When Crocodiles Ruled banner

Teacher's Guide


World Change Central has three basic concepts. Physical earth itself has changed over time; climate has changed over time; and animals and plants, through evolution, have changed throughout time.

The centerpiece of World Change Central is the World Change Theater, where visitors watch a twelve-minute show that illustrates the sweeping changes that have happened in the last sixty million years. Younger visitors can appreciate these same changes by exploring a large Walk-Through Timeline, complete with big, touchable sculptures of extinct animals.



Early paleontologists were able to find relative ages for most fossils long before complex dating methods were developed. They could claim that fish appeared before the dinosaurs, and mammals were more abundant after the dinosaurs disappeared. This is known as relative dating. Relative dating is accomplished though stratigraphy, which is the sequencing of rock layers. The Past in Layers component shows how no single place on earth tells a complete story of time. Visitors can line up rock strata from several locations to piece together a time progression as recorded in the rocks. Index fossils are also important in relative dating. Such fossils are unique to specific time periods, so their presence in a rock holds valuable clues. Mammal teeth are often good index fossils. Visitors can tell time on the Fossil Clock in the World Change Central area by using mammal teeth.

Absolute dates (an actual numerical figure) could not be established until scientists discovered radioactive decay. Visitors can find out more about the methods used in absolute dating in the World Change Central area.

Radioactive elements are atoms that change over time. Sometimes they become different forms (or isotopes) of the same element, and sometimes they become entirely different elements. A radioactive element's half-life is the time necessary for half of the atoms to make such a change. After another half-life, half of the remaining atoms will change, and so on. The original isotope is called the parent, and the breakdown product is known as the daughter. By comparing the ratio of parent to daughter isotopes, scientists can date certain rock layers. The ratio determines how many half-lives have passed since the rock was formed.

For example, say we have 32 atoms in an imaginary parent isotope. After one half-life passes, there will be equal parts of both parent and daughter isotopes (16 of each). A second half-life passes, and we are left with one-fourth the original (8 atoms) and three- fourths of the daughter isotope (24 atoms). Scientists do not actually count the atoms; they just look at the ratios. The illustration below may help.

Illustration of parent isotope and half life illustration

The tricky thing about radiometric dating is that it can only be applied to certain rocks, usually volcanic ones. Fossils are not found in such rocks, and the element isn't present in the fossil itself. Fortunately, some sedimentary rock sections contain volcanic ash layers (called bentonites) which are very widespread and are dateable. If a fossil is entombed in a rock layer with an ash layer above it and below it, a minimum and maximum age can be assigned to the fossil.

A common misconception is that scientists date all fossils using Carbon-14 dating. Carbon-14 has a half-life of only 5,730 years. Because most fossils and the rocks they are found in are so old, too many half-lives have passed for any of the parent Carbon-14 to be detected. If the isotopes are in quantities too small to measure, the method is useless. Carbon-14 dating is widely used on younger remains, especially human ones. (Unlike other dating methods, it can be applied to bones that have not been replaced with minerals). Carbon-14 dating is useful for determining dates since the Ice Ages, but little before that. In paleontology, scientists use elements with longer half-lives. The table below lists some of the elements that paleontologists use in radiometric dating.

Parent Isotope Daughter Isotope    Half-Life
Uranium 238 Lead 206    4.5 billion years
Potassium 40 Argon 40    1.5 billion years
Uranium 235 Lead 207    713 million years

The number following the element is its atomic weight, or the number of protons and neutrons the element has;e.g., Uranium 238 has 92 protons and 146 neutrons.

All radioactive dating methods have a margin of error. This is why you will occasionally see the age of a fossil followed by a symbol that indicates plus or minus several million years. The older the material being dated, the wider the margin of error. A margin of error that is a few million years is still fairly precise in terms of geological time. In order to increase the accuracy of the date, many different dating methods are applied (there are more than the ones mentioned here). If they correlate, or give the same results, the age assigned is considered reliable.

With all that said, there are simpler ways to determine the age of a fossil. Geologists have already gone through the trouble of dating many exposed rock sections in the United States (one reason why it is so important to record the location of a removed fossil). This information is published in books and maps produced by the U. S. Geological Survey, and is often common knowledge. For example, it is already known that most sediments in the Twin Cities area are Ordovician in age, so if a fossil common to that period is brought in to the museum, it's a safe guess that it's about 450 million years old, without looking at isotopic ratios. In fact, the Science Museum of Minnesota (and many other museums and universities) does not own the expensive equipment necessary for radiometric dating. If Science Museum staff were truly perplexed about the age of a fossil, they would have to collect dateable samples from the area in which it was recovered, and send them off to another lab.



