Skip to main content.

Smithsonian National Museum of Natural History
Illustration of ancient flora

How do scientists learn about fossils, extinct animals and plants, and ancient ecosystems? As you explore this website, you will find links to the questions listed below. If you prefer to view the answers here, scroll down and click.

Studying Fossils

  1. How do paleontologists identify dinosaur teeth? (VIDEO)

  2. Photo of a jaw fragment with five teeth.  Each tooht has several tall, pointed cusps arranged in a line.

    Many mammal species have very distinctive tooth shapes. Click to zoom.

    The discovery of even a single mammal tooth can be exceptionally useful to scientists trying to identify the animals that lived in an ancient ecosystem. Because the shape of the crown (the part of the tooth above the gum) varies a lot between mammal species, it is often possible to know the exact species a fossil mammal tooth came from even if no other teeth or bones are found with it. This level of identification is usually impossible for isolated dinosaur teeth, which do not have such complex shapes.

  3. It is extremely uncommon for an animal's fossilized bones to be preserved along with its final footprints, and this makes it challenging to identify the exact species of animal that produced any particular print. However, scientists can narrow down the identities of dinosaur trackmakers by comparing the foot shape, pattern of movement and evidence of body size revealed by the tracks with what is known about dinosaur species living at the time the track was made. A small dinosaur running on two legs, for example, will leave very different prints than a large dinosaur moving slowly on four legs.

    Fossil tracks of a nodosaur and a theropod.  The theropod print has  impressions left by three long toes.  The nodosaur print is rounder and shows the positions of five stubby digits.  Sketches show the positions of the bones in each foot.

    Nodosaur and theropod footprints (left, top and bottom) and drawings of their foot bones (right). Click to zoom.

    Several lines of evidence tell us that the print shown on the top row if this image was made by the front foot of an ankylosaur. The print has five blunt digits ("fingers") arranged in an arch, and behind them there is a faint impression of the "palm". The arrangement of hand bones and hoofed fingers in ankylosaurs, which we know from skeletons, could have created a print that looks like this. In addition, similar prints have been discovered in other places where ankylosaur bones have been found. Other kinds of dinosaurs had fewer digits, or claws instead of hooves, and their prints would reflect that. The bottom row of this image shows a theropod track and an outline of theropod foot bones for comparison.

    Narrowing the identity down further, it is likely that the ankylosaur print was made by a nodosaur because this is the only kind of ankylosaur known from these deposits, and they are the most common Early Cretaceous ankylosaurs worldwide. The print is small, suggesting it came from a young animal.

  4. Bone fragment of an ornithomimid dinosaur.

    Fossil bone fragments can be hard to identify, but if certain diagnostic features are present on a bone, it may be possible to determine the type of dinosaur it belonged to. The process is similar to the identification of fossil teeth, shown in the video.

  5. CT scan of Arundelconodon jaw. The computer image combines many separate images taken as the scanner focussed downward through the jaw.

    CT scan of the Arundelconodon jaw. Click to zoom. Image courtesy of

    CT scan of the skull of the turtle Arundelemys.

    CT scan of the skull of the turtle Arundelemys. Click to zoom. Image courtesy of

    Industrial, high Resolution X-ray CT scanners can be used to create detailed 3-D images of fossils. The scanners operate similarly to the medical CT scanners found in hospitals, but generate a more intense X-ray bombardment that is capable of penetrating the fossils. The X-ray "slices" through the fossils, allowing scientists to view and study them from the inside. Two Cretaceous fossils from the Maryland have been scanned at the Digital Morphology facility at the University of Texas at Austin.

    Visit the DigiMorph website to view high resolution CT scans of the turtle skull, including rotating 3 dimensional images.

    Visit the DigiMorph website to view high resolution CT scans of the mammal jaw.

    View a video story from the National Science Foundation to learn more about the DigiMorph scanning process.

  6. Fossil branches and cones of a conifer. Both are embedded in rock.  The branches are covered with short needles.  The cones look like flattened spheres.

    Fossil foliage and cones of a Cretaceous "Sequoia." Click to zoom.

    Foliage of a modern sequoia tree.  Short needles cover the branches.

    Living Sequoia foliage. Click to zoom. National Park Service photo.

