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Post by Melissa Foxworthy on Dec 2, 2007 17:57:19 GMT -5
Definition: "Archeology" is an alternate spelling for Archaeology. Both spellings are accepted by most scholars today, although the print version of the Oxford English Dictionary (OED) insists on 'archæology', with the ligature in the middle. The origins of the word archeology are found in Old English, and derived from the Greek 'arkhaios' meaning ancient. The OED says that the first occurrence of the word 'archæology' was in 1607, in English bishop and satirist Joseph Hall's Holy Observations Lib. I., in which Hall wrote, "God loveth adverbs", whatever the heck that means. At any rate, during Hall's time the vowel sound in the middle of archæology would have been a flat a, as in, well, flat. During the Great Vowel Shift, the flat "a" shifted to an "ee" sound. An attempt was made in the 20th century to simplify the spelling to 'archeology', but archaeologists, being stodgy and fond of old things, still cling to the old spelling. Now, you probably came here for a definition of archeology, and not a linguistic side track. Archeology is the study of the human past, including everything from yesterday's garbage in the landfill to the impressions of footprints in the mud at Laetoli by our ancestor Australopithecus. Whether studied in a classics department as part of ancient history, or in an anthropology department as part of human cultures, then, archeology is always about people and our immediate ancestors, and never about dinosaurs, "intelligent design", or space aliens.Alternate Spellings: archaeologyExamples: In other languages, archeology is spelled: archéologie (French), 考古学 (simplified Chinese), Archäologie (German), археология (Russian), arqueología (Spanish), archeologia (Italian), 고고학 (Korean), and αρχαιολογία (Greek).The first discoveries The clade Dinosauria was originally defined by Sir Richard Owen in 1842, in a two hour speech that reportedly held the audience captivated. The original dinosaurs of this new group were Megalosaurus, Iguanodon and Hylaeosaurus. However, each of these animals was known only from fragmentary specimens. It wasn't until the discoveries of dinosaurs in North America in the mid-19th century that people began to get a clearer picture of what dinosaurs looked like.It is generally accepted that the first discovery of dinosaur remains in North America was made in 1854 by Ferdinand Vandiveer Hayden during his exploration of the upper Missouri River.Near the confluence of the Judith and Missouri Rivers (shown above) Hayden's party recovered a small collection of isolated teeth which were later described by the Philadelphia paleontologist Joseph Leidy in 1856, in the Proceedings of the Academy of Natural Sciences of Philadelphia.An important, more complete specimen A short two years later, Leidy had the honor of describing the first reasonably complete dinosaur skeleton the world would know, Hadrosaurus foulkii. Named after its discoverer William Parker Foulke, this specimen was recovered during quarrying of a sand pit in Haddonfield, New Jersey.The real significance of this specimen was in its limb proportions. For the first time scientists studying these animals could see that some dinosaurs were bipedal, walking on two legs instead of on all fours. Bipedalism was a revolutionary thought for a reptilian posture.This specimen, now on display at the Academy of Natural Sciences of Philadelphia, was originally mounted in a free-standing bipedal pose by Benjamin Waterhouse Hawkins in 1868.The records for attendance at the Academy show that visitation increased three-fold with this new exhibit, testimony to the public's long-standing and intense fascination with dinosaurs.For many years, Hadrosaurus foulkii was the only dinosaur on public display. Duplicate casts of the skeleton were made for other institutions both in the United States and in Europe. The dinosaur feudsDinosaur skeletons were found for the first time in abundance in the Garden Park area of Colorado and at Como Bluff, Wyoming, in the late 1870s. These specimens initiated the First Great Dinosaur Rush in North America, driven largely by the efforts of a Philadelphia paleontologist, Edward Drinker Cope (on the left), and Othniel Marsh (on the right), a paleontologist from Yale University.These two men started as friends but became bitter rivals in a feud of legendary proportions. The stories surrounding these two include tales of armed field parties, spies, and intercepting shipments of fossils intended for the other. Many of these stories have been exaggerated with time but they clearly point out the bitterness of this rivalry. As a result of the tremendous collecting efforts of these two paleontologists, the public became aware of the fascinating world of the Late Jurassic, and was presented the opportunity to know the largest of all dinosaurs, the sauropods.The Second Great Dinosaur Rush took place in the badlands of the Red Deer River in southern Alberta. Dinosaur remains had been known from this region as early as 1884 but it wasn't until 1910 that this region became an active collecting area. It was here that the second great collecting rivalry took place between Barnum Brown of the American Museum of Natural History in New York and C.H. Sternberg of the Geological Survey of Canada. However, unlike the rivalry between Cope and Marsh, this rivalry resembled more of a friendly competition. Also unlike the earlier efforts, the result of this collecting provided insights into the world of the Late Cretaceous.Diplodocus carnegii. Artwork © 1998 Michael Skrepnick The DinosauriaDinosaurs, one of the most successful groups of animals (in terms of longevity) that have ever lived, evolved into many diverse sizes and shapes, with many equally diverse modes of living. The term "Dinosauria" was invented by Sir Richard Owen in 1842 to describe these "fearfully great reptiles," specifically Megalosaurus, Iguanodon, and Hylaeosaurus, the only three dinosaurs known at the time. The creatures that we normally think of as dinosaurs lived during the Mesozoic Era, from late in the Triassic period (about 225 million