Triassic Dinosaurs Lecture Notes

TRIASSIC DINOSAURS The chapter discusses the evolution of dinosaurs and continents during the Triassic period. The Triassic, from 250 to 210 million years ago, marked the beginning of the Mesozoic era. This period is named for the sequence of redbed and limestone strata in central Germany. All continents merge into one large continent called Pangaea, which supports a distinctive set of land plants and animals. In North America, continental sediments and redbeds add to the rugged beauty of Wyoming, Colorado, and Utah (Fig.130). By the end of the period, Pangea began to break up into current continents, and large amounts of basalt spilled into the landscape. During the late Triassic, large families of land animals died out in record numbers. The extinction lasted less than a million years and was responsible for killing nearly half the reptile family. The demise of species forever changed the character of life on Earth and initiated the emergence of dinosaurs (Figure 131), one of biology's greatest success stories. DINOSAUR ERA At the beginning of the Mesozoic, the continents were consolidated into supercontinents; At the midpoint, they begin to crack; And at the end of the day, they are us on our way to their current location. The breakup of Pangea created three new bodies of water, including the Atlantic, Arctic and Indian Oceans. The climate is very mild for a very long time. One particular group of animals that excels during these exceptional conditions are reptiles. In addition to conquering land, some species go to sea and others rise into the air, occupying almost every corner of the globe. Forests are also vast and cover most of the world with lush vegetation. In the Petrified Forests of Arizona lie the fossil remains of primitive Triassic conifers (Fig. 132) that once flourished in highland areas. At the beginning of the Triassic, Earth recovered from a major ice age and extinction event that claimed the lives of more than 95 percent of all species. Thus, the beginning of the Mesozoic marked the rebirth of life, and 450 new families of plants and animals appeared. However, instead of creating entirely new body plans as happened during the Cambrian explosion at the beginning of the Paleozoic, species developed new variations on established themes. Therefore, fewer experimental organisms appeared, and many species lines today evolved. Several major groups of terrestrial vertebrates made their debut. It consists of the direct ancestors of modern animals, including reptiles, mammals, and possibly the predecessors of birds. (True birds don't appear in the fossil record for another 50 million years.) Amphibians declined substantially when large swamps began to dry out towards the end of the Paleozoic. Amphibian populations continued to decline during the Mesozoic.All large flat-headed species became extinct by the beginning of the Triassic. The group after that is represented by more modern forms. Although amphibians did not achieve complete power over the land, their descendants of reptiles became the undisputed rulers of the world. Reptiles were the main form of animal life in the early Triassic. The unusually warm Mesozoic climate contributed greatly to the success of reptiles. However, not all reptiles are small creatures; Some grow upwards to a length of 16 feet. Reptiles are more suitable for continuous living on dry land. They successfully inhabit desert areas in the interior of the continent and other remote places where they could thrive on a small amount of food. Many reptiles walk on four legs but are raised on their hind legs when running down prey, thus freeing up the front legs to attack other animals. The earliest mammals saw their beginnings in Triassic.At early in the era of dinosaurs, mammals coexisting with large animals. At this time, mammals are small mouse-like creatures with little consequence. However, in the Mesozoic mammals experienced some remarkable adaptations that in many ways dwarfed their larger reptilian neighbors. Mammals developed special teeth, highly sensitive hearing and vision, and enlarged brains that would later help them outlive dinosaurs. Teeth are often the only remains of small extinct mammals. However, they reveal as much about animals as their diet over the past 220 million years. Chief among these early mammals were multituberculates, which first appeared in the upper Triassic 210 million years ago. Unfortunately, after surviving the extinction of the dinosaurs at the end of the Cretaceous, they all died out in the Eocene more than 30 million years ago. Insects can easily claim the title of the most prosperous creatures in the world. Since animals left the oceans and lived on dry land, insects and their arthropod relatives have taken control of the planet. Paleontologists found the missing chapter in insect evolution in an unusually rich fossil mine in southern Virginia, near the North Carolina border. The site yielded wellpreserved insects dating back to the late Triassic 210 million years ago, including the discovery of the oldest recorded records of several types of insects. Insects are difficult to fossilize because their delicate bodies tend to disintegrate at burial. Then insects are preserved in resin, which modified the sap of trees that bury animals and allow them to withstand the rigors of time. The oldest dinosaurs came from the southern continent of Gondwana when the last glaciers of the great Permian ice age set off and the region is still recovering from the frigid conditions. Dinosaurs are descended from thecodonts (Fig. 133), ancestors of crocodiles and birds, their distant living relatives. The earliest thecodonts were small to medium-sized predators that lived during the PermianTriassic transition. One group of thecodonts descended into the