Dinosaur - biology.

Dinosaur - biology. I INTRODUCTION Dinosaur, one of a group of extinct reptiles that lived from about 230 million to about 65 million years ago. The word dinosaur was coined in 1842 by British anatomist Sir Richard Owen, derived from the Greek words deinos, meaning "marvelous" or "terrible," and sauros, meaning "lizard." For more than 140 million years, dinosaurs reigned as the dominant animals on land. Owen distinguished dinosaurs from other prehistoric reptiles by their upright rather than sprawling legs and by the presence of three or more vertebrae supporting the pelvis, or hipbone. Dinosaurs are classified into two orders according to differences in pelvic structure: Saurischia, or lizard-hipped dinosaurs, and Ornithischia, or birdhipped dinosaurs. Dinosaur bones occur in sediments that were deposited during the Mesozoic Era, the so-called era of middle animals, also known as the age of reptiles. This era is divided into three periods: the Triassic (251 million to 200 million years ago), the Jurassic (200 million to 145 million years ago), and the Cretaceous (145 million to 65 million years ago). Historical references to dinosaur bones may extend as far back as the 5th century BC. Some scholars think that Greek historian Herodotus was referring to fossilized dinosaur skeletons and eggs when he described griffins--legendary beasts that were part eagle and part lion--guarding nests in central Asia. "Dragon bones" mentioned in a 3rd century AD text from China are thought to refer to bones of dinosaurs. The first dinosaurs studied by paleontologists (scientists who study prehistoric life) were Megalosaurus and Iguanodon, whose partial bones were discovered early in the 19th century in England. The shape of their bones indicates that these animals resembled large, land-dwelling reptiles. The teeth of Megalosaurus, which are pointed and have serrated edges, indicate that this animal was a flesh eater, while the flattened, grinding surfaces of Iguanodon teeth indicate that it was a plant eater. Megalosaurus lived during the Jurassic Period, and Iguanodon lived during the early part of the Cretaceous Period. Later in the 19th century, paleontologists collected and studied more complete skeletons of related dinosaurs found in New Jersey. From these finds they learned that Megalosaurus and Iguanodon walked on two legs, not four, as had been thought. In the late 19th and early 20th centuries, as the science of paleontology grew and the search for dinosaur remains was extended around the world, new kinds of dinosaurs were discovered. Hundreds of different varieties of dinosaur have been identified from bones found on all of the continents as well as on the islands of Greenland, Madagascar, and New Zealand. II ANCESTRY Dinosaurs belong to a group of advanced reptiles called archosaurs, which appeared late in the Permian Period. Archosaurs survive today in the form of crocodiles and birds. In addition to dinosaurs, extinct archosaurs included pterosaurs (flying reptiles) and a number of other types of reptiles formerly grouped together under the name thecodonts. During the Triassic, two distinct groups of archosaurs evolved, one related to crocodiles and the other related to dinosaurs, birds, and pterosaurs. Scientists were once confused about the early ancestors of dinosaurs because some of the archosaurs related to crocodiles developed a number of dinosaur-like features, including walking upright on two hind legs. This phenomenon is known as parallel evolution, in which animals that are not directly related come to resemble each other because they have similar life styles or diets. New fossils have allowed paleontologists to better recognize the true ancestors of dinosaurs. The ancestors of dinosaurs also walked on two hind legs, but had S-shaped necks; simple, hingelike ankle bones; and other distinctive features. Fossil evidence of the earliest dinosaurs dates from about 230 million years ago. This evidence, found in Madagascar in 1999, consists of bones of an animal about the size of a kangaroo. This dinosaur was a type of saurischian and was a member of the plant-eating prosauropods, which were related to ancestors of the giant, longnecked sauropods that included the Apatosaurus. Prior to this discovery, the earliest known dinosaur on record was the Eoraptor, which lived 227 million years ago. Discovered in Argentina in 1992, the Eoraptor was an early saurischian, 1 m (3 ft) long, with a primitive skull. Scientists have identified remains of a few small dinosaurs representing ornithischians dating from the end of the Triassic Period before about 200 million years ago. By the middle of the Jurassic Period, around 180 million years ago, most of the basic varieties of saurischian and ornithischian dinosaurs had appeared, including some that far surpassed modern elephants in size. Dinosaurs had evolved into the most abundant large animals on land, and the dinosaurian age had begun. III HABITAT Earth's