years ago) until the end of the Cretaceous (about 65 million years ago). But we now know that they actually live on today as the birds. Running Deinonychus. © 1995 B. Cunningham Some things to keep in mind about dinosaurs:• Not everything big and dead is a dinosaur. All too often, books written (or movies made) for a popular audience include animals such as mammoths, mastodons, pterosaurs, plesiosaurs, ichthyosaurs, and the sail-backed Dimetrodon. Dinosaurs are a specific subgroup of the archosaurs, a group that also includes crocodiles, pterosaurs, and birds. although pterosaurs are close relations, they are not true dinosaurs. Even more distantly related to dinosaurs are the marine reptiles, which include the plesiosaurs and ichthyosaurs. Mammoths and mastodons are mammals and did not appear until many millions of years after the close of the Cretaceous period. Dimetrodon is neither a reptile nor a mammal, but a basal synapsid, i.e., an early relative of the ancestors of mammals.• Not all dinosaurs lived at the same time. Different dinosaurs lived at different times. Despite the portrayals in movies like King Kong and Jurassic Park, no Stegosaurus ever saw a Tyrannosaurus, because Tyrannosaurus didn't appear on the scene until 80 or so million years following the extinction of stegosaurs. The same goes for Apatosaurus ("Brontosaurus") — it's bones were already well-fossilized by the time T. rex came along.• Dinosaurs are not extinct. Technically. Based on features of the skeleton, most people studying dinosaurs consider birds to be dinosaurs. This shocking realization makes even the smallest hummingbird a legitimate dinosaur. So rather than refer to "dinosaurs" and birds as discrete, separate groups, it is best to refer to the traditional, extinct animals as "non-avian dinosaurs" and birds as, well, birds, or "avian dinosaurs." It is incorrect to say that dinosaurs are extinct, because they have left living descendants in the form of thingyatoos, cassowaries, and their pals — just like modern vertebrates are still vertebrates even though their Cambrian ancestors are long extinct. Surely ever since the first fossils of obviously extinct animals were found, humankind has wondered: "Why did they die?" A poignant question, for it has relevance to us — if extinct animals were wiped out by some catastrophe, couldn't that just as easily happen to us? Could we be found as fossils someday, and would no one know why we died?History: Until recently, people simply knew that dinosaurs went extinct — their fossils were found throughout the Mesozoic era, but were not located in the rock layers (strata) of the Cenozoic era. So, we knew that dinosaurs went extinct some 64-66 million years ago, but that was all. Many wild ideas about how the dinosaurs were rendered extinct were presented over the years.1980: Few satisfactory answers to the mystery behind the extinction of dinosaurs were offered until 1980, when a group of scientists at the University of California at Berkeley — Luis and Walter Alvarez, Frank Asaro, and Helen Michel — proposed a stunning and convincing mechanism for the "K-T extinction" (meaning the extinction of dinosaurs at the boundary between the Cretaceous period (K) and the Tertiary period (T)). This hypothesis is discussed later. Since the Alvarez hypothesis was first proposed, the search for the "perpetrator" of the K-T extinction has been a thriving area of scientific research. It incorporates scientists from many different fields including astrophysics, astronomy, geology, paleontology, ecology, geochemistry, and so on. The mystery has drawn extensive media coverage over the last 15 years, as you may know; some paleontologists have since lost interest in the issue, preferring to study how the dinosaurs and their contemporaries lived rather than why they died.Mass Extinctions: But before we dive into the complex issue of the K-T extinction, we need essential background information to understand the basics of the controversy. The "great dying," as it is sometimes called, is an example of a mass extinction: an episode in evolutionary history where more than 50% of all known species living at that time went extinct in a short period of time (less than 2 million years or so).Other Mass Extinctions? We know of several mass extinctions in the history of life; the great dying is not nearly the largest! The largest would be the "Permo-Triassic" extinction, between the Permian and Triassic periods, of the Paleozoic and Mesozoic eras. In this obviously catastrophic event, life on Earth nearly was wiped out — an estimated 90% of all species living at that time were extinguished. We are fairly sure that the extinction was due to many changing global conditions at that time, but even that is not solved yet. The issue has not received much press because the dinosaurs were not involved, but another familiar group, the trilobites, were wiped out among others.Who Died? How does the K-T extinction compare to this debacle? Well, about 60% of all species that are present below the K-T boundary are not present above the line that divides the "Age of Dinosaurs" and the "Age of Mammals." In fact, dinosaurs were not among the most numerous of the casualties — the worst hit organisms were those in the oceans. Large groups of organisms, including some members of Foraminifera, Echinodermata, Mollusca, and the marine Diapsida all were devastated by the K-T event. On land, the Dinosauria of course went extinct, along with the Pterosauria. Mammals and most non- dinosaurian reptiles seemed to be relatively unaffected. The terrestrial plants suffered to a large extent, except for the ferns, which show an apparently dramatic increase in diversity at the K-T boundary, a phenomenon known as the fern spike.ComplicationsNow we're heading into the tough stuff; the reasons why we have no conclusive answer to the mystery of the K-T event. Several complications that make work hard for the scientist/detectives trying to crack this case:The Fossil Record: It's not perfect, as you may know; that's why paleontologists keep finding new fossils: so much is hidden in the rocks! Most data on the K-T event comes from North America, which is