The Handy Dinosaur Answer Book. Patricia Barnes-Svarney

too: tracks are generally the traces of distinct footprints, whereas trails may have been produced by an animal dragging its feet or some other appendage as it moved. Tracks, therefore, are more distinctive, and different animals can be distinguished by their own particular footprints. Trails can seldom be associated with a particular animal.

      What can some trace fossils of dinosaur tracks tell us?

      Numerous fossilized dinosaur footprints, called a trackway, indicate much about dinosaurs’ speeds. One such trackway is located north of Flagstaff, Arizona, on the Navajo Reservation. This site was first discovered by Barnum Brown of the American Museum of Natural History in the 1930s, but had been lost until recently. It includes an example of a running dinosaur that left tracks with an 8-foot (2.4-meter) space between the right and left prints; from these prints, scientists calculated that the dinosaur ran at speeds of 14.5 miles (23.3 kilometers) per hour—one of the faster dinosaurs known. The record for fastest dinosaur, however, is presently held by a Jurassic carnivore that left a 16-foot (5-meter) gap between the right and left tracks in a Glen Rose, Texas, trackway. The calculated speed of this dinosaur was about 26.5 miles (42.8 kilometers) per hour, much faster than the speediest human.

DINOSAUR FOSSILS

      How do paleontologists identify species of dinosaurs from other fossils?

      One of the best ways to identify dinosaur fossil bones is by size, as many of the bones are huge. For example, the upper leg bone, or femur, of an adult Apatosaurus often measures over 6 feet (1.8 meters) long.

      But size is not everything, as many dinosaurs were the same size as a chicken or cat. The way scientists detect the differences between dinosaurs and other animal species is by the construction and orientation of their bones, including heads, tails, and hipbones. In addition, dinosaur fossils are often found in association with other dinosaurs at a site. Many times these fossils represent dinosaurs—from meateaters to plant-eaters—that gathered together along the shore of a lake or ocean. The dinosaurs were all searching for food along the banks of the water, a place that would attract many animals and plants.

      Of course, not all dinosaurs are found in the conventional way. In 1998, an amateur fossil collector saw the movie Jurassic Park, and recognized that a fossil he had (which he thought was a bird) was actually a dinosaur. The specimen was found in Italy and measures only 9.5 inches (24 centimeters) long; scientists now know it was a young dinosaur (a theropod named Scipionyx samniticus) that had just hatched before it died. In this case, the dinosaur remains probably washed into oxygen-starved waters, where it was quickly buried. This may be one of the most important dinosaur fossils ever found, as many of its soft body parts were preserved, including the intestines, muscle fibers, and what appears to be the liver.

      When was the first dinosaur bone collected and described?

      The fossilized bones of dinosaurs have probably been found throughout human history, but for much of this time people did not realize what they were. Therefore, no records or descriptions were kept until fairly recently. References to fossilized sharks’ teeth and shells are recorded from the European medieval period, but because they believed that no animal or plant made by God could become extinct, they explained them in other ways. For example, many of the fossils were interpreted as the remains of modern species as opposed to ancient, extinct species; others were thought of as merely pebbles that resembled the remains of animal and plant species.

      The first recorded description of a dinosaur bone was made in 1676 by Robert Plot (1640–1696), a professor of chemistry at the University of Oxford, England, in his book The Natural History of Oxfordshire. Although he correctly determined that it was a broken piece of a giant bone, Plot did not know the bone came from a dinosaur. Instead, he felt it belonged to a giant man or woman, citing mythical, historical, and biblical sources. In 1763, the same bone fragment was named Scrotum humanum, by R. Brookes, to describe its appearance, but the name never gained wide, or serious, acceptance. Based on Plot’s illustration, modern scientists believe the bone fragment is actually the lower end of a thigh bone from a Megalosaurus, a meat-eating dinosaur from the Middle Jurassic period that roamed the area now known as Oxfordshire.

      In 1787, American physician Caspar Wistar (1761–1818) and Timothy Matlack (better known as a statesman and patriot during the American Revolution; 1730–1829) discovered a large fossil bone in the state of New Jersey. Although they reported their finding, it was ignored and unverified; it may have been the first dinosaur bone ever collected in North America.

      What are some of the oldest dinosaur fossils found to date?

      There are several dinosaur fossils claimed to be the oldest ever found, a claim many scientists have made in recent years. The oldest dinosaur bones and skulls found to date may be of a small genus of meat-eating animal measuring about 6.5 feet (2 meters) in length, but some scientists believe it was another type of reptile called a thecodont. The fossils were discovered in Brazil and are thought to be 235 to 240 million years old.

      Previously, an Eoraptor, a 228 million-year-old meat-eating dinosaur, was found in Argentina. More recently, paleontologists also discovered the fossilized bones of three prosauropods, plant-eating dinosaurs that lived approximately 220 million years ago in Santa Maria, an area of southern Brazil. And finally, another claim to the oldest-known “dinosaurs” (or precursors) were prosauropods from Madagascar thought to be 230 million years old. The Eoraptor was dated using radio-isotope analysis, whereas index fossils—or the use of surrounding fossils to date a “dinosaur”—were used to date the fossil in Madagascar.

      

      What is a dinosaur?

      Dinosaur is a term used to describe certain types of animals that lived during the Mesozoic era in geologic history. It is difficult to generalize about dinosaurs, but two things are definitely agreed upon. They were, in general, the largest creatures to ever walk on Earth, even though there were many smaller species of dinosaurs. In addition, these animals were some of the most successful organisms that ever lived, existing as species for at least 160 million years.

      What does the term dinosaur mean?

      Dinosaur comes from the term dinosauria, which is a combination of the Greek words deinos and sauros. It means “terrible reptiles” or “terrible lizards.” The term was invented by the well-known British anatomist Sir Richard Owen (1804–1892). He coined the term in 1842 to describe the 175-million-year-old fossil remains of two groups of giant reptiles that corresponded to no known living creatures. In 1854, Owen prepared one of the first dinosaur exhibits for display at the Crystal Palace in London, England.

DINOSAUR ANCESTORS

      How did life evolve after the early one-celled organisms?

      Over hundreds of millions of years after the evolution of single-celled organisms, the oceans abounded with a huge variety of life. The first soft-bodied animals, such as worms and jellyfish, evolved toward the close of the Pre-Cambrian (also seen as Precambrian) era roughly 600 million years ago; the first animals with hard parts, such as shelled mollusks, evolved during the first period after the Pre-Cambrian known as the Cambrian period of the Paleozoic era.

      What are vertebrates?

      The first vertebrates, or animals with backbones, evolved during the late Cambrian to early Ordovician periods as jawless freshwater fish that looked much like today’s hagfish and lampreys. By the Devonian period (the “age of fishes”), jawed and armored fishes dominated the oceans. Around 380 million years ago, a line of fish with bony skeletons developed air-breathing lungs and “limbs” strong enough to support