Evolution, simply put, is the idea that life has changed, and continues to change over time. The process through which life changes is known as natural selection. The idea behind natural selection is that more organisms are born than will actually survive. Within any population, some organisms will have unique traits that make them more successful at surviving and reproducing. The organisms that are more likely to reproduce are more likely to pass on those successful traits to their offspring. Over time, these traits will become more frequent within the population.

Genetics plays a role in natural selection. New traits usually occur because of a mutation. Mutations are not necessarily harmful; some may have no practical effect, while others may actually be beneficial. Geneticists have found that slight mutations on one gene that controls development can change how an entire structure, like an arm or a tail, is built. A beneficial mutation is likely to become more common over generations. Accumulation of such changes over time can lead to entirely new species. This doesn't mean that all organisms become inherently "better" as time progresses. An adaptation that suits an organism in one environment could become a liability in another. Organisms adapt as best they can to the environment that surrounds them.

Visitors can learn about how various adaptations help animals to succeed in the World Change area. In the Competition Game--similar to "Hungry Hippos"--visitors use different mouthpieces to capture marbles. Certain mouthpieces are better at catching small marbles, others can get larger marbles, and some can capture marbles of all sizes. The Wannagan Yearbook panel allows visitors to check up on the animals found at Wannagan Creek and to see which ones adapted to the changing climate and are still around today. A media kiosk lets visitors listen to scientists describe how to classify species and study changes in biodiversity.



Individual species die off all around us, but their absence goes largely unnoticed (unless they happen to be cute, cuddly animals). Paleontologists define a mass extinction as a certain point in geologic time beyond which numerous groups of organisms never appear in the fossil record again. What makes extinction events so strange is that they include the apparently simultaneous extinction of many organisms, often both on land and in the sea. This does not mean they were instantaneous; very few things in geologic time are, or can be proven to be, rapid. Many extinction events happened over the course of thousands, even millions, of years. There have been several mass extinctions recorded in geologic history, some worse than the one responsible for the extinction of the dinosaurs, in terms of the number of species that went extinct. We will probably never know exactly what caused these extinctions, although changes in global sea level, which are accompanied by dramatic regional changes in temperature and climate, have aroused the most suspicion. When climates begin to change, animals must adapt or migrate elsewhere. This isn't always possible. In the World Change Area of the exhibit, visitors can test their knowledge of what survived the extinction that killed the dinosaurs and what did not at the Who Survived--And Who Died Out? component. The Extinction Times panel presents a humorous approach to extinction, in the style of a newspaper's front page.



Geographers and other scientists have always noticed how some of the earth's continents appear to "fit" together like pieces of a puzzle. Some even suggested that they once did. Fossil evidence also seemed to support this idea. Fossil flora and fauna found in countries very far apart seemed to be identical, as if they once lived on some great landmass. Near the end of the nineteenth century, Edward Seuss pointed to geologic similarities between Australia, Antarctica, Africa, and South America. He suggested they were all once part of a Southern supercontinent that he named Gondwonaland. Fossils found in these continents were also strikingly similar; for instance, the fossil plant Glossopteris was found on all four. Alfred Wegener, who is considered to be the founder of plate tectonic theory, suggested that another supercontinent Pangea, incorporated all known landmasses during the time of the dinosaurs. Although we accept this idea today, there was very little evidence to support a mechanism for moving continents during Wegener's life.

Support for moving continents really didn't gain momentum until the 1960s. The technology for mapping the seafloor was improved and geologists discovered that certain ridges on the ocean floor were actually sites of seafloor spreading. New molten rock wells up through the earth's lithosphere (or outer crust) along these trenches and pushes the old lithosphere away on either side of the ridge. Geologists figured this out by studying magnetic fields along the ridges. The basalt formed during seafloor spreading has magnetic properties that indicate the orientation of Earth's magnetic field at the time they were created. The magnetic signatures of the rock on either side of the rift matched, indicating uniform spreading.

Illustration of magnetic signatures of shifting rock
Continents, along with the seafloor, make up the lithosphere, and pieces of the lithosphere are called plates. These plates "float" on the earth's asthenosphere (the plastic-like layer underneath the lithospheric crust that actually circulates along convection currents). New rock formation along ridges pushes plates away from each other and causes them to move around. If pieces of continental crust collide, they may form mountain ranges. This is what is happening in the Himalayas today, where India is still pushing inland and causing the Tibetan Plateau to rise.
Illustration of plates colliding to form mountain ranges
Sea floor crust is denser than continental crust, so when the two collide, the seafloor is subducted, or dragged underneath, the continental crust.