one of the few areas known that has a somewhat continuous fossil record (remember, fossils are only formed under certain rare conditions, and are only found in sedimentary rocks). The infamous Hell Creek locality in Montana is one such continuous site enclosing the K-T boundary. UCMP researchers have led and continue to lead expeditions to Hell Creek, gathering fossils from the rich fossil beds. The secret to the K-T event may lie within our collections; who knows! Anyway, we don't know much about what was occurring in the rest of the world at the time of the K-T event. The marine fossil record gives us great hints about what was occurring within the sea, but how applicable is that to what went on in the terrestrial realm?The Nature of Extinction: Extinction is not a simple event; it is not simply the death of all representatives of a group. It is the cessation of the origination of new species that renders a group extinct; if species are constantly dying off and no new ones originate through the process of evolution, then that group will go extinct over time no matter what happens. New dinosaur species ceased to originate around the K-T boundary; the question is, were they killed off (implying causation, especially a catastrophe), or were they not evolving and simply fading away (perhaps implying gradual environmental change)?Time Resolution: Determining the age of rocks or fossils that are millions of years old is not easy; carbon dating only has a reasonable resolution when used with organic material that is less than about 50,000 years old, so it is useless with the 65 million year old K-T material. Other methods of age determination are often less accurate or less useful in certain situations. So we don't know exactly when the dinosaurs went extinct, and matching events precisely to give a picture of what was happening at a specific moment in the Mesozoic is not easy. Thus, the ultimate question of a gradual decline of dinosaurs vs. a sudden cataclysm is almost intractable without a wealth of good data.Reconstruction: To truly understand the situation of the dinosaurs around the K-T boundary, we need to understand the paleoecology of that time on Earth. Paleoecology is an extension of the discipline of ecology, attempting to understand the interactions of organisms with their environment, using geological (the rocks tell you what the soil was like, and thus tell a lot about the abiotic (non-living) environment) and paleontological (what plants and animals are found as fossils tell you a lot about the biotic (living) environment) evidence. With the problems of the fossil record and time resolution, it is difficult to understand the paleoecology of a region at a specific time in the past.The Signor-Lipps Effect: Proposed by Phil Signor and UCMP's own Jere Lipps, this concept helps us to understand the limitations of the fossil record. The theory states that groups of organisms may seem to go extinct in the fossil record before they actually do; this is an artifact of the fickle nature of the fossil record rather than actual extinction. Thus, it is possible that some groups of organisms did not go extinct at the K-T boundary, and also possible that some organisms that seemed to have gone extinct earlier may have survived up to the boundary, and then gone extinct. This matter further complicates the important issue of the selectivity of the K-T extinction (discussed later).Falsifiability: Sad but true: many hypotheses about dinosaur extinction sound quite convincing and might even be correct, but, as you know, are not really science if they cannot be proven or disproved. Even with the best hypothesis, such as the impact hypothesis, it is very difficult to prove or disprove whether the dinosaurs were rendered extinct by an event that occurred around the K-T boundary, or whether they were just weakened (or unaffected) by the event. This is not to say that all extinction hypotheses are not science; many are excellent examples of good science, but a linkage of direct causation is a problem. "Why" questions, such as "Why did the dinosaurs die out?" or "Why did dinosaurs evolve?" are among the most difficult questions in paleontology. Ultimately, a time machine would be required to see exactly what killed the dinosaurs.The first Alaskan North Slope dinosaur bones were discovered in the mid-1980s. They were from the "duck-billed" Edmontosaurus, which as fully-grown adults could reach 10 feet tall, 40 feet long, and weigh three tons. These plant-eaters are thought to have lived in social groups or even herds. The question of how they survived so far north initially raised two possibilities: either they stayed in the north and perhaps lived by slowing their metabolism (maybe even hibernating), or they migrated southward to continuous food supplies and warmer climates.Newer discoveries along the Colville River have thrown doubt on the migration theory. Several of the newer dinosaur types, including Troodon and Dromaeosaurus (both smaller flesh-eaters) as well as juvenile hadrosaurs, probably could not have physically migrated a round-trip distance estimated at more than 5,000 miles (8,000 km). This distance would have been greater than that covered by today's caribou which migrate less than half distance within a 250-mile-wide area.Instead, the North Slope dinosaurs may have survived year-round in ancient long-gone river systems which supported lush summer vegetation. Enough seasonal plant matter may have grown during the 24-hour sunlit summers to last during the cool-to-cold dark days of winter (though not as harsh as today's North Slope winters). The plant-eating dinosaurs, in turn, would have been the over-wintering food source of the meat-eaters.While recent discoveries have made it more difficult to present a universally agreed-upon list of dinosaurs' distinguishing features, nearly all dinosaurs discovered so far share certain modifications to the ancestral archosaurian skeleton. Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical across Dinosauria; the earliest dinosaurs had them and passed them on to all their descendants. Such common structures across a taxonomic group are called synapomorphies.