environment during the dinosaurian era was far different than it is today. The days were several minutes shorter than they are today because the gravitational pull of the sun and the moon have over time had a braking influence on Earth's rotation. Radiation from the Sun was not as strong as it is today because the Sun has been slowly brightening over time. Other changes in the environment may be linked to the atmosphere. Carbon dioxide, a gas that traps heat from the Sun in Earth's atmosphere--the so-called greenhouse effect--was several times more abundant in the air during the dinosaurian age. As a result, surface temperatures were warmer and no polar ice caps could form. The pattern of continents and oceans was also very different during the age of dinosaurs. At the beginning of the dinosaurian era, the continents were united into a gigantic supercontinent called Pangaea (all lands), and the oceans formed a vast world ocean called Panthalassa (all seas). About 200 million years ago, movements of Earth's crust caused the supercontinent to begin slowly separating into northern and southern continental blocks, which broke apart further into the modern continents by the end of the dinosaurian era. As a result of these movements of Earth's crust (see Plate Tectonics), there was less land in equatorial regions than there is at present. Deserts, possibly produced by the warm, greenhouse atmosphere, were widespread across equatorial land, and the tropics were not as rich an environment for life forms as they are today. Plants and animals may have flourished instead in the temperate zones north and south of the equator. The most obvious differences between dinosaurian and modern environments are the types of life forms present. There were fewer than half as many species of plants and animals on land during the Mesozoic Era than there are today. Bushes and trees appear to have provided the most abundant sources of food for dinosaurs, rather than the rich grasslands that feed most animals today. Although flowering plants appeared during the dinosaurian era, few of them bore nuts or fruit. The animals of the period had slower metabolisms and smaller brains, suggesting that the pace of life was relatively languid and the behavior patterns were simple. The more active animals--such as ants, wasps, birds, and mammals--first made their appearance during the dinosaurian era but were not as abundant as they are now. IV BEHAVIOR AND PHYSIOLOGY The behavior of dinosaurs was governed by their metabolism and by their central nervous system. The dinosaurs' metabolism--the internal activities that supply the body's energy needs--affected their activity level. It is unclear whether dinosaurs were purely endothermic (warm-blooded), like modern mammals, or ectothermic (cold-blooded), like modern reptiles. Endotherms regulate their body temperature internally by means of their metabolism, rather than by using the temperature of their surroundings. As a result, they have higher activity levels and higher energy needs than ectotherms. Ectotherms have a slower metabolism and regulate their body temperature by means of their behavior, taking advantage of external temperature variations by sunning themselves to stay warm and resting in the shade to cool down. By determining whether dinosaurs were warm- or cold-blooded, paleontologists could discover whether dinosaurs behaved more like modern mammals or more like modern reptiles. Gradual changes in dinosaur anatomy suggest that the metabolic rates and activity levels of dinosaurs increased as they evolved, and some scientists believe this indicates that dinosaurs became progressively more endothermic. In general, dinosaur body size decreased throughout the latter half of the dinosaurian era, increasing the dinosaurs' need for activity and a higher metabolism to maintain warmth. Smaller animals have more surface area in proportion to their volume, which causes them to lose more heat as it radiates from their skin. Well-preserved fossils show that many small dinosaurs were probably covered with hair- or feather-like fibers. Dinosaurs' tooth batteries (many small teeth packed together) became larger, enabling them to chew their food more efficiently, their breathing passages became separated from their mouth cavity, allowing them to chew and breathe at the same time, and their nostrils became larger, making their breathing more efficient. These changes may have helped the dinosaurs digest their food and change it into energy more quickly and efficiently, thereby helping them maintain a higher metabolism. Fossils from Australia, Antarctica, and Alaska indicate that some dinosaurs lived in colder regions where sunlight would have been absent for part of the year. Although polar climates were much milder than today, freezing temperatures sometimes occurred. The central nervous system of dinosaurs affected their behavioral flexibility--how much they could adapt their behavior to deal with changing