    In the late 1800's when many Cretaceous plant fossils were first being described, paleobotanists named species based on observations of overall similarity. This fossil foliage, for example, which looks similar to that of modern sequoia (Sequoia sempervirens, the coast redwood of California), was named as a new species in the same genus, Sequoia ambigua.

    Today paleobotanists are more cautious when they name fossil plants. They know that distantly related plants can look generally similar, and that even the best fossils preserve only some of the features that botanists consider when naming and classifying living species. If the Sequoia ambigua fossils were being described for the first time today, they probably would not be placed in the genus Sequoia, either because of small differences between the fossils and living Sequoia, or out of caution that the few features visible in the fossils don't provide enough information to place them in Sequoia.

    As of this writing (2011), no one has published a scientific paper renaming these fossils, so Sequoia ambigua remains the correct scientific name. We have put quotation marks around the generic name, "Sequoia" to indicate we are skeptical that these fossils really belong in the living genus for redwood.

  7. Scientists combine several well-tested techniques to find out the ages of fossils. The most important are Relative Dating, in which fossils and layers of rock are placed in order from older to younger, and Radiometric Dating, which allows the actual ages of certain types of rock to be calculated.

    Relative Dating. Fossils are found in sedimentary rocks that formed when eroded sediments piled up in low-lying places such as river flood plains, lake bottoms or ocean floors. Sedimentary rock typically is layered, with the layers derived from different periods of sediment accumulation. Almost any place where the forces of erosion - or road crews - have carved through sedimentary rock is a good place to look for rock layers stacked up in the exposed rock face.

    A man stands in front of a vertical rock face that is taller than he is.  The rock is layered, and the layers can be seen as horizontal stripes of slightly different colored rock.

    These rock layers formed from sediments deposited in a lake. Click to zoom. Photo courtesy of Rod Benson,

    When you look at a layer cake, you know that the layer at the bottom was the first one the baker put on the plate, and the upper ones were added later. In the same way, geologists figure out the relative ages of fossils and sedimentary rock layers; rock layers, and the fossils they contain, toward the bottom of a stack of sediments are older than those found higher in the stack.

    Radiometric Dating. Until the middle of the last century, "older" or "younger" was the best scientists could do when assigning ages to fossils. There was no way to calculate an "absolute" age (in years) for any fossil or rock layer. But after scientists learned that the nuclear decay of radioactive elements takes place at a predictable rate, they realized that the traces of radioactive elements present in certain types of rock, such as hardened lava and tuff (formed from compacted volcanic ash), could be analyzed chemically to determine the ages, in years, of those rocks.

    Putting Relative and Radiometric Dating Together. Once it was possible to measure the ages of volcanic layers in a stack of sedimentary rock, the entire sequence could be pinned to the absolute time scale. In the Wyoming landscape shown below left, for example, the gray ash layer was found to be 73 million years old. This means that fossils in rock layers below the tuff are older than 73 million years, and those above the tuff are younger. Fossils found embedded within the ash, including the fossil leaves shown below right, are the same age as the ash: 73 million years old.

    A photo, taken from a distance, of a cliff-like exposure of rock in which you can see different colors of rock arranged in horizontal layers. The colors range include different shades of tan, red and gray.  A thick layer of gray volcanic ash is labeled on the photo.

    Ash layers from ancient volcanic eruptions are found in many sedimentary rock deposits. Click to zoom.

    A small branch with three lobed brown leaf fossils is visible on the surface of a slab of light gray rock that is made up of comrpressed volcanic ash.

    Fossil leaves embedded in ash. Click to zoom.

Understanding Extinct Animals

  1. A Deinonychus skeleton posed in running position.  It is on its left foot, right leg extended forward. The arms are positioned in front, hands together, and the tail extends straight out behind.

    Skeleton of the dromaeosaur Deinonychusat the Field Museum. Click to zoom. Photo posted on Wikipedia by Scott Anselmo.

    Stance: There are only a few teeth and bone fragments from this site and we don't know what exact species of dromaeosaur they are from, so assembling a life-like image might seem impossible. But scientists studying the skeletons of different dromaeosaur species have learned that the bones of all dromaeosaurs are quite similar. This allowed the artist to use a "typical" dromaeosaur skeleton, such as the one in this photo, as the basis for the reconstruction.