Dinosaur synapomorphies include an elongated crest on the humerus, or upper arm bone, to accommodate the attachment of deltopectoral muscles; a shelf at the rear of the ilium, or main hip bone; a tibia, or shin bone, featuring a broad lower edge and a flange pointing out and to the rear; and an ascending projection on the astragalus, one of the ankle bones, which secures it to the tibia.
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Post by Melissa Foxworthy on Dec 2, 2007 17:58:29 GMT -5
Edmontonia was an "armored dinosaur" of the group Ankylosauria.A variety of other skeletal features were shared by many dinosaurs. However, because they were either common to other groups of archosaurs or were not present in all early dinosaurs, these features are not considered to be synapomorphies. Such shared features include a diapsid skull bearing two pairs of holes in the temporal region; holes in the snout and lower jaw (two characteristics shared by other archosaurs); loss of the skull's postfrontal bone; a long neck incorporating an S-shaped curve;[11] an elongated scapula, or shoulder blade; forelimbs shorter and lighter than hind limbs, coupled to asymmetrical hands; a sacrum composed of three or more fused vertebrae; and an acetabulum, or hip socket, with a hole at the center of its inside surface.[12]
The open, or "perforate", hip joint described above had significant implications for dinosaur movement and behavior. Most notably, it allowed dinosaur hind limbs to be "underslung", or situated directly beneath the animals' bodies; this, in turn, allowed dinosaurs to stand erect in a manner similar to modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side.[13] Vertical limb configuration also enabled dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of "sprawling" reptiles.
Phylogenetic definition Under phylogenetic taxonomy, dinosaurs are usually defined as all descendants of the most recent common ancestor of Triceratops and modern birds.[14] It has also been suggested that Dinosauria be defined as all the descendants of the most recent common ancestor of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria.[15] They are divided into Ornithischia (bird-hipped) and Saurischia (lizard-hipped), depending upon pelvic structure. Ornithischian dinosaurs had a four-pronged pelvic configuration, incorporating a caudally-directed (rear-pointing) pubis bone with (most commonly) a forward-pointing process. By contrast, the pelvic structure of saurischian dinosaurs was three-pronged, and featured a pubis bone directed cranially, or forwards, only.[13] Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia, while Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia.
Stegosaurus skeleton, Field Museum, Chicago.There is an almost universal consensus among paleontologists that birds are the descendants of theropod dinosaurs. Using the strict cladistical definition that all descendants of a single common ancestor are related, modern birds are dinosaurs and dinosaurs are, therefore, not extinct. Modern birds are classified by most paleontologists as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs.[16]
However, referring to birds as 'avian dinosaurs' and to all other dinosaurs as 'non-avian dinosaurs' is cumbersome. Birds are still referred to as birds, at least in popular usage and among ornithologists. It is also technically correct to refer to birds as a distinct group under the older Linnaean classification system, which accepts paraphyletic taxa that exclude some descendants of a single common ancestor. Paleontologists mostly use cladistics, which classifies birds as dinosaurs, but some biologists of the older generation do not.
For clarity, this article will use 'dinosaur' as a synonym for 'non-avian dinosaur', and 'bird' as a synonym for 'avian dinosaur' (meaning any animal that evolved from the common ancestor of Archaeopteryx and modern birds). The term 'non-avian dinosaur' will be used for emphasis as needed.