situations. Scientists believe that the ratio of dinosaurs' brain size to their body weight increased as the animals evolved. As a result, their behavioral flexibility increased from a level comparable to that of modern crocodiles, in the primitive dinosaurs, to a level comparable to that of modern chickens and opossums, in some small Cretaceous dinosaurs. Imprints of the skin of large dinosaurs show that the skin had a textured surface without hair or feathers. The eyes of dinosaurs were about twice the diameter of those of modern mammals. The skeleton of one small dinosaur was found preserved in windblown sand. Its head was tucked next to its forelimbs, resembling the posture of a modern bird, and its tail was wrapped around its body, resembling the posture of a cat. Many, if not all, dinosaurs laid eggs, and extensive deposits of whole and fragmented shells have been found in China, India, and Argentina, suggesting that large nesting colonies were common. A very few eggs have been identified from the skeletons of embryos contained within them. In proportion to the body weight of the mother, dinosaurs laid smaller eggs in greater numbers than do birds. Scientists have found what they believe is a typical nest dug into Cretaceous streamside clays in Montana. The nest is a craterlike structure about 2 m (6.6 ft) in diameter--thought to be about the diameter of the mother's body. The large number of bones of small dinosaurs that have been found in nesting colonies indicates that the mortality rate of juveniles was very high. The growth rings preserved in dinosaur bones suggest that primitive dinosaurs grew more slowly than later dinosaurs. The growth rings in some giant dinosaurs suggest that these dinosaurs may have grown to adulthood rapidly and had shorter life spans than some large modern turtles, such as the giant tortoise, which can live 200 years in captivity. V SAURISCHIAN DINOSAURS Saurischian dinosaurs were characterized by a primitive pelvis, with a single bone projecting down and back from each side of the hips. This pelvis construction was similar to that of other ancient reptiles but, unlike other reptiles, saurischians had stronger backbones, no claws on their outer front digits, and forelimbs that were usually much shorter than the hind limbs. There were three basic kinds of saurischians: theropods, prosauropods, and sauropods. A Theropods Nearly all theropods were bipedal flesh eaters, a successful design already found in Coelophysis from the late Triassic Period. Some theropods, such as Tyrannosaurus of the late part of the Cretaceous Period, reached lengths of 12.5 m (41 ft) and weights of 5 metric tons. In large theropods the huge jaws and teeth were adapted to tearing prey apart. Fossil trackways reveal that these large theropods walked more swiftly than large plant-eating dinosaurs and were more direct and purposeful in their movements. Well-known large theropods include Allosaurus and Giganotosaurus. Other theropods, such as Compsognathus, were small and gracefully built, resembling modern running birds such as the roadrunner. Their heads were slender and often beaked, suggesting that these theropods fed on small animals such as lizards and infant dinosaurs. Some of them possessed brains as large as those of modern chickens and opossums. Some theropods such as Ornithomimus resembled ostriches and had toothless beaks that may have been used in a fashion similar to the feeding of large living ground birds. One unusual group of theropods called therizinosaurs became plant-eaters. Therizinosaurs looked similar to giant ground sloths and had enormous claws on their hands. Scientists are particularly interested in a group of theropods called dromaeosaurids, often dubbed raptors. They include Velociraptor and Deinonychus. These agile meat-eaters bore powerful claws, like those of an eagle, on their hands and feet and used their flexible tails as balancing devices to increase their agility when turning. The second toe on each foot was shaped like a sickle. These animals appear to have hunted in packs. Many paleontologists believe that birds may have arisen from small, primitive theropods that were also ancestors of the raptors. Remarkable fossils from China of small raptors show that some had feathers very similar to those of birds. One small form called Microraptor had long flight-feathers on its hind legs as well as its arms, and may have launched itself from trees to glide from the canopy to hunt at ground level. Other theropods show evidence of behavior remarkably similar to that of modern birds. A nest of dinosaur eggs discovered in the Gobi Desert contains the fossil bones of an Oviraptor sitting on its brood of about 15 eggs. B Prosauropods Unlike the primitive theropods, the prosauropods had relatively small skulls and spoon-shaped, rather than blade-shaped, teeth. Their necks were long and slender and, because they were bipedal, the prosauropods could browse easily on the foliage of bushes