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Teacher's Guide

WHAT IS PALEONTOLOGY AND GEOLOGIC TIME?

Paleontology is the study of ancient life (life that is typically older than 10,000 years). Paleontologists study fossils and the rock layers in which they are found. Fossils are the remains or traces of ancient life. A fossil can be a seed from an ancient plant, the bone of an animal, or even prehistoric footprints or feces. Fossils are most commonly preserved in sedimentary rocks (rocks that are formed from the eroded grains of older rocks or minerals). Some of the most common sedimentary rocks are sandstone, shale, and limestone. In the Fossil or Not? component, visitors view a variety of fossils and non-fossils and see if they can determine the difference.

Geologic time refers to the length of time the earth has existed. It is a scale used internationally as a reference to certain key events in the history of the planet. Scientists currently believe that the earth formed between 4.5 and 5 billion years ago. As more knowledge is gained, the story of how the earth actually formed is revised and rewritten. It is important to recognize that geologic time represents such an enormous span that it is unobservable with conventional methods.

In order to organize events in Earth's history, scientists have broken down geologic time based on characteristic rocks and fossils. No one species has existed since the beginning of life on Earth. Therefore, different fossils are present in rock layers of different ages. Long before scientists were able to use radiometric techniques to determine the absolute age of fossils, they noticed something helpful in discerning the relative age of fossils. Rock layers in their original, horizontal position (meaning processes like mountain building have not overturned them) can be put in sequence. The youngest rocks and fossils are those on top. As one moves down through rock layers, they get progressively older. You can envision the rock layers, or strata, as parts of a layer cake, in which the bottom is made first. This is known as the law of superposition. It sounds elementary, but it was a crucial concept to the earliest paleontologists who worked to determine the relative succession of different life forms on earth. Without actually having dates for the ages of certain fossils, they divided up geologic time into categories starting with Eons, which are further subdivided into Eras, Periods, Epochs, and even finer divisions. Visitors to the Field Camp area can learn about stratigraphy, or how a time progression is built from rock sequences. Look through the Stratigraphy Viewmaster to understand what a paleontologist sees while looking at layers of rock.

 

HOW ARE FOSSILS FORMED?

Not everything that dies becomes a fossil. In fact, fossilization is a rare exception to the normal postmortem processes, and we estimate that far less than 0.1 percent of the earth's former inhabitants are preserved at all. Most animals die and are scavenged by other animals or decomposed by bacteria and other microorganisms. Exposed remains are also left to the weathering elements of temperature, wind, and water. At What Becomes a Fossil?, visitors look at illustrations of several pairs of organisms, and try to decide which ones will become fossils. Many unique conditions play a role in determining whether an organism becomes a fossil. Listed below are a few of these conditions:

1) Rapid Burial
If an organism is buried very soon after death, it might avoid the effects of weathering, scavengers, and decay. How something becomes quickly buried is dependent on location. Animals that live in or near water are much more likely to be preserved. This is because sediments settling down through the water sometimes bury them. An animal might also be buried in an ash flow from a volcano or become trapped in muddy or swampy sediments.

2) Dying in a place devoid of oxygen
Many decomposers thrive on oxygen. Therefore, a lack of oxygen delays or inhibits decay. Anoxic (oxygen deprived) conditions can be found in waters with poor circulation, such as stagnant lake bottoms, or bogs. Sometimes the excessive growth of microorganisms, like an algal bloom, can deplete the dissolved oxygen in a body of water.

3) Having Hard Parts
The soft parts of organisms decay or are rapidly eaten by scavengers. But animals with bones, teeth, or shells are more common in the fossil record, since these parts can endure the elements longer. Only in unusual circumstances are soft-bodied animals or plants preserved.

METHODS OF PRESERVATION

Fossils most typically form from a process called permineralization (see below). But there are a few other unique ways in which fossils can form.

1) Actual, original remains
Complete preservation of an entire organism is very rare, but it can happen. For example, some Ice Age mammals, such as mammoths, have been found frozen in ice. No animals older than these have been found preserved this way, since temperatures have changed so much over geologic time. Small creatures, like insects, or plant remains, may become trapped in tree resin that hardens to form amber.

