Skip to main content.

Smithsonian National Museum of Natural History
{search_item}
Image of a dinosaur

The painting of the Cretaceous wetland ecosystem.

Why do we think this depiction of a Cretaceous wetland environment is accurate? Click to zoom.

During the Early Cretaceous, the Washington, D.C. area was a swampy lowland traversed by meandering rivers and streams. The terrain probably resembled areas of Georgia's Okefenokee Swamp or the lower Mississippi River delta today. How do scientists figure out what past environments were like? The fossils and the rocks that contain them provide a lot of clues.


How do we know what the terrain was like?

Aerial photo of a wetland with a meandering river winding through it. Locations where sand and silt would be deposited are labled

Scientists learned to interpret ancient landscapes by studying the ways that water- and wind-carried sediments are deposited in modern landscapes, such as in this meandering river system. Click to zoom.

Sedimentary rocks hold clues about the ancient environments in which they formed. Geologists who study the accumulation of sediments in the present observe that fine clays build up on the bottoms of ponds and other bodies of still water and on the floodplains of meandering rivers during periods of high water. Sandy sediments, on the other hand, build up in river channels and along river banks. Because the Cretaceous fossils found in the Washington, D.C. region are embedded in claystones and fine-grained sandstones, scientists conclude that a meandering river system flowed through this area during the Cretaceous. The claystone and sandstone deposits we find today point to the positions of ponds, river channels and swamps in the ancient landscape.

Careful attention paid to the precise locations of fossils in collection sites can give scientists additional clues about the physical structure of an ecosystem. The distribution of fossils in the local Cretaceous deposits tell us that shrubs and ferns grew in open sites along river banks and around ponds, while conifer trees were dominant both in swampy forests and in drier upland forests set back from the river.


How do we know what the climate was like?

A map of the Early Cretaceous Earth showing areas of high and low rainfall.  The region around present-day Washington D.C. is relatively dry.

In this climate model of the Early Cretaceous, the density of light blue dots indicates the abundance of rainfall. The region that now encompasses Washington D.C. is predicted to have been dry. Click to zoom. Paleoclimatic reconstruction by C.R. Scotese, PALEOMAP Project

Earth was warmer during the Cretaceous than it is now. Coal beds and fossils show that lush forests grew in Antarctica and in the far north, and many kinds of warm temperature-loving animals, including dinosaurs, lived near the poles. Another indication of globally warm temperatures can be found in the shells of single-celled marine organisms called foraminiferans (FOR-am-in-IF-eh-ranz); fossils dating from the Early Cretaceous show the chemical signature of growth in ocean waters much warmer than today's seas. In the Washington, D.C. region, temperatures were probably subtropical. Climate models suggest that the area did not receive a lot of rainfall, and the local discovery of abundant fossilized charcoal confirms that conditions were, at times, dry enough for forest fires despite the swampy environment.

RELATED VIDEOS

How Do We Understand Ancient Environments?

Why do we think that some dinosaurs cared for their young?

Dr. Anna K Behrensmeyer, Curator of Vertebrate Paleontology, visits a modern wetland in search of clues about this Cretaceous environment.



Ask the Scientist

Do you have a question about dinosaurs, ancient environments or fossil collecting?

We regularly answer selected questions sent to us from our Visitors' Questions page.



Explore More

Allosaurus skull

Allosaurus skull on exhibit at the National Museum of Natural History.

Visit the Department of Paleobiology website.

Explore interactive virtual tours, watch videos, read our blogs, learn more about dinosaurs, geologic time, and much more!

[ TOP ]