and trees that were well beyond the reach of other herbivores. A large clawed, hooklike thumb was probably used to grasp limbs while feeding. The feet were broad and also heavily clawed. When prosauropods appeared in the fossil record along with the earliest known theropods, they had already reached lengths of 3 m (10 ft). By the end of the Triassic Period, the well-known Plateosaurus had attained a length of 9 m (30 ft) and a weight of 1.8 metric tons. During the late Triassic and early Jurassic periods, prosauropods were some of the largest plant-eating dinosaurs. C Sauropods Sauropods, which include giants such as Apatosaurus (formerly known as Brontosaurus) and Diplodocus, descended from prosauropods. The earliest sauropods appeared near the end of the Triassic. Their relatively rare fossils suggest they were not common, but some were already the largest animals on land at the time. By the middle of the Jurassic Period sauropods had far surpassed all other dinosaurs in size and weight. Some sauropods probably reached lengths of more than 30 m (100 ft) and weights well in excess of 50 metric tons. Sauropods walked on four pillar-like legs. Their feet usually bore claws on the inner toes, although they otherwise resembled the feet of an elephant. The sauropod backbone was hollow and filled with air sacks similar to those in a bird's vertebrae, and the skull was small in proportion to the animal's size. The food they ate was ground by stones in their gizzard, a part of their digestive tract. Indeed, sauropods may be compared to gigantic elephants, with the long necks of sauropods performing the function of an elephant's trunk, and their gizzard stones acting as the strong teeth of an elephant. Some sauropods, such as the late Jurassic Apatosaurus, used their long, thin tails as a whip for defense, while others used their tails as clubs. Some unusual sauropods had bony armor or spines along their backs or necks. VI ORNITHISCHIAN DINOSAURS In ancestral ornithischians the bony structure projecting down and back from each side of the hips was composed of two bones, so that their hips superficially resembled the hips of birds. Early ornithischians were small bipedal plant eaters, about 1 m (3 ft) in length. One of the most complete fossils of such a primitive ornithischian was found in South Africa. Called Eocursor, the fox-size dinosaur lived during the late Triassic Period. These agile early plant-eaters gave rise to five kinds of descendants: stegosaurs, ankylosaurs, ornithopods, pachycephalosaurs, and ceratopsians. A Stegosaurs Some ornithischians quickly became quadrupedal (four-legged) and relied on body armor and other physical defenses rather than fleetness for protection. Plated dinosaurs, such as the massive Stegosaurus of the late Jurassic Period, bore a double row of triangular bony plates along their backs. These narrow plates contained tunnels through which blood vessels passed, allowing the animals to radiate excess body heat or to warm themselves in the sun. Many also bore a large spined plate over each shoulder. Stegosaurs resembled gigantic porcupines, and they probably defended themselves by turning their spined tails toward aggressors. B Ankylosaurs The armored ankylosaurs appeared during the Jurassic Period, but their early fossils are less common than those of stegosaurs. During the Cretaceous Period, stegosaurs were supplanted by armored dinosaurs such as Ankylosaurus. These animals were similar in size to stegosaurs but otherwise resembled giant horned toads. Some even possessed a bony plate in each eyelid as well as large tail clubs. Their necks were protected by heavy bony rings and spines, indicating that these areas needed protection from the attacks of carnivorous dinosaurs. C Ornithopods Ornithopods increased in variety from the middle Jurassic through the Cretaceous periods and became the most abundant plant-eating dinosaurs. They ranged in size from small runners that were 2 m (6 ft) long and weighed 15 kg (33 lb), such as Hypsilophodon, to elephantine cows that were 10 m (32 ft) long and weighed 4 metric tons, such as Edmontosaurus. These animals had flexible jaws and grinding teeth, which eventually surpassed those of modern cows in their suitability for chewing fibrous plants. The beaks of ornithopods became broader, earning them the name duck-billed dinosaur. Their tooth batteries became larger, their backs became stronger, and their forelimbs lengthened until their arms became elongated walking sticks, although ornithopods remained bipedal. The nose supported cartilaginous sacks or bony tubes, suggesting that these dinosaurs may have communicated by trumpeting. Fossil evidence from the late Cretaceous Period includes extensive accumulations of bones from ornithopods drowned in floods, indicating that duck-billed dinosaurs often migrated in herds of thousands. A number of spectacularly preserved fossils found in parts of North America preserve skin impressions, traces of muscles