2) Replacement or permineralization
Often times the remains of organisms are "petrified", or become like stone. This occurs when ground water deposits minerals (that are transported in solution) inside the fossil itself. This can happen to various degrees. Sometimes, with bones, the original mineral content of the bone itself (hydroxyapatite) remains, and only the organic parts (collagen and water) are replaced. In petrified wood or plants, the structure of the cell walls can sometimes remain, and minerals fill in everything else.

3) Carbonization
In very fine-grained rocks, the liquid or gaseous components of the organic material may be driven off, leaving a thin film of organic carbon, which gives a delicate representation of the original organism. Carbonized fossils often reveal fine details, or even the soft parts of animals.

4) Natural molds and casts
A shell or other hard part of an organism, which has been buried in sediments, may be later dissolved by percolating water, leaving behind a mold or space in the rock bearing the impression of the organism. Later filling of the cavity by other sediments or minerals forms a cast. You can imagine the mold itself as a "Jell-O" mold, and the cast as the "Jell-O" itself.

 

WHAT IS WANNAGAN CREEK? HOW WAS IT DISCOVERED AND WHAT HAS THE SCIENCE MUSEUM FOUND THERE?

Wannagan Creek is the name of the site in western North Dakota (Billings County) where most of the fossil specimens in the When Crocodiles Ruled exhibit were recovered. Fossil crocodiles, turtles, fish, and other animals were all buried in a layer of sandstone and siltstone known as the Bullion Creek Formation. Wannagan Creek animals lived, died, and were buried during the Paleocene Epoch, about 60 million years ago.

The Science Museum of Minnesota's paleontology department first became aware of the Wannagan Creek fossils when the University of Minnesota gave several crocodile bones to museum curator Bruce Erickson for study. Evelyn Sheldrup brought the bones to the University. Evelyn's sister-in-law, Jean Adams, owned ranchland on the fossil site and had been finding fossil bones there since she was a child.

Not all potential field sites turn out to be success stories like Wannagan Creek. Paleontology curator Bruce Erickson had a hunch that there might be some good fossil finds at this site worth looking into. Among the fossil bones Evelyn Sheldrup brought in were six crocodilian occipital condyles. Occipital condyles are bones on the back of the head of a crocodile that connect with the first vertebra of the neck, also known as the atlas. Since an individual crocodile only has one occipital condyle, Erickson knew that he was looking at the remains of at least six crocodiles. Wannagan Creek seemed like a place worth visiting.

Test excavations in 1970 revealed many interesting fossils. There were fragments of fossil birds, mammals, turtles, and fish exposed on the surface of the site. It looked as though there was a lot of good fossil material to be discovered, but no one could have guessed how much would be found in the years to come. Curator Bruce Erickson originally thought the quarry would be good for only a few years, or a couple of field seasons. But just one shallow quarry kept the Science Museum staff busy for more than twenty years! Other nearby sites were examined, but none turned out to be worth excavating.

Excavations took place on the Adams' land, as well as on Theodore Roosevelt National Park; land that belongs to Department of the Interior. The Science Museum of Minnesota received permission to excavate there and was obligated to clean up the excavation sites when finished. It would be illegal for anyone without a special permit to remove fossils from the park. The Can I Collect That? component outlines the rules of fossil collecting. Although the ground at Wannagan Creek still holds more fossils to discover, the Science Museum of Minnesota will probably never excavate there again. Following the excavation in the summer of 1999, the open quarries were filled back in again, smoothed over, and seeded with prairie grasses (to restore parkland and prevent others from disturbing the site).

After more than twenty years of research there the Science Museum of Minnesota has collected fossils representing about 100 species. These include crocodiles, alligators, champsosaurs (swimming reptiles), turtles, fish, amphibians, birds, insects, and plants. The top predator at the site was definitely Leidyosuchus formidabilis. This large crocodile, which grew up to fifteen feet long, was found in abundance. In fact, about eighty individuals were found in rock just a few feet thick! Although crocodiles are the stars of this show, there are perhaps four species and one genus of animal discovered at Wannagan Creek that were previously unknown to science. Most of these fossils are stored in the museum's collection vault and are available for visiting scientists to study. Visitors can enter the Camp Tent in the Field Camp area of the exhibit and learn more about how Wannagan Creek was discovered and what the Science Museum's field crew experienced while working there.

