Investigating Dinosaurs

by David H. Griffing, PhD

What were dinosaurs? The word "dinosaur" was invented by the famous British scientist Sir Richard Owen in 1842, and means "terrible lizard." Dinosaurs were land-dwelling animals closely related to modern reptiles and birds. They lived from approximately 230 million years to 66 million years before the present (in a time we call the Mesozoic Era).

Time scale

All dinosaurs had distinctive hip, legs and ankle bones which allowed them to walk with their legs tucked under their body. Other reptiles, such as modern lizards and crocodiles, walk with their legs held out to the sides, a position similar to a person doing "push-ups." For example, the sail-backed reptile Dimetrodon lived about 270 million years ago (many millions of years before the Mesozoic) and walked with its arms out to the sides: it was not a dinosaur! Mesozoic reptiles that swam (ichthyosaurs, mosasaurs and plesiosaurs) and flew (pterosaurs) were not dinosaurs either. Although we commonly think of all dinosaurs as being bigger than other reptiles, they came in nearly all sizes, from about 70 cm to over 30 m in length.
How do we know about dinosaurs? The only way we know about dinosaurs is from studying fossils. Animals and plants may become fossilized if they are buried in sand or mud soon after death. Even so, only the hard parts of the body (bones and teeth) usually remain to be fossilized because scavengers and bacteria destroy the fleshy parts of the body. In addition to bones and teeth, footprints or tracks of dinosaurs may be preserved in sandstones or mudstones, which were once soft sediments. Scientists can estimate the walking, running or even wading speed of certain dinosaurs from a series of tracks, as well as dinosaurs size and whether they lived alone or traveled in groups. The bones and tracks of a particular dinosaur are rarely preserved in the same place, so paleontologists must compare the shapes of foot bone and the footprints to make a match. Other dinosaur fossils include skin impressions, eggs, stomach stones, and droppings, but these are relatively rare. Some dinosaurs (the sauropods) apparently swallowed stones to help grind up fibrous food in their stomach, much like chickens do! The large stomach stones (or "gastroliths") became smooth and highly polished after grinding food for a while, like rocks in a tumbler. Scientists also study fossilized droppings (or "coprolites") to see what kind of foods the dinosaurs ate.

What are the oldest known dinosaurs and what were they like? The oldest known dinosaur fossil is Eoraptor. It was discovered in Argentina and lived about 228 million years ago (during the first period of the Mesozoic - the Triassic Period). It was a small, light-weight animal that walked on its hind legs (as shown above). Most dinosaurs from the Triassic Period were small, agile predators that lived among more primitive reptiles (crocodilians), reptile ancestors to the mammals (the therapsids), as well as the earliest mammals. It was only after some of the other reptile groups and all the therapsids died off (about 200 million years ago) that larger dinosaurs appeared.

Are birds and dinosaurs related? Yes. Scientists have long considered dinosaurs and birds to have a common ancestor (because, for example, both have scaly skin and lay hard-shelled eggs), but it wasn't until the 1970's that scientists began to fully understand this relationship. Evidence suggests that a certain group of early dinosaurs (the "coelurosaurs") may have been the direct ancestors to birds. The early bird Archaeopteryx (which lived about 150 million years ago) had teeth, a long tail and claws on its arms (features not found on most modern birds but common to coelurosaurs) and it was covered with feathers (a feature only known from birds). They are so similar that one specimen of Archaeopteryx which didn't have the preserved feathers was misidentified as a small coelurosaur! Some scientists guess that some dinosaurs also had feathers, but there is no evidence to support this claim yet

What did dinosaurs look like? Paleontologists can reconstruct the body of most dinosaurs using marks on the bones called "muscle scars." Muscle scars form where muscles attached to the bones by tendons and ligaments. The large the scar, the more massive the muscle. Although rare, dinosaur skin impressions give scientists an idea of what they looked like from the outside. Since pigments do not fossilize, nobody really knows what color dinosaurs were. Artists use the colors of modern dinosaur relatives (reptiles and birds), as well as those of other large animals to guess at the color of dinosaurs.