and other soft tissues, and possibly evidence of internal organs. D Pachycephalosaurs Pachycephalosaurs were small bipedal ornithischians with thickened skulls, flattened bodies, and tails surrounded by a latticework of bony rods. In many of these dinosaurs, such as the Pachycephalosaurus--a large specimen up to 8 m (26 ft) long--the skull was capped by a rounded dome of solid bone. Some paleontologists suggest that males may have borne the thickest domes and butted heads during mating contests. Eroded pachycephalosaur domes are often found in stream deposits from late in the Cretaceous Period. E Ceratopsians The quadrupedal ceratopsians, or horned dinosaurs, typically bore horns over the nose and eyes, and had a saddle-shaped bony frill that extended from the skull over the neck. Some smaller early forms lacked the well-developed horns and frill, and were mainly bipedal, however. The bony frill was well developed in the late Cretaceous Triceratops, a dinosaur that could reach lengths of up to 8 m (26 ft) and weights of more than 12 metric tons. The frill served two purposes: It protected the vulnerable neck, and it contained a network of blood vessels on its undersurface to radiate excess heat. Large accumulations of fossil bones suggest that ceratopsians lived in herds. Ceratopsians found in Asia such as the 1.8-m (6-ft)-long Protoceratops did not reach the large size of North American forms. VII EXTINCTION Controversy surrounds the extinction of the dinosaurs. A traditional point of view held that dinosaurs were slowly driven to extinction by gradual and long-term environmental changes--changes that dinosaurs could not adapt to. Proponents of this theory postulated that dinosaurs dwindled in number and variety over several million years. Other causes once suggested for the end of the dinosaurs included epidemics or even egg-eating mammals. Another group of theories have tied the extinction of the dinosaurs to more sudden, catastrophic events. The best known of these theories proposes that the impact of an asteroid caused catastrophic destruction of the environment, leading to the extinction of the dinosaurs. Evidence to support this theory includes the discovery of a buried impact crater (thought to be the result of a large comet striking the earth) that is 200 km (124 mi) in diameter in the Yucatán Peninsula of Mexico. A spray of debris, called an ejecta sheet, which was blown from the edge of the crater, has been found over vast regions of North America. Minerals found in asteroids and meteorites enriched material from the impact's fiery explosion and were distributed all over the world. With radiometric dating (see Dating Methods: Radiometric Dating), scientists have used the decay rates of certain atoms to date the crater, ejecta sheet, and fireball layer. Using similar techniques to date the dramatic changes in the record of microscopic fossils, they have found that the impact and the dinosaur extinction occurred nearly simultaneously. Although large amounts of ash suggest that vast areas of North and South America were devastated by fire from the impact, the longer-term planetwide environmental effects of the impact were ultimately more lethal to life than the fire. Dust blocked sunlight from Earth's surface for many months. Scorched sulfur from the impact site, water vapor and chlorine from the oceans, and nitrogen from the air combined to produce a worldwide fallout of intensely acidic rain. Scientists postulate that darkness and acid rain caused plant growth to cease. As a result, both the herbivorous dinosaurs, which were dependent on plants for food, as well as the carnivorous dinosaurs, which fed on the herbivores, were exterminated. On the other hand, animals such as frogs, lizards, and small insect-eating turtles and mammals, which were dependent on organisms that fed on decaying plant material, were more likely to survive. Their survival indicates that, in most areas, the surface of Earth did not freeze. An alternative theory links the extinction of the dinosaurs to giant volcanic eruptions in a region of India called the Deccan Traps. These massive eruptions occurred repeatedly over a period of thousands of years around 65 million years ago, releasing large quantities of carbon dioxide and other gases into the atmosphere. Some of the proposed effects of the eruptions would have been similar to those of an asteroid impact, creating a harsh environment in which dinosaurs struggled to survive. Other scientists have suggested a combination of factors and events contributed to the demise of the dinosaurs. The asteroid impact may have been the final blow after climate change and volcanic eruptions had already reduced dinosaur numbers. Field work around the world continues to carefully document the sequence and scale of events that happened at the end of the Cretaceous period. Recent findings preserved in the sea off the coast of northern South America demonstrate that the extinctions did coincide