water and became large fish-eaters. They included phytosaurs, which died out in the Triassic, and crocodiles, which remain successful today. Pterosaurs are also descendants of the Triassic thecodonts. The appearance of feather-like scales ostensibly used for insulation suggests that thecodonts are also the ancestors of birds. Protofeathers helped trap body heat or served as colorful displays to attract mates like modern birds. By the end of the Triassic, dinosaurs replaced thecodonts as the dominant land vertebrates. Each dinosaur species can trace its origins back to one common ancestor called eoraptor, which means "early hunter." It evolved from predatory reptiles in the lower Triassic about 240 million years ago. Early in dinosaur evolution, the animals had to contend with large crocodiles, flying reptiles called pterodactyls, and other ferocious reptiles. In the middle Triassic about 230 million years ago, when mammal-like reptiles dominated the land, dinosaurs represented only a small fraction of all animals. Some species of reptiles that lived during the time of the early dinosaurs still far outnumber them. However, in just 10 million years, dinosaurs quickly became the dominant species. They evolved from medium-sized animals less than 20 feet long to the giants that made them famous. Dinosaurs emerged as the dominant terrestrial species over the next 150 million years. At the end of the Jurassic, dinosaurs were the largest predators to ever roam Earth.Their fossils were found in relative abundance in Triassic to Cretaceous formations (Fig. 134). Dinosaurs traveled to all major continents. Their distribution around the world is strong evidence for continental drift (Table 8). By the time dinosaurs appeared, all continents had gathered to become the supercontinent Pangaea. At the beginning of the Jurassic, it began to crack, and the continents drifted towards their current location. Except for a few temporary land bridges, the ocean filling the gap between the newly formed continents provided a barrier to further dinosaur migration. At the moment, almost identical species live in North America, Europe and Africa. The success of dinosaurs is exemplified by their wide range. They occupy a wide range of habitats and dominate all other forms of land animals. Indeed, had dinosaurs not gone extinct, mammals would never have achieved dominance over Earth and humans would not have existed. This is because dinosaurs would have kept pressing for further advances of mammals, which would have remained small, nocturnal creatures, guarding out from under the dinosaurs' feet. Dinosaurs are classified as sauropods or carnosaurs. Sauropods are long-necked herbivores. Carnosaurs were bipedal carnivores that probably hunted sauropods in groups. Camptosaurs (Figure 135), the ancestors of many later dinosaur species, were herbivores up to 25 feet long. However, not all dinosaurs were giant. Many are about the size of modern mammals. Protoceratops and ankylosaurs were no larger than the largest land mammals alive today. They were very common and ranged in a large space like modern sheep. The smallest known dinosaur footprint is only the size of a penny. Smaller dinosaurs had hollow bones similar to birds. Some have long, slender hind legs; long and smooth front legs; and a long neck. If it were not for the long tail, their skeleton would have been very similar to modern ostriches. Many early small dinosaurs were raised on their hind legs and were among the first animals to establish a successful permanent two-legged stance. They are descendants of diapsid reptiles, which run on two legs, the fastest way to travel. Diapsid is one of the most primitive reptile lineages. They gave rise to the most living dinosaurs, birds, and reptiles, including crocodiles, lizards, and snakes. Bipedalism increases speed and agility. It also frees up the front legs for foraging and other tasks. The hind legs and hips support the entire weight of the animal, while the large tail compensates for the upper part of the body. Dinosaurs walked like birds. Therefore, dinosaurs were classified as ornithischians with bird-like pelvis or as saurischians with lizard-like pelvis. Ornithischians probably arose from the same group of thecodont reptiles that gave rise to crocodiles and birds. Indeed, birds are the only surviving relatives of dinosaurs. The skeletons of many small dinosaurs resemble those of birds (Fig. 136). Many large bipedaled dinosaurs assumed the same appearance simply because of the need to balance themselves on two legs, which required a large tail and small arms. A twolegged sprint will also be the fastest way to travel to hit prey. Some dinosaur species may have acquired a certain level of body temperature control similar to mammals and birds. When the age of dinosaurs began, the climate of southern Africa and the tip of South America where early dinosaurs roamed experienced winters, during which large, cold-blooded animals could not survive without migrating to warmer regions. The stamina required for such a long-distance migration would require a sustained level of energy that only a warm-blooded body could provide. Warm-blooded animals mature faster than cold-blooded reptiles, which continue to grow steadily until death. Analysis of the bones of dinosaurs, crocodiles, and birds, all of which share a common ancestor, shows similarities between bird and dinosaur bones—another sign of possible warmbloodedness. The complex social behavior of dinosaurs that required a high metabolic rate appears to be an evolutionary advance resulting from warm-bloodedness. But at the end of the Cretaceous, when the climate was thought to have grown colder, warmblooded mammals survived while dinosaurs did not. Carnivorous dinosaurs were cunning and aggressive, attacking prey with unusual