Were dinosaurs "cold-blooded" or "warm-blooded"? Animals need a relatively constant body temperature in order to digest efficiently and grow. An animal is sometimes called "cold-blooded" if its body cannot generate enough energy to keep a constant temperature without help from the surrounding environment. For example, an alligator must lie in the sun during the cool morning hours to bring its body temperature up. Animals like mammals and birds can generate enough energy and internal heat to effectively keep their bodies at a constant temperature, and are therefore called "warm-blooded." This means they can generate more energy for long distance movement and for living in cooler places. Some, but not all, paleontologists believe dinosaurs were "warn-blooded" because of their posture and capability to move fast, the structure in their bones, their global distribution, and their close relationship to birds. Just as in modern groups of animals, dinosaurs may have had a range of abilities and needs, and all might not fit into our definition of either "warm-blooded" or "cold-blooded."

What caused the extinction of the dinosaurs? The final and permanent disappearance (extinction) of a group of plants or animals is caused by major changes in the physical environment in which they live. Actually, large groups of dinosaurs went extinct three times. Contrary to many story books, Tyrannosaurus never fed upon Apatosaurus (=Brontosaurus) or Stegosaurus because these herbivores were extinct millions of years before the big predator first appeared. Many reasons have been proposed for the final extinction of the dinosaurs, 65 million years ago. These include a climatic cooling and a change to more seasonal weather, disease spread by migrating dinosaurs, and changes in vegetation. There is not much evidence for or against these possibilities. There is, however, considerable evidence for a collision of one or more large bodies from space (like comets or asteroids) with Earth about 65 million years ago. These impacts may have finished off the dinosaurs by starting global forest fires, and by rapidly cooling the Earth due to the blockage of sunlight by dust and soot.

Where are dinosaur fossils found? Dinosaur fossils are found on every continent (even Antarctica). Dinosaurs lived from about 230 to 65 million years ago, so only rocks deposited as soft sediments during that time period may contain dinosaur fossils. Rocks of this age are common in the western United States (and western Canada) but much less common in the eastern United States.

Why are there so few dinosaur fossils in the eastern United States? Most sedimentary rocks (those made of solidified mud, sand and gravel) in the eastern United States are either too old or too young to contain dinosaur fossils. The only dinosaur-bearing rocks in this area of the country come from an ancient system of valleys that opened up when the continent split apart about 245-200 million years ago (Triassic and Early Jurassic times). These valley deposits are very common in Connecticut, Massachusetts and New Jersey, and many dinosaur fossils (mostly footprints) have been found in those states. Dinosaurs probably lived in most of the eastern United States, but there were few places to permanently preserve their skeletons. One probable inhabitant was the small, bird-like coelurosaur Coelophysis. Most sedimentary rocks from the later Mesozoic Era (Cretaceous Period) found in the eastern United States were deposited in oceans formed as the continents moved apart, so there are very few dinosaur fossils.

Are dinosaurs being discovered? Many of the 500 or more known species of dinosaur have only been discovered in the last twenty years, and paleontologists are finding never-before-seen fossils every day! Many of the newest dinosaur fossils come from areas not yet fully explored because they are hard places to travel to. such as Mongolia or the Sahara Desert in northern Africa.


Sources of more information

Gardom, Tim with Angela Milner (scientific advisor), 1993. The Book of Dinosaurs: The Natural History Museum guidebook. Prima Publishing, Rocklin, CA. 128p. ISBN1-55958-350-9.

Horner, John R., and Lessem, Don, 1993. The Complete T. rex: How stunning new discoveries are changing our understanding of the world's most famous dinosaur. Simon and Schuster, New York, NY 239p. QE862.S3H66 1993 ISBN 0-761-74185-3.

Lucas, Spenser, 1994. Dinosaurs: The textbook. WCB Publishers, Dubuque, IA. 290p. ISBN 0-697-14429-1.

McGowan, Christopher, 1991. Dinosaurs, Spitfires, & Sea Dragons. Harvard University Press, Cambridge, MA. 365p. QE861.M38 1991 ISBN 0-674-20769-6.

Munsart, Craig, 1993. Investigating Science with Dinosaurs. Teacher Ideas Press. Englewood, CO. 249p. LB1585.3.M86 1993 ISBN 1-56308-008-7.

Norman, David, 1985. The Illustrated Encyclopedia of Dinosaurs. Crescent Books, New York, NY. 208p. QE682.D5N67 1985 ISBN 0-517-46890-5.

PRI Educational Leaflet No. 2, version 8/1/96

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