with a single asteroid impact on Yucatán. VIII STUDYING DINOSAURS Fossilized dinosaur remains are usually buried in sediments deposited on land. These remains are likely to be found in regions where the silts and sands spread by rivers of the Mesozoic Era are exposed. Fossils are easier to find in arid badlands--rugged, rocky areas with little vegetation, where the sediments are not covered by soil. The excavation of large skeletal fossils involves painstaking procedures to protect the fossils from damage. Although thousands of dinosaur specimens have been collected to date, only a small percentage are complete or near complete skeletons. Probably less than 10 percent of the varieties of dinosaurs that once lived have been identified. The shape of dinosaur bones provides clues to how these animals interacted with each other. These bones also reveal information about body form, weight, and posture. Surface ridges and hollows on bones indicate the strength and orientation of muscles, and rings within the bones indicate growth rates. Diseased, broken, and bitten bones bear witness to the hazards of life during the dinosaurian age. Cavities in bones reflect the shape of the brain, spinal cord, and blood vessels. Delicate ossicles, or small bony structures in the skull, reveal the shape of the eyeball and its pupil. The structure of the skull and fossilized contents of the abdominal region provide clues to diet. Organic molecules are also preserved within bones in trace quantities. By studying isotopes of atoms within these molecules, scientists can gather evidence about bodyheat flow and about the food and water consumed by dinosaurs. Impressions in sediment depict skin texture and foot shape, and trackways provide evidence about speed and walking habits. A 113-million-year-old fossil called Scipionyx samniticus, discovered in southern Italy in the late 1980s, is the first fossil identified that clearly shows the structure and placement of internal organs, including the intestines, colon, liver, and muscles. The fossilized internal organs of Scipionyx samniticus provide paleontologists with information about how dinosaurs metabolized their food, as well as other general information about dinosaurs. Beginning in the late 19th century, the field of paleontology grew as scientific expeditions to find fossil remains became more frequent. American paleontologist Othniel Charles Marsh and his collectors explored the western United States for dinosaurian remains. They identified many genera that have since become household names, including Stegosaurus and Triceratops. In the early part of the 20th century, American paleontologists Barnum Brown and Charles Sternberg demonstrated that the area now known as Dinosaur Provincial Park in Alberta, Canada, is the richest site for dinosaur remains in the world. Philanthropist Andrew Carnegie sponsored excavations in the great Jurassic quarry in Utah, which subsequently became Dinosaur National Monument. Beginning in 1922, explorer Roy Chapman Andrews led expeditions to Mongolia that resulted in the discovery of dinosaur eggs. More recently, Luis Alvarez, a particle physicist and Nobel laureate, and his son, geologist Walter Alvarez, discovered evidence of the impact of an asteroid or comet that coincided with the extinction of the dinosaurs. Among foreign scholars, German paleontologist Werner Janensch, beginning in 1909, led well-organized dinosaur collecting expeditions to German East Africa (modern Tanzania), where the complete skeletal anatomy of the gigantic Brachiosaurus was documented. IX DINOSAUR DISCOVERIES AROUND THE WORLD One of the most important fossil-rich regions is located in China. In a group of sites about 400 km (about 200 mi) northeast of Beijing there are a series a fossil formations that have yielded many fossilized specimens of primitive birds and birdlike dinosaurs, as well as soft parts such as feathers and fur. Many of these specimens were preserved in lake sediments covered by volcanic ash. Some scientists believe these fossils provide evidence that birds may have evolved from dinosaurs. Among the recent finds in the Yixian formation is an eagle-sized animal with barracuda-like teeth and very long claws named Sinornithosaurus millenii. Although this dinosaur could not fly, it did have a shoulder blade structure that allowed a wide range of arm motion similar to flapping in birds. Featherlike structures covered most of the animal's body. Argentina is another area rich in fossils. In 1995 a local auto mechanic in Neuquén, a province in the Patagonia region on the eastern slopes of the Andes Mountains, found the fossils of Giganotosaurus, a meat-eating dinosaur that may have reached a length of more than 13 m (43 ft). In 2006 paleontologists reported discovering the fossil remains of another large meat-eating dinosaur species in the same Argentine province, near the town of Plaza Huincul. They named the species Mapusaurus roseae and estimated that it exceeded 