gluttony. The cranial capacity of some carnivores suggests that they had relatively large brains and were quite intelligent (Fig. 137), capable of reacting to various environmental stresses. Velociraptor, which means "fast hunter," has sharp claws and powerful jaws. They are vicious killing machines whose voracious appetite shows that they are warm-blooded. Many species of dinosaurs were fast and agile, requiring a high metabolic rate that only a warmblooded body could provide. The anatomy of some dinosaurs implies that they were active, upright, and constantly eating, behaviors that exhibit a high metabolic rate commensurate with warm-blooded animals. A comparison of the ratio of oxygen 18 to oxygen 16 in dinosaur teeth with cold-blooded crocodiles shows dinosaurs had a constant body temperature regardless of outside temperature. They breathed with diaphragm-driven lungs for extra respiration necessary for rapid movement like modern mammals. However, when at rest, they may have switched to rib-based respiration similar to reptiles, giving them metabolic functions unlike any living animal. Comparison of pelvic bone density in fossil dinosaur embryos with those of modern crocodiles and birds shows that juvenile dinosaurs were highly mobile, requiring energy that only warm-blooded bodies could provide. Evidence for rapid juvenile growth, which is common among mammals, is also present in the bones of some dinosaur species, possibly providing another sign of warm-bloodedness. The newborn dinosaur did not seem to be a helpless dependent like most birds, but jumped from its shell, ready to run and protect itself from the dangerous world. Baby dinosaurs had the bone and muscle power to move quickly and run away from danger just like modern young crocodiles do. However, the discovery of the Maiasaura fossil, which literally means "good mother lizard," suggests that adult dinosaurs were gentle, keeping parents whose cubs remained in their nests for feeding and protection. The density of dinosaur bone blood vessels is higher than that of living mammals, another characteristic feature of warm-bloodedness. Some dinosaur skulls show signs of sinus membranes, which are present only in warm-blooded animals. Respiratory turbinates, which act as common thermal exchangers in the nostrils of warm-blooded species, appear to have been present in dinosaurs as well. Bones of carnivorous dinosaurs found in Antarctica suggest that they adapted to cold and dark by warm-blooded or migrated to nearby South America across land bridges. The dinosaur tracks (Figure 138) are the most impressive of all fossil footprints because the large weight of many species produces deep indentations in the ground. Their footprints exist in relative abundance in Mesozoic age terrestrial sediments around the world. Studies of dinosaur tracks show that some species were very gregarious and gathered in groups. Large carnivores such as Tyrannosaurus rex (Figure 139), whose name means "monstrous lizard," were fast and agile predators that could run as fast as horses in their tracks. Females of some dinosaur species may have had live births like mammals. Many nurture and vigorously protect their young until they can fend for themselves, allowing larger numbers to mature into adults, thus ensuring the continuation of the species. Parents may have brought food to their children and regurgitated seeds and berries just like modern birds. Parental care for these babies shows strong social bonds and may explain why dinosaurs were so successful for so long. Some dinosaurs may have developed complex mating rituals. In addition to regulating her body temperature, a large screen on the dimetrodon's back may have been used to attract females. Other dinosaurs may have worn elaborate headgear like present-day birds for the same purpose. Dinosaurs may have used complex vocal resonance devices for mating calls. They seem to produce sounds with cartilage oscillating in their throats. The larger the cartilage the lower the frequency of sound, allowing some species to make sounds somewhat like foghorns, which are carried for long distances. One 75-million-year-old herbivore had a 4.5-footlong hollow crest that curved backwards from the skull. It was probably used as a sexual display device or as a resonating chamber to produce unusual sound effects. The period between the end of the Triassic and the beginning of the Jurassic is one of the most interesting times in the history of land vertebrates. At the end of the Triassic, all continents were joined only at their western end with Laurasia in the north and Gondwana in the south. The land bridge connecting Laurasia to the Indochinese microcontinent may be the last link, allowing animal migration as two blocks collide as the Triassic draws to a close. At the end of the Triassic, about 210 million years ago, a large meteorite slammed into Earth, creating a 60-mile-wide Manicouagan impact structure in Quebec, Canada (Fig. 140). The giant explosion apparently coincided with a mass extinction over a period of less than a million years that killed 20 percent or more of all animal families, including nearly half the reptile family. In the ocean, ammonoids and bivalves disintegrate, and conodonts completely disappear. Extinction forever changed the character of life on Earth and paved the way for the rise of dinosaurs. Almost all modern animal groups, including amphibians, reptiles, and mammals, debuted at the current stage of evolution. This was also when dinosaurs achieved dominance over Earth and defended their land for the next 145 million years. After that, another huge meteorite seemed to hit Earth with the explosive force of 100 trillion tons of dynamite, turning the planet into an inhospitable world. Thus, dinosaurs were probably created and destroyed by meteorites.