Giant Dinosaurs Academic Notes

GIANT DINOSAURS Dinosaurs reached their largest size and longest life span during the Jurassic. The largest dinosaurs occupied Gondwana, which covered all the southern continents. The Jurassic Morrison Formation, a famous sedimentary layer in the Colorado Plateau region, has yielded some of the largest dinosaur fossils. Many of the finest specimens are on display at Dinosaur National Monument near Vernal, Utah (Fig. 148). More than 500 genera of dinosaurs have been discovered over the past two centuries. Among the largest dinosaur species are sauropods. This giant creature fully deserves the title of "thunder lizard." They have long, slender tails and necks, and the front legs are generally longer than the back legs. Their fossils are found in Colorado and Utah, southwestern Europe, and East Africa, with some species possibly having traveled to Africa via Europe. Dinosaurs are often depicted as sluggish and unintelligent. But the fossil record shows that many species, such as velociraptors, are fast and intelligent. All dinosaurs were not giants, and many species were no larger than most mammals today. Protoceratops—a parrot-billed, shield-headed, dinosaur—and ankylosaurus, whose name means "rigid lizard"—an elephant-sized dinosaur heavily covered in protective bones and swinging a club-shaped tail—were very common and scattered around the world. Some large bipedal dinosaurs later returned to the four-foot position as their weight increased. They eventually evolved into giant, long-tailed, long-necked sauropods like apathosaurs (Figure 149), which belonged to the same dinosaur family as brontosaurs. Others, such as Tyrannosaurus rex, perhaps the fiercest land carnivore of all time, retained a permanent two-legged stance with strong hind legs, a muscular tail for counterweight, and arms shortened to almost useless appendages. However, Tyrannosaurus rex was at a disadvantage as a galloping predator. It was bipedaled with very strong legs, but its weak front legs could not break the fall. If prey of a 7-ton T. rex charging at a top speed of 25 miles per hour tripped and fell, the accident could be fatal. Most likely, dinosaurs traveled in hunting packs that surrounded their prey. This will require much less speed, resulting in fewer crashes. About 90 million years ago, when the continents became isolated and evolved separate fauna, Tyrannosaurus rex began roaming the American West. Oviraptor is a fleet-legged predator, whose name, which literally means "egg hunter," is a misnomer because it was originally thought to have invaded the nests of other dinosaurs. A fossil oviraptor was found sitting in a nest filled with as many as two dozen eggs neatly arranged in a circle, with the thinner end pointing outward. The dinosaur resembled a wingless version of an ostrich with a short neck and a long tail. It sits with its pelvis in the center of the nest and its long arms wrapped around the nest like a bird. The oviraptor probably protected the egg against a giant sandstorm that apparently engulfed and fossilized it along with its coupling 70 to 80 million years ago. The oviraptor is in the right position, a chicken will sit on the nest. Whether dinosaurs kept eggs warm like birds, shaded them from the scorching sun, or protected them is unknown. The clutch may be a communal nest similar to that used by ostriches, in which chickens deposit their eggs and take turns incubating them. Giant herbivores may have traveled in large groups, with the largest adults leading and juveniles placed in protective centers. Duckbilled hadrasaurs (Fig. 150), among the most successful of all dinosaur groups, reached 15 feet tall and lived in arctic regions. There they have to adapt to cold and darkness or migrate in large groups over long distances to warmer climates. Triceratops, whose large livestock roamed around the world towards the end of the Cretaceous and were among the last to leave during the extinction of the dinosaurs, may have contributed to the decline of other dinosaur species possibly due to extensive habitat destruction or the spread of disease. Fossils of western North American triceratops have been found in sediments deposited between 73 and 65 million years ago, when the region was covered by warm shallow seas. Parallel sets of footprints and bony beds with a large number of remains suggest that triceratops gathered in large flocks that may have migrated long distances. Instead of walking with a broad crocodile-like gait as originally thought, the animal walked upright similar to a rhinoceros. Maybe it can run as fast as one. It can also lock its knees like cows and horses when they sleep standing up. Dinosaurs were one of the dominant herbivores long before grass evolved. It has a very large head, weighs almost half a ton and uses its tortoise-like beak and knife-like teeth to graze on flowering shrubs, palm leaves, and small trees. Among the largest dinosaur species were apathosaurs and