    This skeleton holds several important clues to stance: Scientists determined that the arm and wrist bones allowed only certain motions, so they know the hands were held palms-inward. The arms are much shorter than the legs and end in thin claws, showing that it stood on the hind legs only. The end of the tail was stiffened by long bony rods, so it was not very flexible. The huge claw on the inside toe was too big to rest on the ground, and is shown held up high.

    Feathers and colors: Every dromaeosaur specimen ever found in sediment fine enough to preserve soft tissue impressions has shown evidence of feathers. This is why the dromaeosaur was painted with feathers. The colors of the feathers were chosen by the artist, Mary Parrish, who prefers to color her reconstructions with a conservative palette.

  2. How do we know how extinct animals moved? (VIDEO)
  3. How do we know that some dinosaurs cared for their young? (VIDEO)
  4. How do we know what dinosaurs and other extinct animals ate? (VIDEO)

  5. Painting of pterosaurs in flight

    Pterosaurs in flight. Click to zoom.

    The main clue we have that pterosaurs flew, rather than glided, is that their bones had enlarged attachment sites for the muscles that, in birds, control flight. Scientist interpret this as meaning that the muscles were large and powerful in pterosaurs, a characteristic unlikely to have evolved unless they were capable of active flight. Another characteristic that pterosaurs shared with birds is hollow bones with internal struts. This adaptation increases bone strength while reducing weight, and is regarded as critical for the evolution of flight. Light, strong bones would have been especially important in pterosaurs, given the enormous sizes attained by some species.

  6. The ornithomimid dinosaur is depicted with a beak

    The ornithomimid is shown with a beak. Click to zoom.

    These hadrosaur and chicken skulls show similar alterations to the bone texture in the area where the beaks grew

    Skulls of a hadrosaur,a type of dinosaur (left), and a modern chicken (right). Click to zoom.

    Beaks are made of keratin, like claws or fingernails, and do not fossilize often, but the bones they are attached to do fossilize, preserving evidence of the beak. In many plant-eating dinosaurs, the front of the jaw has a different texture than the rest of the skull, and has many small openings for blood vessels and nerves. This structure is typical of the jaws of of living beaked animals. It provides a rich blood supply to the beak, allowing it to grow with the animal. The areas highlighted in the photos are the beak attachment sites of a hadrosaur dinosaur, left, and a chicken, right.

  7. A hadrosaur skull. Compared with the surrounding bone, the parts of the jaw where the teeth grew are set in toward the center of the skull.  This creates the same kind of gap you feel on your face if you press against your cheek bones and lower jaw, then slide your fingers inward to feel your teeth.

    The teeth of this hadrosaur are set inward from the side of the skull. Click to zoom.

    Most reptiles do not have cheeks, and this limits how much food they can chew before swallowing. Without cheeks, food tends to fall off the teeth and out of the mouth - very counterproductive! Cheeks help animals to keep food in place while they chew it. Because well-chewed food is easier to digest it supplies more nutrients. Cheeks are made of soft flesh, which typically doesn't fossilize, but scientists believe that indirect evidence of cheeks can be found in the shape and arrangement of the skull and jaw in many plant-eating dinosaurs. You can see that the teeth of this hadrosaur, for example, are set in from the side of the skull and jaw, creating a gap that might have been covered by cheeks.

  8. Painting of baby Propanoplosaurs, showing armor.

    The artist's reconstruction of the baby nodosaru includes armor typical of adults. Click to zoom.

    The fossil of the baby Propanoplosaurus has bones, but no evidence of armor.

    Baby Propanoplosaurus fossil with a sketch of the probable position in death. The fossil shows no evidence of armor. Click to zoom.

    All known nodosaurs were armored, but the impression fossil of the Propanoplosaurus baby doesn't appear to have any armor. What's going on? The hard, bony armor of nodosaurs was readily preserved and has been found associated with the bones of adults of all species, so the lack of armor on the baby's skeleton is surprising. Scientists have two hypotheses. It could be that the skeleton of the dead baby nodosaur slipped out of its armored skin before the skeletal impression formed. It is also possible that nodosaurs were born without armor and developed it as they grew. We know that other types of dinosaurs were born without some of the features that they had as adults. For example, Pachycephalosaurus, famous for its domed skull with knobby protuberances, lacked the dome when it was young. Likewise, the horns and neck frill of Triceratops species were different in young, mature, and old individuals.