Illustration of seafloor being subducted under the crust

Other plates are rifted, or torn apart, as is happening in East Africa today. Where plates come in contact with each other they are likely to cause earthquakes or the birth of volcanoes.

Illustration of plate shifting to cause earthquake

Besides clues from fossils and the shapes of continents, geologists have another way to help them determine where continents were once located: the magnetic polarity of rocks. Some mineral grains, like magnetite, have magnetic properties. When they crystallize in igneous rocks, or settle as components of a sedimentary rock, they "freeze" in directions that line up with the earth's prevailing magnetic field. Ancient rocks are found whose magnetic fields do not line up with the earth's current magnetic field. This means either the earth's magnetic poles shifted or the continents did. All the other evidence points to continental drift. By studying the direction the mineral grains in undisturbed rock point today, geologists can estimate where they once were in relation to the magnetic poles.



Climate has a lot to do with the position of the continents on the globe. As continents change, so does the circulation pattern of seawater and wind. By studying the previous locations of land masses, probable weather patterns can be reconstructed based on knowledge of climate today. By manipulating the variables involved in climate change, visitors can see how North America might have been. At the Climate Change computer interactive, visitors can see how the continent has changed, and can zero in on specific places at specific points in time.

Rocks also hold important clues. Certain rocks form only in certain conditions. Dead and decaying vegetation from ancient swamps has formed coal over millions of years. Layers of sand, silt, and mud with occasional fossil footprints or traces of ancient roots often indicate the sediments were deposited in flood plain environments. Limestone forms in warm, shallow waters, and shale forms in deeper waters. Anyone hoping to reconstruct an ancient environment must have a good understanding of rocks, minerals, soils, and the conditions under which they are created. The Rocks Can Tell Amazing Stories component helps visitors to the World Change Central area of the exhibit learn more about various rocks and how scientists see them as evidence of ancient events and environments.

Fossils give important clues to ancient environments. Certain plants and animals in the fossil record have living relatives today. We can make assumptions about the habitats of extinct animals by looking at their living relatives. For example, at Wannagan Creek the presence of organisms that cannot withstand cold or dry temperatures--such as crocodiles, amphibians, and cypress trees--indicates the climate was most likely warm and wet. Fossil leaves are used frequently in reconstructing climates. Large, broad leaves with smooth edges are common in tropical areas, whereas leaves with jagged edges usually indicate more seasonal temperatures. What kind of leaves might you expect to find at Wannagan Creek?



The Wannagan Creek fossils are from the Paleocene Epoch, which was the first epoch of the Cenozoic Era. The Cenozoic Era is commonly known as the "Age of Mammals." The very end of the era is often called the "Ice Age" (although there were certainly other times in Earth's history when glaciations occurred). With the extinction of the dinosaurs, there was a window of opportunity for other animals to move into the various niches left vacant. Mammals and birds filled these ecological roles as herbivores, carnivores, omnivores, and scavengers. Flowering plants and insects continued to evolve and colonize new niches.

The first epoch of the Cenozoic was the Paleocene. The climate was warm, humid, and subtropical-- similar to Louisiana or Florida today. Many types of small squirrel-like and rat-like mammals came to North America from Europe. A few small carnivores and some primitive meat-eaters called creodonts threatened the tiny mammals, but that was about it for terrestrial predators. The creodonts were different from modern carnivores because their carnassial (slicing) teeth were placed further back in the jaw than they are in modern carnivores, such as cats and dogs. The first rodents and the earliest primates also appeared in the Paleocene, along with many other obscure mammals that have no living ancestors today.

During the following epoch, the Eocene, the climate in North America was briefly tropical, which encouraged the growth of forests. These forests were home to a variety of browsing mammals. Many new families of mammals appeared and existing families produced larger species. True carnivores became more common. Alongside them were some of the first ungulates (animals with hooves). More species lived in North America during the Eocene than any time since.

