Our knowledge of dinosaurs comes from what they left on earth as fossils. But very little dinosaur material has been fossilized by nature, and then nature itself often becomes the fossils' enemy. Wind and weather can damage fragile fossils, but erosion also uncovers them for collectors and paleontologists.
Scientists study dinosaur skeletons to learn about the animals' behavior and appearance. The skull is especially important. A dinosaur's teeth can tell us whether it was an advanced or primitive dinosaur, whether it ate plants or meat, or even whether it ate soft or tough plants. Its skull will tell if it had a large or small brain or if it had good or bad eyesight, hearing, and sense of smell. The shape of a dinosaur skull can also tell us what other dinosaurs it was related to and where it fits in the family tree.
Though the skull is important, paleontologists can tell much about a dinosaur skeleton without a head; many skeletons are found without skulls. And there are other clues that dinosaurs left behind, all giving information about their lifestyles and habits. Scientists have discovered dinosaur trackways (fossilized footprints), nests, and eggs. But even with this information, there is much that will never be known. The fossil record is incomplete and can be misleading. Paleontologists continue to search, hoping to uncover other clues that will give them more information about dinosaurs.
How Dinosaurs Became Fossils
The term fossilized is important. It is not the dinosaur bones themselves that paleontologists find, but their stony replicas. Fossil bones are created by the slow replacement of bone molecules with the molecules of minerals in ground water. This process sometimes preserves even the delicate cell structure of the bones. Other times the bones dissolve completely and only their imprint is left on the rocks.
Teeth and bones are the most durable parts of a vertebrate's body. They may survive long after the animal's soft parts have rotted or been eaten. Except for fossilized footprints, skeletal remains are the most common fossils. Unfortunately, few animals ever become fossilized, compared to the millions that lived.
Animals seldom die where they can be fossilized. A recently deceased animal that remains above ground will be dissolved by the chemicals in soil and converted into plant food. This is why the bodies of even large animals are seldom found in the woods-and why fossils are most often found in deserts and other "bone-dry" places where there are not many plants.
In order for a skeleton to become a fossil, a layer of sediment must quickly cover the body. This prevents the skeleton from being destroyed by other animals or nature. Over the following thousands and millions of years, this sediment hardens into a protective layer of rock, such as shale, siltstone, mudstone, or sandstone.
Dinosaur bones could have been covered by sediment in still water, such as at the bottoms of lakes, slow-moving rivers and river deltas, shallow seas, and seacoasts. The sediment could also have been brought by the air, such as sand from dust storms or volcanic ash falls. The places where conditions are right for skeletons to become fossils are only a small percentage of the world's land area at any time. Many species of dinosaurs lived where no fossils could form. Partly for this reason, there are many evolutionary "missing links" between groups of dinosaurs. So there is an incomplete picture of dinosaurs from the fossil record.
Problems with the Fossil Record
The fossil record can give us large amounts of knowledge. But there are many other ways that the fossil record is incomplete or misleading. Scientists must keep these problems in mind when they are reading the fossil record to come to conclusions about dinosaurs and their lifestyles.
Large, heavy bones are more likely to be preserved than small, lightly built bones. So, although small vertebrates usually outnumber large, fossils of small vertebrates are scarce. This might seem to suggest that big dinosaurs dominated the landscape and that there were fewer small animals. This was probably not the case.
For dinosaur fossils to be found, the rocks where the fossils are located must be accessible. Many dinosaur-bearing formations were eroded by wind and weather long before humans were around, so their fossils have been lost forever. Other dinosaur-bearing formations are buried beneath hundreds of feet, even miles, of rock. These fossils are waiting to be excavated in the future. Only dinosaur-bearing formations that are now at the earth's surface are ready to be researched. So there is much information about the fossil record that has already been lost to erosion, but there is still much that today's paleontologists will never be able to find and evaluate.