brachiosaurus, which lived about 100 million years ago. Paralititan, meaning "tidal giant," was the second largest dinosaur, measuring 100 feet long and weighing 75 tons. It resembled apathosaurs and lived in fertile coastal areas 94 million years ago. Perhaps the tallest and heaviest dinosaur so far discovered is the 80ton Ultrasaurus, which could tower over a five-story building. Seismosaurus, meaning "earthshaker," is the longest known dinosaur. It reaches more than 140 feet in length from its head, which is supported by a long, slender neck, to the tip of its longer whiplike tail. Giganotosaurus (Figure 151) even challenged Tyrannosaurus rex as the most ferocious terrestrial carnivore that ever lived. Large reptiles have an almost unlimited growth power. Adults never stop growing completely but keep getting bigger until disease, accidents, or predation take their lives. The giant Komodo dragon lizard in Southeast Asia, for example, grows to more than 300 pounds and preys on monkeys, pigs, and deer. Reptiles with continuous growth they reach youthful size, whereas mammals grow rapidly to adulthood and then slowly degenerate and die. The massive body allows the cold-blooded reptile to maintain its body temperature for a long time. Large bodies inhibit heat loss better than small ones because they have a better surface area to volume ratio. Thus, the animal is less susceptible to short-term temperature variations such as cold nights or cloudy days. In contrast, large reptiles take longer to warm up from a long cold period than small ones. Muscles also produce body heat, although for reptiles they produce only about a quarter of mammals during exertion. High stable body temperature maintains an efficient metabolism and increases muscle output. Therefore, the performance of some large dinosaurs may be able to match that of large mammals. The generally warm Mesozoic climate produced excellent growing conditions for lush vegetation, including ferns and cycads, to cater to the diet of planteating dinosaurs. Much information can be obtained about dinosaur diets by studying coprolites (Fig. 152), which are fecal masses preserved as fossils and are usually modular, tubular, or pellet shaped. Dinosaur coprolites could be very large, even bigger than a loaf of bread. Coprolites are often used to determine the eating habits of extinct animals. For example, the coprolites of herbivorous dinosaurs were black and blockshaped and were usually filled with plant material. Carnivorous dinosaurs were spindle-shaped and contained pieces of bone that were broken from eating other animals. Some species of dinosaurs swallowed rocky rocks called gizzard stones, similar to the sand used by modern birds to grind vegetation in their stomachs into mush. Polished cobblestones called gastroliths were left in piles where dinosaurs died. Sometimes deposits of these rocks are found on exposed Mesozoic sediments. Giant herbivorous dinosaurs such as apathosaurs and stegosaurs (Fig. 153) developed large stomachs to digest hard fibrous leaves, requiring very large bodies to carry. Dinosaurs grew into such giants, probably for the same reason that large ungulates like rhinos and elephants were so large. Most large dinosaurs were herbivores that consumed large amounts of coarse cellulose that took a lot of time to digest. The digestive juices further break down the coarse material, and the long fermentation process requires a large storage capacity. The sheer size of herbivores spurred the evolution of giant carnivorous dinosaurs to prey on them like Tyrannosaurus rex, perhaps the most ferocious carnivore of them all. Giant dinosaurs were prevented from growing larger due to the force of gravity. When an animal doubles its size, its bone weight is four times greater. The only exceptions are dinosaurs that lived permanently in the sea. As with modern whales, some of which were even larger than the largest dinosaurs, the buoyancy of seawater kept their bone weight down. If the animal accidentally strands itself, as whales sometimes do, it suffocates because its bones cannot support the weight of its body, crushing its lungs. Many large dinosaur families, including apathosaurs, stegosaurus, and allosaurus (Figure 154) became extinct by the end of the Jurassic.Allosaurus was a carnivorous dinosaur with a thick skull to withstand highimpact attacks on prey. The dinosaur would ambush its victim, slam it at high speed with its jaws wide open, and point its sharp teeth into the flesh. After the extinction of the great dinosaurs, the population of small animals exploded, as the species occupied niches vacated by large dinosaurs. Most of the surviving species are aquatic, confined to freshwater lakes and marshes, and also small animals living on land. Many small nondinosaur species are the same types that survived the extinction of dinosaurs at the end of the Cretaceous, possibly due to their large populations and ability to seek refuge during climatic disturbances.