Natural history
Origins and early evolution Dinosaurs diverged from their archosaur ancestors approximately 230 million years ago during the Middle to Late Triassic period, roughly 20 million years after the Permian-Triassic extinction event wiped out an estimated 95% of all life on Earth.[17] [18] Radiometric dating of the rock formation that contained fossils from the early dinosaur genus Eoraptor establishes its presence in the fossil record at this time. Paleontologists believe Eoraptor resembles the common ancestor of all dinosaurs;[19] if this is true, its traits suggest that the first dinosaurs were small, bipedal predators.[20] The discovery of primitive, dinosaur-like ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators.
When dinosaurs appeared, terrestrial habitats were occupied by various types of basal archosaurs and therapsids, such as aetosaurs, cynodonts, dicynodonts, ornithosuchids, rauisuchias, and rhynchosaurs. Most of these other animals became extinct in the Triassic, in one of two events. First, at about the boundary between the Carnian and Norian faunal stages (about 215 million years ago), dicynodonts and a variety of basal archosauromorphs, including the prolacertiforms and rynchosaurs, became extinct. This was followed by the Triassic-Jurassic extinction event (about 200 million years ago), that saw the end of most of the other groups of early archosaurs, like aetosaurs, ornithosuchids, phytosaurs, and rauisuchians. These losses left behind a land fauna of crocodylomorphs, dinosaurs, mammals, pterosaurians, and turtles.[10]
The first few lines of primitive dinosaurs diversified through the Carnian and Norian stages of the Triassic, most likely by occupying the niches of groups that became extinct. Traditionally, dinosaurs were thought to have replaced the variety of other Triassic land animals by proving superior through competition. This now appears unlikely, for several reasons. Early considerations of dinosaur evolution had dinosaurs as polyphyletic, with multiple groups of unrelated "dinosaurs" evolving due to similar pressures, but dinosaurs are now known to have formed a single group. Early conceptions also had a long, drawn-out period of competition beginning in the Middle Triassic, but more work has shown that dinosaurs did not appear that early and had a sudden diversification. Dinosaurs do not show a pattern of steadily increasing in diversity and numbers, as would be predicted if they were competitively replacing other groups; instead, they were very rare through the Carnian, making up only 1-2% of individuals present in faunas. In the Norian, however, after the extinction of several other groups, they became significant components of faunas, representing 50-90% of individuals. Also, what had been viewed as a key adaptation of dinosaurs, their erect stance, is now known to have present in several contemporaneous groups that were not as successful (aetosaurs, ornithosuchids, rauisuchians, and some groups of crocodylomorphs). Finally, the Late Triassic itself was a time of great upheaval in life, with shifts in plant life, marine life, and climate.[10]
Classification Main article: Dinosaur classification Dinosaurs (including birds) are archosaurs, like modern crocodilians. Archosaurs' diapsid skulls have two holes, called temporal fenestrae, located where the jaw muscles attach. Most reptiles (including birds) are diapsids; mammals, with only one temporal fenestra, are called synapsids; and turtles, with no temporal fenestra, are anapsids. Anatomically, dinosaurs share many other archosaur characteristics, including teeth that grow from sockets rather than as direct extensions of the jawbones. Within the archosaur group, dinosaurs are differentiated most noticeably by their gait. Dinosaur legs extend directly beneath the body, whereas the legs of lizards and crocodylians sprawl out to either side. All dinosaurs were land animals.
Many other types of reptiles lived at the same time as the dinosaurs. Some of these are commonly, but incorrectly, thought of as dinosaurs, including plesiosaurs (which are not closely related to the dinosaurs) and pterosaurs, which developed separately from reptilian ancestors in the late Triassic period.
Collectively, dinosaurs are usually regarded as a superorder or an unranked clade. They are divided into two orders, the Saurischia and the Ornithischia, on the basis of their hip structure. Saurischians ('lizard-hipped', from the Greek sauros (σαυρος) meaning 'lizard' and ischion (ισχιον) meaning 'hip joint') are dinosaurs that originally retained the hip structure of their ancestors. They include all the theropods (bipedal carnivores) and sauropods (long-necked herbivores). Ornithischians ('bird-hipped', from the Greek ornitheios (ορνιθειος) meaning 'of a bird' and ischion (ισχιον) meaning 'hip joint') is the other dinosaurian order, most of which were quadrupedal herbivores. (NB: the terms "lizard hip" and "bird-hip" are misnomers — birds evolved from dinosaurs with "lizard hips".)