12 m (40 ft) in length. The fossilized bones indicate that the dinosaur's jaw was shaped like scissors, suggesting it used its teeth to dissect prey. The sharp, blade-shaped teeth are typical of meat-eating dinosaurs. From the distribution of the fossil bones, paleontologists speculated that Mapusaurus roseae may have lived in packs. A pack-hunting strategy may have enabled the two-legged Mapusaurus roseae to prey on the four-legged Argentinosaurus, the largest-known plant-eating dinosaur, which lived in the same region. Argentinosaurus grew to a length of about 38 m (125 ft). New discoveries in North America include many skeletons of an early therizinosaur called Falcarius that lived during the early Cretaceous Period. The skeletons were found piled together at sites in Utah. Such "bone beds" allow scientists to study how dinosaurs grew and lived in groups. One of the largest known sauropods was discovered in Oklahoma in 1994. Named Sauroposeidon, this giraffe-like plant-eater also lived during the early Cretaceous and had vertebrae in its neck that were each 1.2 m (4 ft) long. Amateur paleontologists found two superbly preserved specimens of duck-billed dinosaurs that preserve soft anatomy and internal organs as well as skin. One specimen is an Edmontosaurus that was found in South Dakota in 1999 and nicknamed Dakota. The other fossil is a Brachylophosaurus that was found in Montana in 2000 and dubbed Leonardo. Scientists hope to learn much about dinosaur anatomy, physiology, and life style from these amazingly intact fossils. Other new insights into dinosaur physiology have come from soft tissues found inside the bones of a Tyrannosaurus from Montana. Scientists announced in 2005 that the 68-million-year-old tissues had not turned to stone and still contained blood vessels and cells. Calcium deposits within its thigh bone indicate that the animal was a female about to lay eggs. In early 2000 scientists used X rays to view the chest cavity of a dinosaur fossil found in South Dakota. Computerized three-dimensional imaging revealed the remains of what is thought to be the first example of a dinosaur heart ever discovered. The heart appears to contain four chambers with a single aorta, a structure that more closely resembles the heart of a bird or mammal than the heart of any living reptile. The structure of the heart suggests that the dinosaur may have had a high metabolic rate that is more like that of an active warm-blooded animal than that of a cold-blooded reptile. Another important dinosaur discovery in North America made in 1993 strengthens the evolutionary relationship between dinosaurs and birds. A 14-year-old boy who was hunting for fossils near Glacier National Park in northern Montana found a fossil of a nearly complete skeleton of a small dinosaur, later named Bambiraptor feinbergi. The fossil is of a juvenile dinosaur only 1 m (3 ft) long with a body that resembles that of a roadrunner. It has a number of physical features similar to those of early birds, including long, winglike arms, birdlike shoulders, and a wishbone. Some scientists propose that Bambiraptor feinbergi may be a type of dinosaur similar to those from which birds evolved. Other scientists believe that the animal lived too late in time to be ancestral to birds, while still other scientists hypothesize that dinosaurs may have given rise to flying dinosaurs ancestral to birds more than once in evolutionary time. Many important new dinosaur finds have been made in Europe, including in Portugal, Spain, France, Germany, and Italy. Late Jurassic dinosaurs very similar to those that lived in North America have been found in Portugal, including new species of Allosaurus and Stegosaurus. A dwarf sauropod called Europasaurus was found in Germany--its small size (only 6 m [20 ft] long) was likely the result of living on an island. A number of unusual dinosaur fossils found in the Sahara in North Africa appear to be related to dinosaur fossils discovered in South America, indicating that the two continents were connected through most of the dinosaurian period. Among the findings is another large meat-eating dinosaur known as Spinosaurus, which may have been larger than Giganotosaurus. These findings, along with other studies of the environments of dinosaurs and the plants and animals in their habitats, help scientists learn how the world of dinosaurs resembled and differed from the modern world. Surprising dinosaur finds have been made in Antarctica. Cryolophosaurus was a meat-eater with a strange curved crest on its skull and Glacialisaurus was a large prosauropod that weighed as much as 6 tons. Both dinosaurs were up to 7 m (25 ft) long and lived during the early Jurassic Period, when Antarctica lay further north and was not covered in ice. The fossils were found in the Transantarctic Mountains, where scientists dug out the bones under freezing conditions. Contributed By: Dale Alan Russell Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.