A FRONTIER OF FOSSILS

By Dr. Giulio Panascí, Montana State University

Cretaceous Rocks Compose

the backbone of the Rocky Mountains of North America, and they have been intensively studied for their outstanding fossil record since the 19th Century when paleontologists began to explore the western regions of this continent. The young science of paleontology, then confined to the fields of Europe, eventually grew roots in North America and became a main research target thanks to the discoveries of spectacular fossil remains. The second half of the 19th Century was marked by the great rush to unearth the bones of the “terrible lizards”: the dinosaurs. Paleontologist Othniel Charles Marsh was one of the first who pointed out the extraordinary richness of dinosaur fossils in the Rocky Mountain region stating that: “ a narrow belt of strata can be trace for several hundred miles, marked always by the bones of gigantic Dinosaurs”. Likewise, Joseph Leidy mentioned the presence of Cretaceous fossil-rich deposits extensively exposed to the west of the Wind River Range in what is now a region comprising the States of Wyoming, Idaho, and Utah. Ever since their first description, the Cretaceous rocks of North America have provided a great number of scientific contributions, revealing the history of a unique ecosystem of the past.

Cretaceous World

The Cretaceous was a long period of the Earth`s history that lasted about 80 Ma. It was characterized by warm temperatures, absence of ice caps in the polar regions, and sea levels so high that great portions of continents were submerged by water. At this time North America was a land roamed by dinosaurs, while small mammals lived their existences under the shadow of reptiles. These ancient organisms flourished until an asteroid hit the Earth about 66 Ma ago marking one of the most catastrophic mass extinction events that ever occurred on the planet. Although very different from today`s Earth, flora and fauna were nevertheless characterized by animals and plants that also survived the notorious mass extinction of the Cretaceous, becoming pivotal in the economy of modern terrestrial ecosystems. For example, Eosuchians, the ancestors of modern crocodiles, first appeared during the Cretaceous, as » CONTINUED ON PAGE 14

» CONTINUED FROM PAGE 12 well as important groups of freshwater and terrestrial turtles. The soil was colonized for the first time by insects and invertebrates capable of constructing sophisticated nests, pupation chambers, and burrow systems, which greatly contributed to the development of soils. More importantly, the Cretaceous marks an important change in the terrestrial flora with the appearance of the first flowering plants, the angiosperms, that added colors to the green landscapes of our planet.

In this world of changes, North America was indeed an interesting land to live in. The Jurassic dinosaur community that populated these lands was replaced over the Cretaceous by the arrival of newcomers. The long-necked sauropods, the spiky stegosaurians, and the sharp-toothed allosaurs were substituted by novel dinosaur groups that included the armored ankylosaurs, the horned ceratopsians, and the duck-billed ornithopods. Similarly, carnivorous dinosaurs became highly diversified and by the end of the Cretaceous some groups, that also included the ancestors of the famous Tyrannosaurus rex, become the top predators of these terrestrial ecosystems. By the end of the Cretaceous, these changes produced the iconic fauna dominated by duck-billed dinosaurs, tyrannosaurs, and triceratops famously depicted by books, paintings, and movies. But dinosaurs were not the only organisms who contributed to these changes. In fact, a variety of new crocodiles and turtle groups also participated in the reorganization of these terrestrial ecosystems.

Dark Ages On Land

Fossils strongly suggest that the ancestors of this archetypical fauna dispersed from Asia roughly around 125 and 95 Ma. As it happened for the Ice Age faunal interchanges between Asia and North America, dispersals occurred crossing a land bridge approximately located where the Bering Strait is now separating Alaska from Russia. But how did these organisms evolve after their arrival and became so successful and abundant at the end of the Cretaceous? Unfortunately, the history of Cretaceous terrestrial ecosystems in North America is still not entirely understood because the study of fossils is incomplete and often focused on specific rock formations thus limiting our perception of history in space and time. For example, fossils collected from the renowned Hell Creek and the Two Medicine Formations of Montana have provided a plethora of fossils, but their record represents only the last 16 Ma of Cretaceous history known in North America. The time the first organisms arrived from Asia until the establishment of the well-known fauna that dominated the lands of North America at the end of the Cretaceous spans a period of about 20 Ma that we still know very little about. Consequently, it is still difficult to assess how this important transition took place. This obscure period is known among paleontologists as the Middle Cretaceous (Figure 1). To give an idea of the amount of time represented by the Middle Cretaceous, we can imagine compressing 20 Ma into two centuries. Seen in this prospective, it would be as if after the French Revolution (1789), we would miss the entire 19th and 20th Centuries of human history (Figure 2). In other words, we would miss a critical period to understand our present. Similarly, the Middle Cretaceous would represent, in ecological and evolutionary terms, the temporal gap that clarifies the evolution of terrestrial ecosystems at the end of the Cretaceous period.

Over the last 20 years, great efforts have been made to implement the information on the Middle Cretaceous and numerous research projects have been carried out to understand the geology and the paleontology of these rock formations in North America. For example, in Utah and New Mexico attempts to retrieve new fossil material have provided detailed geological