We can also get a wrong idea about dinosaurs when scientists and paleontologists make mistakes. In the past, researchers were interested mainly in well-preserved skeletons that could be displayed in museums, so they sometimes ignored dinosaur bones unless they were new dinosaurs. Other times, they removed only the most interesting parts of the skeleton, such as the skull. They may have done this because they did not have the time or the money to excavate the whole skeleton, so they left the rest to erode.
Some museum specimens are useless because important information was recorded incorrectly or not at all. Dinosaurs have occasionally been mounted in museums with the bones in the wrong places. Museum workers have sometimes used too much plaster, restoring missing parts incorrectly. This may have mislead other researchers who relied on the published drawings and photographs of the restored skeletons. Modern methods of fossil collection and preparation have eliminated many of these problems with newly collected material. But museums have many fossils in their collections that need to be restudied.
So, everything we know about dinosaurs comes to us through an incomplete fossil record and the imperfect people who investigate it. But there is still much information that this fossil record gives us.
Dinosaur Appearance and Behavior
Dinosaurs with almost complete fossil skeletons give us clues about what they were like. We get a good idea of the dinosaurs' size, weight, and appearance in life. The cell structure of fossilized dinosaur bones can tell us about the biology of a dinosaur. We then have information about how rapidly they grew, and perhaps about whether they were warm-blooded or cold-blooded. Fossilized bones sometimes leave evidence of bone diseases and the tooth marks of predators. Muscle tissue is almost never preserved (only two good "dinosaur mummies," with soft parts intact, have been discovered). But we can still tell how dinosaurs moved from the traces of ligaments and muscle scars on the bones.
Dinosaurs' teeth can tell us what kinds of foods they ate. Occasionally the actual stomach contents are preserved, so scientists can study what a dinosaur had for its last meal. The smooth "stomach stones" with which some dinosaurs ground up their food are sometimes preserved, and even fossilized dinosaur droppings, which are known as coprolites, have been found. All this gives us more information on dinosaur diets.
Fossilized skin impressions can be seen on very well preserved skeletons. The fine-grained sandy rocks of Dinosaur Provincial Park in Alberta, Canada, have preserved the skin impressions of duckbilled and horned dinosaurs. These provide an idea of what dinosaur skin looked like. No dinosaur has ever been found with feather traces, so there is no evidence that any dinosaur had feathers.
Scientists study locations and distribution of dinosaur bones for information about the dinosaurs' environments. The direction in which bones are pointed and the way they are arranged in dinosaur bone beds are clues about the size and strength of rivers dinosaurs had to wade through. Scientists compare the number of different types of dinosaurs from one location to find out about dinosaur habits and lifestyles. For example, if there were few predators and many animals that would have been their prey, it would show that the predatory dinosaurs had a quick metabolism and were probably warm-blooded.
Fossilized dinosaur footprints are more common than dinosaur skeletal remains. This is not surprising because each dinosaur could have left hundreds of thousands of prints over its lifetime. Unfortunately, footprints give few details about what the dinosaur looked like and what species it belonged to, though the skin texture of the soles of its feet is sometimes imprinted. The tracks can tell us an enormous amount about how the trackmakers walked, cruised, ran, sprinted, and even swam.
We know from their footprints, for example, that large theropod dinosaurs either roamed by themselves or in small groups. Smaller dinosaurs, both plant-eaters and meat-eaters, often gathered in herds that left many directionless footprints. Large sauropods also probably lived in herds. After more study by paleontologists, trackways may show that dinosaurs migrated along thousand-mile-long "highways" in search of food.
By measuring stride lengths and footprint sizes, and comparing them with modern animals, it is possible to estimate how fast dinosaurs walked and ran. Typical walking speeds were about four or five miles per hour, though large sauropods may have walked two or three miles per hour. Since animals do not run at top speed for any length of time, tracks of fast-running dinosaurs are rare. Estimated theropod speeds may be more than 30 miles per hour, and ornithomimids may have been faster than ostriches.