  9. Painting showing several snails and mussels living in shallow water.

    The behaviors and habitat preferences of modern mussels and snails influenced the placement of the mussels and snails in this painting. Click to zoom.

    Snails and fresh-water mussels alive today have very conservative life styles, meaning that there is little variety among the thousands of species in the ways they feed and live. This makes scientists think that Cretaceous species probably behaved in the same way as their modern descendants. If modern snails and mussels had evolved a variety of different lifestyles, it would be hard to make a convincing argument about how their ancient ancestors lived.

Understanding Ancient Environments and Ecosystems

  1. How do paleontologist reconstruct environments form the ancient past? (VIDEO)

  2. painting of a Cretaceous Crocodile

    Cretaceous crocodile. Click to zoom.

    Climate is one of the factors that determines where different species of plants and animals can live, so paleontologists look for clues to a location's ancient climate in the types of fossil plants and animals they find there. For example, no modern crocodile species lives in a climate with long periods of freezing temperatures, so scientists hypothesize that ancient crocodiles had the same requirement for year round warmth. That leads them to consider the 110-million-year-old crocodile fossils from the Washington, D.C. to be part of a large body of circumstantial evidence that temperatures there were warm year round during the Early Cretaceous. Similarly, coal beds and fossil trees in the Arctic Slope of Alaska are among the many clues that Alaskan temperatures were very warm during the Late Cretaceous.

  3. Photo of a distant mountain, an erosional environment, with a lake, where silty sediments are accumulating, in the foreground.

    Organisms that die and fall to the bottom of this lake are much more likely to be buried and fossilize than those that die in the distant mountains. Click to zoom. Photo by K. Lee. From Wikipedia.

    Scientists expect that the plants and animals living in some environments will be underrepresented in the fossil record, or even completely absent from it. This is because the places where they live are not well suited for the deposition of sediments that bury plant and animal remains, or to the long term preservation of fossils. In mountainous areas, for example, most eroding sediments are carried away by wind or by water that rushes down steep slopes. Few remains are buried, and those that are will be uncovered again as erosion continues to wear away at the mountains. The result is that we have very few fossils of animals and plants from mountain ecosystems. Organisms that live in and around lowland lakes and streams, on the other hand, have a much higher chance of being buried in accumulating sediments - and of staying buried for long periods of time.

  4. painting of a Cretaceous bird

    Cretaceous bird. Click to zoom.

    Painting of a green katydid insect being eaten by the small mammal, Arundelconodon.

    Katydid. Click to zoom.

    Panting of a leaping green frog.

    Frog. Click to zoom.

    The quick answer to this question is that an animal or plant had to be very "lucky" to become a fossil, and even luckier to have its fossil remains discovered. Fossilization is an infrequent occurrence that is highly dependent on chance. In the past, like today, the remains of most organisms were eaten by animals, consumed by microorganisms, or weathered away. Only dead organisms that are buried in sediment quickly can escape these destructive natural processes and become fossils. After remains have been buried and preserved, they may still be destroyed by geological processes, or exposed and weathered away before people can find them.

    Organisms that were very rare in their environment might never have been fossilized simply because the odds of preservation favor more numerous organisms. Animals with small and delicate bones, such as small birds and amphibians, would be less likely to be preserved - and discovered - than larger organisms with tougher bones. Soft-bodied organisms like worms are even more poorly represented in the fossil record because they had no hard parts that could resist decay. We usually learn of their existence only if we find fossilized burrows, or trackways or impressions left by their bodies in soft sediments.

    Scientists studying ancient ecosystems such as this one try to collect fossils from as many types of organisms as possible, but they never expect to find fossil evidence of everything that lived there.

  5. painting of a Cretaceous bird

    Cretaceous bird. Click to zoom.