Western North America contains abundant fossil mammals from the Oligocene Epoch. The climate in the early Oligocene was warm, but dryer and less tropical than the middle Eocene. Fauna in Europe and North America were fairly similar, but starting to show divergence. There were plenty of carnivores around, such as primitive saber-toothed cats. The larger carnivores of the Oligocene had larger prey animals to pursue as well. There were several primitive horses, like the three-toed Mesohippus. Rhinos roamed North America, as did other types of hoofed animals called titanotheres. These creatures resembled rhinos, and their heads were adorned with strange horns. Some were the size of a small elephant. Other types of hoofed animals related to modern deer, sheep, and antelope became more common in North America during the Oligocene.

titanotheres illustration

In the Miocene Epoch, temperatures became more seasonal. Most of the archaic types of animals had become extinct, and almost all modern families of animals were well established. One of the most important new things to evolve in the Miocene was grass. It is hard to imagine a world without grass, but until the Miocene, the ground was covered with bushes, herbs, and other plants. Grasses, which can recover after having been grazed, had an advantage over other sorts of ground cover and proliferated quickly. Grass has silica within its cell walls, which makes it tough on teeth. However, animals evolved in tandem with grasses, developing both teeth with high crowns and elaborate digestive systems. Animals that were able to chew and digest grass became very abundant during the Miocene. Examples include horses, primitive deer and cattle, as well as relatives of the pronghorn antelope.

The Pliocene was a very short epoch. The Isthmus of Panama formed, leading to the migration of many species. Some South American species, like the sloth and the armadillo relatives, moved north. Many more North American species moved south, devastating the native populations of marsupials and other strange beasts that had been evolving in isolation for millions of years. Another important event in the Pliocene was the evolution of the australopithecines; these were probably our earliest biped ancestors (they walked on two feet). The specimen known as "Lucy" is one of the most famous australopithecine fossils. The following epoch, the Pleistocene, is best known for the Ice Ages. Major continental glaciation began in the Pleistocene.

Temperatures during the Pleistocene Epoch were much cooler. There were four major intervals of glaciation during this time, with somewhat warmer climates intervening. (Many scientists believe that we are currently living during one of these warmer, interglacial periods.) As the glaciers grew, sea level dropped, because so much water was incorporated into the ice. Sea level drops exposed land bridges and allowed animals to migrate to new areas. Some mammals migrated between North and South America, and it is presumed that a land bridge from Asia to Alaska encouraged the migration of other species, including rodents, rabbits, and humans.

The Pleistocene was an epoch of very large mammals. There were mammoths and mastodons, along with the giant ground sloth, large cave bears, and deer with antlers three to five meters across. The genus Homo, to which the human race belongs, also appeared at this time.

Near the end of the last glacial interval (18,000-10,000 years ago) the large mammals began to disappear. Their extinction may have been caused by the changing climate, but it is difficult to ignore the fact that the appearance of advanced, tool making hominids coincided with their disappearances. Our ancestors may have had a role to play in the extinction of the large mammals of the Pleistocene.



No one really knows for sure. In fact, it is not entirely clear what caused the extinction of the dinosaurs and some of their contemporaries. The most popular explanation for this extinction is that a comet or asteroid slammed into the Yucatan Peninsula 65 million years ago. In addition to the immediate destruction caused (impact, wildfires, and tsunamis), such an impact would have thrown a huge amount of dust and debris into the atmosphere, which would block out the sun for an extended period of time and prevent photosynthesis. This would cause the death of many plants, and in turn, the animals that relied on them. Although there is geologic evidence that such an impact did occur, it does not answer all the questions surrounding the extinction event. There may have been other, multiple causes for the extinctions, which are difficult to identify. In fact, some paleontologists believe that dinosaurs were already scarce and disappearing prior to any impact. Changes in climate and mountain building events may have fragmented species populations and made it difficult for them to survive.

If we assume a large impact was the main cause of death, we can explain why some mammals survived. Most mammals from the time of the dinosaurs were burrowing, nocturnal creatures. They could have avoided some of the catastrophe by hiding underground. Being nocturnal, the absence of light would not have had such a dramatic effect on them. They could have survived longer by scavenging on all the dead creatures above.

But what about creatures like amphibians, which are extremely sensitive to environmental pollution, and crocodiles and turtles, whose eggs are sensitive to slight temperature changes? That is where the explanation starts to fall apart. Although some attempts have been made to explain this predicament, none have done so satisfactorily.


Immerse yourself in a subtropical habitat as you explore three detailed dioramas: environments with heavy vegetation, pools of water, and animals. Some plants and animals may seem familiar: magnolias, crocodiles, fish, and turtles. Other species--squirrel-like and dog-like animals--you've probably never seen before.

Just beyond the lush perimeter of the dioramas is an array of skeletons and specimens of the habitat's plants and animals. Panels feature interviews with scientists, and comparisons with modern animals help you understand how this extinct ecosystem was re-created. Interactive exhibits invite you to learn more about how these animals lived.

Teacher's Guide | After the Dinosaurs | Exhibits