Randall Irmis, curator of paleontology at the Natural History Museum of Utah and assistant professor at the University of Utah, is investigating the rise of dinosaurs in southeastern Utah.
Friday, Sept. 7
It is often said that geological and paleontological investigations are like trying to put together a puzzle with many of the pieces missing. Yet, while we can?t travel back in time to observe every detail, the geologic record contains a rich archive of past life and its environments.
As I?ve mentioned in previous posts, we?re interested in not only in the animals and plants that lived 210 million years ago, but also their physical and climatic surroundings. The rocks we?re working in contain many clues to this past environment, if you just know where to look.
Even the color is important; in our area, the lower part of the Chinle Formation is greenish-gray, whereas the younger Chinle rocks are orange and red.
All of these sediments contain trace amounts of iron. When iron oxidizes, or rusts, it typically turns shades of red, orange or purple. In contrast, unrusted iron means the sediments stay more muted ? gray, brown and green.
Because rusting occurs when sediments are exposed to the air, it tells us that the lower part of the rock sequence was deposited in a wetter environment than the upper part.
These rocks also preserve chemical signatures that provide an even more detailed record of climate. Our team is collecting rock samples from throughout the sequence that will then be taken back to the lab for isotopic analysis. The specific isotopic values of carbon and oxygen preserved in the rock can inform us about how temperature and aridity changed through time. Similarly, we will also extract microscopic spores and pollen from these samples. The types and concentrations of these tiny plant remains will inform us about how plant communities changed through time in relation to climate.
Yet much of our geological work just requires careful observation rather than sophisticated laboratory analysis. The presence of wavy layers in sandstone indicates the sediments were deposited by a stream or river moving at slow to moderate speed, whereas layers with lots of pebbles indicate a larger and very fast river.
Putting these clues together, we can tell that the early dinosaurs and crocodilian relatives whose fossils we find lived on floodplains with rivers and streams that shifted back and forth between slow- and fast-moving currents. This environment became drier through time, and may have had annual wet and dry seasons.
Our work in the field and the lab will continue to refine this picture, so that we can compare it with records across North America. We want to know how 210-million-year-old southern Utah compares with other places where we find similar fossil animals and plants, and to determine how climate change through time affected these organisms.
This research is supported by a grant from the National Geographic Society?s Committee for Research and Exploration, and is conducted with research permits from the Bureau of Land Management and the State of Utah.
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