Antoine Bercovici is a Peter Buck postdoctoral fellow in the Department of Paleobiology at the National Museum of Natural History. He spent most of the past summer in the field. This is his third post about his work in North Dakota on the Late Cretaceous rocks and fossils of the Hell Creek Formation.
Time-travel 66 million years back in time to explore the Late Cretaceous Hell Creek ecosystem, and you might be surprised to see a world that looks somewhat familiar: you walk across a landscape of wetlands and meandering rivers, making your way among palm trees, pines, cypress, redwoods, and other deciduous trees and flowering plants with tropical-looking leaves. The ground is not covered by grass (grasses will only evolve to form vast prairies 35 million years later). Instead, you walk on a carpet of mosses and ferns. You hear insects buzzing around your ears as you move around, frogs, and occasional bird calls. Near a pond, a lizard sunbathes peacefully on the bark of a dead tree while a salamander paddles its way across lilies and water ferns. At a distance, a crocodile watches your intrusion into its territory, eyes and nostrils raised just above the surface of the water. Then, if you are lucky (or unlucky, depending on which species!) you might encounter a dinosaur.
How do we know the Hell Creek was like this? Reconstructing an entire extinct ecosystem is no easy task for paleontologists. Fossils, trapped in rocks and sediments like time capsules, provide direct evidence of past animal and plant life, but there are a lot of other data to be assembled. The rocks and sediments, themselves, provide evidence about the environment, as does the condition of the fossils and their distribution in the modern landscape. Our group working in the Hell Creek Formation this past summer was part of a long-standing collaboration between scientists working in several disciplines within paleontology and geology.
While finding large fossils is always a thrill, the smaller ones can be more relevant when it comes to environment reconstruction: smaller animals like frogs and salamanders, are climate sensitive and tied to specific environments. After their death, the small remains of these animals may be transported by water and accumulate on river floodplains or in other places within a river system where the water slows enough for them to settle to the bottom. These ancient accumulations of small remains form what we call a microsite. Spending several hours collecting small fossils from microsites can rapidly give an image of animal diversity.
Left. Embedded in the sediment, this beautiful crocodile tooth waits to be picked up. Right. These small teeth are from little "raptor-like" theropod dinosaurs. The largest one is just 1cm (less than half an inch) long. Click photos to zoom.
Left. Our holy grail of fossil finds in the Hell Creek Formation: this rare fragment of a mammal jaw still has three molars embedded in the tooth sockets. The asteroid impact at the end of the Cretaceous ended the reign of the dinosaurs, but some mammals survived, including an ancestor of all primates, including humans. This jaw fragment is barely 1cm long. Right. These little white nodules are fossilized poop (called coprolites). They are common finds in the Hell Creek Formation. Click photos to zoom.
Sedimentology is the science that studies how and where sedimentary rocks are formed. It provides information about the existence of rivers, lakes, floodplains and soils. Taphonomy, the science of how animal and plants remains were accumulated and buried after death and how they became fossils, gives even more insight into the ancient landscapes where the fossils formed.
Left. Sediments deposited by river systems show many structures that help us understand the conditions under which they were deposited. Here, this "pipe" of cemented sandstone shows angled bedding which indicate the direction in which the river was flowing. Right. Water is a force of nature, and rivers can carry large objects in their streams. Here, this large log, petrified in ironstone, is still embedded in the river sandstone deposits. Click photos to zoom.
The animals living in this part of North Dakota today are adapted to a dry environment. They wouldn't begin to recognize the lush conditions that existed here 66 million years ago.
A rattlesnake, left, and horned lizard, right, were some of the many living animals we encountered during our weeks in the field. Click photos to zoom.
As paleontologists traveling the modern landscape in search of clues about the ancient one, we get to experience both. Visitors to the Last American Dinosaurs exhibit at the National Museum of Natural History can experience both environments, as well, through fossils, artists' reconstructions, and photos from other recent expeditions to the Hell Creek Formation.
Photos by Antoine Bercovici.
Read earlier posts about collecting fossils in the Hell Creek Formation.