 

HOW WERE THE FOSSILS LOCATED AND BROUGHT BACK TO THE MUSEUM?

Surface prospecting is one of the best ways to locate a good place to start digging. As surface features are eroded by weather, fossils are revealed. Broken bits and pieces of bone eroding out of the ground often lead to a larger fossil still protected by the earth surrounding it. The Science Museum's crew didn't always rely on such delicate measures, however. A bulldozer was brought in to the field site to help remove the topsoil. Over the course of many field seasons, more than 4,000 cubic yards of shale and mudstone were removed, resulting in a cliff face about eighteen feet deep. Only about three feet of the cliff face yielded valuable fossils.

Large fossil bones need to be encased in a protective covering known as a field jacket. To make field jackets, the field crew spreads wet newspaper over the top of the fossil. Next, plaster-soaked strips of burlap are applied on top of the newspaper. When the top side is dry, the fossil is turned over, and the process is repeated. The resulting field jacket protects the fossil like a cast on a broken arm. The Pack it in a Jacket component tells the story of the origins of the field jacket and shows the steps used to make one. Visitors can examine a field jacket in the Field Camp area. Once the protected fossil arrives at the museum, volunteers remove the top of the field jacket and work to clear the surrounding matrix (rock and dirt) away from it with small tools such as dental picks and toothbrushes. They stabilize fragile bits of the fossil with hardener as they go, and glue together any fragments they find that fit.

Locating microfossils, such as tiny mammal teeth or pollen, requires the use of a sieve. But because searching for microfossils can be so labor intensive, it is often done at the museum. Members of the field crew bring bags of matrix back to the museum, where they can be sifted through later. Volunteers removing matrix from larger field jackets also keep a sharp watch out for any microfossils. Visitors to the Field Camp area can view real microfossils through magnifiers at Tiny Fossils Add to the Big Picture and look for microfossils by sifting through matrix.

Visitors can also learn about the various tools used in the field with the Paleo Toolkit media interactive in the Field Camp area. Visitors get to pick a field site, clothing, and equipment to bring along. If they make appropriate choices, their character will get to dig up bones or microfossils.

 

WHAT HAVE THE FOSSILS OF WANNAGAN CREEK "TOLD" US ABOUT ANCIENT NORTH DAKOTA?

At this time, not many scientists know about Wannagan Creek. Over 100 Paleocene sites throughout Wyoming, Utah, New Mexico, Colorado, and Montana also have yielded fossils. One of the goals of this exhibit is to make the sites in North Dakota better known to the world.

Seventy million years ago, Wannagan Creek was underneath a huge inland sea that cut through the center of the United States. As the sea retreated, lakes and rivers left many sediments behind that tell the geologic history of the region. The climate of the United States was warmer and less seasonal than it is today. Parts of the western states, including North Dakota, were basically subtropical, like South America is today. The sediments of Wannagan Creek are primarily sand, silt, and shale, with sediment bands of different colors. The dark bands indicate times when there was dense vegetation in the area (these bands are lignite or coal rich). They were formed in swampy, oxygen-deprived conditions. Lighter colored silty layers are sediments that were deposited on wet flood plains. Sediments and rocks such as these are crucial to interpreting the story of Wannagan Creek.

Animal remains found at Wannagan Creek, such as those of crocodiles, tell us that the temperature was fairly warm year-round and that there was an abundance of water. Much of this environment may have been swampy, as suggested by the findings of fossil cypresses, descendents of which grow in swampy regions today. The presence of amphibians also indicates reliable year-round water, since amphibians cannot reproduce without it. Although a swamp-like environment is clearly represented by the fossils, there was also a fresh water lake at Wannagan Creek, as indicated by the presence of fish such as gar, pike, and bowfins. A forest understory nearby probably provided shelter to the small fossil mammals that were found at Wannagan Creek, as well as dry land for the reptiles to lay their eggs on. Three areas in the diorama display represent these findings: the crocodile pool, the lake, and the forest understory. Although they are separate dioramas in the exhibit, they were not discrete environments at Wannagan Creek. For example, crocodiles probably lived in the lake, swampy pools, and the forest understory.

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