Dinosaur evolution after the Triassic follows changes in vegetation and the location of continents. In the Late Triassic and Early Jurassic, the continents were connected as the single landmass Pangaea, there was a worldwide dinosaur fauna mostly composed of coelophysoid predators and prosauropod herbivores.[21] Gymnosperm plants (particularly conifers), a potential food source, radiated in the Late Triassic. Prosauropods did not have sophisticated mechanisms for processing food in the mouth, so must have employed other means of breaking down food farther along the digestive tract.[22] The general homogeneity of dinosaurian faunas continued into the Middle and Late Jurassic, where most localities had predators consisting of ceratosaurians, spinosauroids, and carnosaurians, and herbivores consisting of stegosaurian ornithischians and large sauropods. Examples of this include the Morrison Formation of North America and Tendaguru Beds of Tanzania. Dinosaurs in China show some differences, with specialized sinraptorid theropods and unusual, long-necked sauropods like Mamenchisaurus.[21] Ankylosaurians and ornithopods were also becoming more common, but prosauropods had become extinct. Conifers and pteridophytes were the most common plants. Sauropods, like the earlier prosauropods, were not oral processors, but ornithischians were evolving various means of dealing with food in the mouth, including potential cheek-like organs to keep food in the mouth, and jaw motions to grind food.[22] Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from maniraptoran coelurosaurians.[16]
By the Early Cretaceous and the ongoing breakup of Pangaea, dinosaurs were becoming strongly differentiated by landmass. The earliest part of this time saw the spread of ankylosaurians, iguanodontians, and brachiosaurids through Europe, North America, and northern Africa. These were later supplemented or replaced in Africa by large spinosaurid and carcharodontosaurid theropods, and rebbachisaurid and titanosaurian sauropods, also found in South America. In Asia, maniraptoran coelurosaurians like dromaeosaurids, troodontids, and oviraptorosaurians became the common theropods, and ankylosaurids and early ceratopsians like Psittacosaurus became important herbivores. Meanwhile, Australia was home to a fauna of basal ankylosaurians, hypsilophodonts, and iguanodontians [21] The stegosaurians appear to have gone extinct at some point in the late Early Cretaceous or early Late Cretaceous. A major change in the Early Cretaceous, which would be amplified in the Late Cretaceous, was the evolution of flowering plants. At the same time, several groups of dinosaurian herbivores evolved more sophisticated ways to orally process food. Ceratopsians developed a method of slicing with teeth stacked on each other in batteries, and iguanodontians refined a method of grinding with tooth batteries, taken to its extreme in hadrosaurids.[22] Some sauropods also evolved tooth batteries, best exemplified by the rebbachisaurid Nigersaurus.[23]
There were three general dinosaur faunas in the Late Cretaceous. In the northern continents of North America and Asia, the major theropods were tyrannosaurids and various types of smaller maniraptoran theropods, with a predominantly ornithischian herbivore assemblage of hadrosaurids, ceratopsians, ankylosaurids, and pachycephalosaurians. In the southern continents that had made up the now-splitting Gondwana, abelisaurids were the common theropods, and titanosaurian sauropods the common herbivores. Finally, in Europe, dromaeosaurids, rhabdodontid iguanodontians, nodosaurid ankylosaurians, and titanosaurian sauropods were prevalent.[21] Flowering plants were greatly radiating,[22] with the first grasses appearing by the end of the Cretaceous.[24] Grinding hadrosaurids and shearing ceratopsians became extremely diverse across North America and Asia. Theropods were also radiating as herbivores or omnivores, with therizinosaurians and ornithomimosaurians becoming common.[22]
The Cretaceous–Tertiary extinction event, which occurred approximately 65 million years ago at the end of the Cretaceous period, caused the extinction of all dinosaurs except for the line that had already given rise to the first birds. Some other diapsid groups, such as crocodylians, lizards, snakes, sphenodontians, and choristoderans, also survived the event.[25]
Study of dinosaurs Knowledge about dinosaurs is derived from a variety of fossil and non-fossil records, including fossilized bones, feces, trackways, gastroliths, feathers, impressions of skin, internal organs and soft tissues.[26][27] Many fields of study contribute to our understanding of dinosaurs, including physics, chemistry, biology, and the earth sciences (of which paleontology is a sub-discipline). Two topics of particular interest and study have been dinosaur size and behavior.