Fossilized dinosaur eggs were first unearthed in Mongolia during the 1920s. Dinosaur eggs usually are rare fossils, but they have been found in China, Mongolia, India, France, and South America. Besides shedding light on dinosaur nest structure, the eggs seemed to tell little.
But the recent discoveries of dinosaur nests, nurseries, eggs, embryos, hatchlings, nestlings, and juveniles in Montana have greatly increased our knowledge. Dinosaurs that began life as helpless hatchlings-the embryos show whether the animal could walk once it hatched-must have required parental care to survive. Other dinosaur hatchlings that could have walked once they hatched probably were left by themselves. To confirm this, the eggs of dinosaurs that required parental care are found smashed to bits, as if the hatchlings lived in their nests. The eggs of dinosaurs that were able to take care of themselves are usually found unbroken. By studying the sizes of the dinosaurs in the Montana nurseries, scientists may find out how many seasons it took for a particular species to grow big enough to leave the nursery and venture into the dangerous world. Unfortunately, nest structures and eggs have been found for only a small number of species.
Dinosaur Skeletal Structures
Dinosaurs are known for the special skeletal structures they evolved. These include the horns and frills of the ceratopsians; the sailbacks of certain iguanodontids and theropods; the domed skulls of the pachycephalosaurians; the cranial crests of the hadrosaurs and certain theropods; and the plates and spines of the stegosaurians and ankylosaurians. These special skeletal structures would have been bold and obvious, so these structures may have been for display. Many also had other uses, including regulation of body heat, making noise, and defense.
Sight was an important sense to dinosaurs; their eyes were large. Dinosaurs were closely related to birds and crocodiles; both see colors and have a wide range of visual signals. So, these special skeletal structures of dinosaurs may have been brightly colored or boldly patterned. Closely related but different species may have been differently colored so they could be told apart during mating season.
Some of these structures, such as stegosaurian plates and ceratopsian frills, were made of thin bone that had many passages for blood vessels. Blood must have run through these passages, either to cool the animal off (if the dinosaur were warm-blooded) or to warm it up (if the dinosaur were cold-blooded). By regulating the blood flow to these structures, a dinosaur could regulate heat loss or heat gain and maintain a reasonably constant body temperature. The dinosaur may also have been able to change the structure's color by changing the blood flow. This would have given dinosaurs a way of expressing their emotional state to others of their kind.
Birds and crocodiles have excellent hearing, and their relatives the dinosaurs probably did, too. Some dinosaur skulls have the delicate bony structures of their middle and inner ears preserved. By examining these, scientists can confirm that most dinosaur groups could hear very well. Since that is true, they may have been able to vocalize. The head crests of the lambeosaurines were part of their nasal passages. An animal inhaled air into its crest, which then went down through the back of its head into the lungs. This was a very complicated pathway, but it may have been used for making sounds. The lambeosaurine could have inhaled air and vibrated its looping nasal passage, emitting a deep, low-frequency call. As far as we know, no two lambeosaurines had exactly the same crest shape, so no two would have made the same call. Their crests may have vocally distinguished individuals from one another.
The function of the horns of ceratopsids seems obvious: To fight an attacking tyrannosaurid. Also, rival ceratopsids probably locked horns in shoving matches over mates. Today's moose and elk use their antlers like this and fight predators by using their sharp-hoofed feet, not their antlers. Ceratopsids could also have used their sharp beaks to inflict painful, sometimes fatal wounds on predators.
It is difficult to understand that the world was a far different place 65 to 240 million years ago and that it was populated by huge and strange-looking creatures. All the large animals in today's zoos, from elephants to zebras to kangaroos (except crocodiles), evolved well after dinosaurs were gone. We are fortunate that some of the dinosaurs left their traces in rocks, which scientists have been able to study and compare with living animals. Scientists can then make educated assumptions about dinosaur lifestyles. In a world with a history that spans four and a half billion years, perhaps the most interesting thing about dinosaurs is that we have been able to learn anything about them at all.