysicists Walter Alvarez and his father, Luis. They discovered an anomalously high concentration of iridium at the Cretaceous–Paleogene boundary. Because iridium is an element very rare in the Earth’s crust and relatively abundant in extraterrestrial bodies such as asteroids, they proposed that the demise of the non-avian dinosaurs at the end of the Cretaceous was linked to a large meteorite impact. Testing this hypothesis required good fossiliferous rocks spanning the Cretaceous–Paleogene boundary, like the Hell Creek Formation, to document how animal life responded to the impact. At that time, it was still a mystery as to how plants responded to the Cretaceous–Paleogene mass extinction event, but the Alvarez publication pushed a then-aspiring PhD student from Yale, Kirk Johnson, to go to North Dakota to test the extinction hypothesis from the plant point of view.
Fossil leaves are different than fossil bones. They are two dimensional objects that preserve as fine carbon imprints within sedimentary rock strata, while bones are three dimensional and harden in the fossilization process due to mineral replacement. Erosion and weathering destroy leaf fossils, so prospecting for leaf sites cannot be done by casually looking for fossils exposed on the ground. Because of this, the Hell Creek Formation, which had been prospected since the 19th Century starting with vertebrate paleontology pioneers such as Edward Cope, Othniel Marsh and Barnum Brown, had only sporadically yielded the remains of fossil plants and leaves.
Kirk Johnson's approach to finding plant sites was to use geology to identify the right sedimentary rock with the potential to preserve fossil leaves and then wield a pick axe and rock hammer to excavate and split large slabs of rocks to reveal the leaves trapped within.
Pick axe prospecting allowed him to find hundreds of fossil plant localities in the Hell Creek Formation. Slowly building up a large dataset, he was able to observe changes in floral composition and diversity throughout the Hell Creek Formation associated with changes in climate. But most importantly, he saw an abrupt and drastic change at the Cretaceous–Paleogene boundary. From a diversity of over 100 species in many Cretaceous plant sites, most of the earliest Paleocene sites yield no more than 4-5 species. Plants are essential in supporting terrestrial ecosystems, forming the bases of food chains and providing shelter for many animals. The sudden and drastic reduction in plant diversity observed in association with the asteroid impact shows that the essential foundations of the food chain were affected. A chain reaction affecting all animal life depending on it must have followed.
Incidentally, finding numerous leaves opened a window onto another group of animals that is poorly know due to poor preservation potential: insects. Insects interact with leaves in various ways including feeding, galling, laying eggs, and drilling mines, and evidence of these behaviors can be found in fossil leaves. Using this evidence, Conrad Labandeira from the Smithsonian Institution was able to draft a detailed pattern of insect diversity in the Hell Creek Formation and throughout the Cretaceous–Paleogene boundary. The fossil leaf shown at left, from the Pioneer Trails Regional Museum collection in Bowman, North Dakota, has holes left by an insect that fed on it when the leaf was alive.
Sometimes, parts of plants can also be fossilized as three dimensional objects. It is the case for seeds, pine cones, and petrified wood.
To collect fossils for my research, I also dig into the ground to expose and collect unweathered rock, but the preserved pollen is microscopic and will not become visible until the rock is processed back in the lab. Analyzing the samples will occupy me through the coming winter. Learn more about collecting fossil pollen from this video interview recorded this summer at one of my excavation sites.
Click here to learn more about the different types of fossils recovered in the Hell Creek Formation during last year’s field season.
All photos by Antoine Bercovici except the panorama, which was photographed by John Hankla.