Size Comparative size of Diplodocus; human figures provide scale.Main article: Dinosaur size While the evidence is incomplete, it is clear that, as a group, dinosaurs were large. Even by dinosaur standards, the sauropods were gigantic. For much of the dinosaur era, the smallest sauropods were larger than anything else in their habitat, and the largest were an order of magnitude more massive than anything else that has since walked the Earth. Giant prehistoric mammals such as the Indricotherium and the Columbian mammoth were dwarfed by the giant sauropods, and only a handful of modern aquatic animals approach or surpass them in size — most notably the Blue whale, which reaches up to 173,000 kilograms (381,400 lb) and over 30 meters (98 ft) in length.[28]
Most dinosaurs, however, were much smaller than the giant sauropods. Current evidence suggests that dinosaur average size varied through the Triassic, early Jurassic, late Jurassic and Cretaceous periods.[29] Theropod dinosaurs, when sorted by estimated weight into categories based on order of magnitude, most often fall into the 100 kilograms (220 lb) to 1,000 kilograms (2,205 lb) category, whereas Recent predatory carnivorans peak in the 10 kilograms (22 lb) to 100 kilograms (220 lb) category.[30] A rough estimate for average dinosaur weight is about 100 kilograms (220 lb). This contrasts sharply with the size of Cenozoic mammals, estimated by the same source (the National Museum of Natural History) as about 2 kilograms (4 lb) to 5 kilograms (11 lb).[31]
Largest and smallest dinosaurs Only a tiny percentage of animals ever fossilize, and most of these remain buried in the earth. Few of the specimens that are recovered are complete skeletons, and impressions of skin and other soft tissues are rare. Rebuilding a complete skeleton by comparing the size and morphology of bones to those of similar, better-known species is an inexact art, and reconstructing the muscles and other organs of the living animal is, at best, a process of educated guesswork. As a result, scientists will probably never be certain of the largest and smallest dinosaurs.
Comparative size of Brachiosaurus. Comparative size of Eoraptor.The tallest and heaviest dinosaur known from good skeletons is Brachiosaurus brancai (also known as Giraffatitan). Its remains were discovered in Tanzania between 1907–12. Bones from multiple similarly-sized individuals were incorporated into the skeleton now mounted and on display at the Humboldt Museum of Berlin;[32] this mount is 12 meters (39 ft) tall and 22.5 meters (74 ft) long, and would have belonged to an animal that weighed between 30,000 kilograms (66,139 lb) to 60,000 kilograms (132,277 lb). The longest complete dinosaur is the 27 m (89 ft) long Diplodocus, which was discovered in Wyoming in the United States and displayed in Pittsburgh's Carnegie Natural History Museum in 1907.
There were larger dinosaurs, but knowledge of them is based entirely on a small number of fragmentary fossils. Most of the largest herbivorous specimens on record were all discovered in the 1970s or later, and include the massive Argentinosaurus, which may have weighed 80,000 kilograms (176,370 lb) to 100,000 kilograms (220,462 lb); the longest, the 40 meters (131 ft)) long Supersaurus; and the tallest, the 18 meters (59 ft) Sauroposeidon, which could have reached a sixth-floor window. The longest of them all may have been Amphicoelias fragillimus, known only from a now lost partial vertebral neural arch described in 1878. Extrapolating from the illustration of this bone, the animal may have been 58 meters (190 ft) long and weighed over 120,000 kilograms (264,555 lb),[33] heavier than all known dinosaurs except possibly the poorly known Bruhathkayosaurus, which could have weighed 175,000 kilograms (385,809 lb) to 220,000 kilograms (485,017 lb). The largest known carnivorous dinosaur was Spinosaurus, reaching a length of 16 meters (52 ft) to 18 meters (59 ft), and weighing in at 8,150 kilograms (17,968 lb).[34] Other large meat-eaters included Giganotosaurus, Mapusaurus, Tyrannosaurus rex and Carcharodontosaurus.
Not including modern birds, the smallest dinosaurs known were about the size of a crow or a chicken. The theropods Microraptor and Parvicursor were both under 0.6 meters (2 ft) in length.
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Post by Melissa Foxworthy on Dec 2, 2007 18:00:57 GMT -5
Interpretations of dinosaur behavior are generally based on the pose of body fossils and their habitat, computer simulations of their biomechanics, and comparisons with modern animals in similar ecological niches. As such, the current understanding of dinosaur behavior relies on speculation, and will likely remain controversial for the foreseeable future. However, there is general agreement that some behaviors which are common in crocodiles and birds, dinosaurs' closest living relatives, were also common among dinosaurs.
The first direct evidence of herding behavior was the 1878 discovery of 31 Iguanodon dinosaurs which were thought to have perished together in Bernissart, Belgium, after they fell into a deep, flooded sinkhole and drowned.[35] Despite the deposition of those skeletons being now regarded as more gradual,[36] other, well supported, mass death sites were subsequently discovered. Those, along with multiple trackways, suggest that herd or pack behavior was common in many dinosaur species. Trackways of hundreds or even thousands of herbivores indicate that duck-bills (hadrosaurids) may have moved in great herds, like the American Bison or the African Springbok. Sauropod tracks document that these animals traveled in groups composed of several different species, at least in Oxford, England,[37] and others kept their young in the middle of the herd for defense according to trackways at Davenport Ranch, Texas. Dinosaurs may have congregated in herds for defense, for migratory purposes, or to provide protection for their young.