    We know birds were widespread during the Cretaceous because bird fossils have been found in Early Cretaceous and older rock at sites around the world, including sites in North America. But bird bones are delicate, and well preserved bird fossils are rare except in some unusual sites. For these reasons, scientists do not think the failure to find bird fossils in the Cretaceous deposits near Washington, D.C. is evidence that there were no birds here.

  6. Painting of a katydid insect being eaten by the small mammal, Arundelconodon.

    Katydid. Click to zoom.

    We know from fossils preserved in other locations that many insects, including katydids, were widespread in the Cretaceous and would have played important roles in this ecosystem. The Early Cretaceous rocks of the Crato Formation in northwestern Brazil, which are about the same age as the Arundel deposits, contain well-preserved fossil katydids as well as many other insect fossils. Those fossils give us a good sense of what the local insects would have looked like.

Finding Fossils

  1. Geologic map from the United States Geological Survey.  Areas where the bedrock are of different age and type are shown in different colors.

    The colors on this geologic map represent different types an ages of rock. Click to zoom. Map produced by the United States Geological Survey (

    Geologic maps are among the most important tools used by paleontologists and fossil hunters. The maps show the kinds and ages of bedrock found in different locations, which is critical information to have when deciding where to look for fossils.

    Why is rock type important? Ancient plant and animal remains had the best chance of fossilizing in places where they were buried quickly. On land, those were most often places such as flood plains, lakes, ponds and river deltas where sediments (primarily debris from rocks eroding elsewhere) were deposited by water and later hardened into rock. Sedimentary rock also formed on the sea floor, where the hard shells of microscopic marine organisms often made up the majority of accumulating sediments. Before looking for fossils, paleontologists scan geologic maps for locations where sedimentary rock types, including sandstone, mudstone and limestone, are exposed at the surface of the earth. If they are looking for dinosaur fossils, they will narrow their search to sedimentary rock that formed in terrestrial (land) environments because dinosaurs lived on land. Marine deposits occasionally yield the fossilized remains of dinosaurs whose bodies washed out to sea, but scientists would not normally set out to search for dinosaurs in marine rock.

    Why is it important to identify rocks of the right age? If you were an historian looking for newspaper reports about civil war battles that took place in 1863, you wouldn't search in newspapers from before or after that year. Similarly, if you want to find dinosaur fossils, you shouldn't look in rock that formed before the dinosaurs evolved or after they went extinct. You should restrict your search to rock that formed during the Mesozoic Era because that time period, between 252 and 65.6 million years ago, was when the dinosaurs lived.

  2. photo of two crouched people looking for fossils on an eroded hillside.

    Scanning the ground for fossils at the Dinosaur Park. Click to zoom. Photo by Gerald Elgert.

    The Dinosaur Park in Prince George's County, Maryland, is located on a hillside where erosion exposes clays deposited in an extensive wetland during the Early Cretaceous. Frequent fossil finds include lignite, or brown coal, formed from conifer trees that lived in the wetland, and fossilized cones. Sharp-eyed (and lucky) visitors may also discover dinosaur bones and teeth.

    A man and a woman crouched over a fossil limb bone that is partly excavated from the ground.  The bone is about as long as an adult's arm. The man is wrapping it in plaster-soaked burlap while the woman sprays it with a bottle of water

    Smithsonian staff place a field jacket on a dinosaur limb bone discovered embedded in the ground at the Dinosaur Park. Click to zoom. Photo by Dave Hacker.

    Important fossils discovered at the park are transferred to the Department of Paleobiology at the Smithsonian Institution where they are kept available for scientific research. Some of them are on also display in the 'Dinosaurs in Our Backyard' exhibit.

  3. 1. Try to record as much information as you can about where the fossil was found. If possible, use GPS to record the precise location. If you have a camera with you, take pictures of the place. If the fossil is still embedded in the rock, record the kind and color of sediment. If it is on the surface of the ground, try to figure out exactly what rock layer it eroded from. Make careful note of anything else that was in or on the ground where you found the fossil.

    2. Be careful! Fossils are fragile, and break easily. It's important to keep all the pieces together.

    3. If the fossil must be excavated, don't remove it from the ground without asking for help from someone who has collected fossils before.

    4. Follow the Code of Fossil Collecting adopted by the Paleontological Society. Click here to read the code.

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 ]