Fossilized egg of the oviraptorid Citipati, American Museum of Natural History.Jack Horner's 1978 discovery of a Maiasaura ("good mother dinosaur") nesting ground in Montana demonstrated that parental care continued long after birth among the ornithopods.[38] There is also evidence that other Cretaceous-era dinosaurs, like the Patagonian sauropod Saltasaurus (1997 discovery), had similar nesting behaviors, and that the animals congregated in huge nesting colonies like those of penguins. The Mongolian oviraptorid Citipati was discovered in a chicken-like brooding position in 1993, which may mean it was covered with an insulating layer of feathers that kept the eggs warm.[39] Trackways have also confirmed parental behavior among sauropods and ornithopods from the Isle of Skye in northwestern Scotland.[40] Nests and eggs have been found for most major groups of dinosaurs, and it appears likely that dinosaurs communicated with their young, in a manner similar to modern birds and crocodiles.
Artist's rendering of two Centrosaurus, herbivorous ceratopsid dinosaurs from the late Cretaceous fauna of North America.The crests and frills of some dinosaurs, like the marginocephalians, theropods and lambeosaurines, may have been too fragile to be used for active defense, so they were likely used for sexual or aggressive displays, though little is known about dinosaur mating and territorialism. The nature of dinosaur communication also remains enigmatic, and is an active area of research. For example, recent evidence suggests that the hollow crests of the lambeosaurines may have functioned as resonance chambers used for a wide range of vocalizations.
From a behavioral standpoint, one of the most valuable dinosaur fossils was discovered in the Gobi Desert in 1971. It included a Velociraptor attacking a Protoceratops,[41] proving that dinosaurs did indeed attack and eat each other. While cannibalistic behavior among theropods is no surprise,[42] this too was confirmed by tooth marks from Madagascar in 2003.[43]
Based on current fossil evidence from dinosaurs such as Oryctodromeus cubicularis, some herbivorous species seem to have led a partially fossorial (burrowing) lifestyle, and some bird-like species may have been arboreal (tree-climbing), most notably primitive dromaeosaurids such as Microraptor and the enigmatic scansoriopterygids. Since the later mammalian radiation in the Cenozoic produced numerous burrowing and tree-climbing species, e.g., rodents and primates, the lack of evidence for a similar widespread radiation of species among the dinosaurs is somewhat surprising. Because most dinosaur species seem to have relied on land-based locomotion, a good understanding of how dinosaurs moved on the ground is key to models of dinosaur behavior; the science of biomechanics, in particular, has provided significant insight in this area. For example, studies of the forces exerted by muscles and gravity on dinosaurs' skeletal structure have investigated how fast dinosaurs could run,[44] whether diplodocids could create sonic booms via whip-like tail snapping,[45] and whether sauropods could float.[46]
Areas of controversy
Physiology Main article: Physiology of dinosaurs Tyrannosaurus rex skull and upper vertebral column, Palais de la Découverte, Paris.A vigorous debate on the subject of temperature regulation in dinosaurs has been ongoing since the 1960s. Originally, scientists broadly disagreed as to whether dinosaurs were capable of regulating their body temperatures at all. More recently, dinosaur endothermy has become the consensus view, and debate has focused on the mechanisms of temperature regulation.
After dinosaurs were discovered, paleontologists first posited that they were ectothermic creatures: "terrible lizards" as their name suggests. This supposed cold-bloodedness implied that dinosaurs were relatively slow, sluggish organisms, comparable to modern reptiles, which need external sources of heat in order to regulate their body temperature. Dinosaur ectothermy remained a prevalent view until Robert T. "Bob" Bakker, an early proponent of dinosaur endothermy, published an influential paper on the topic in 1968.
Modern evidence indicates that dinosaurs thrived in cooler temperate climates, and that at least some dinosaur species must have regulated their body temperature by internal biological means (perhaps aided by the animals' bulk). Evidence of endothermism in dinosaurs includes the discovery of polar dinosaurs in Australia and Antarctica (where they would have experienced a cold, dark six-month winter), the discovery of dinosaurs whose feathers may have provided regulatory insulation, and analysis of blood-vessel structures that are typical of endotherms within dinosaur bone. Skeletal structures suggest that theropods and other dinosaurs had active lifestyles better suited to an endothermic cardiovascular system, while sauropods exhibit fewer endothermic characteristics. It is certainly possible that some dinosaurs were endothermic while others were not. Scientific debate over the specifics continues.[47]
Complicating the debate is the fact that warm-bloodedness can emerge based on more than one mechanism. Most discussions of dinosaur endothermy tend to compare them to average birds or mammals, which expend energy to elevate body temperature above that of the environment. Small birds and mammals also possess insulation, such as fat, fur, or feathers, which slows down heat loss. However, large mammals, such as elephants, face a different problem because of their relatively small ratio of surface area to volume (Haldane's principle). This ratio compares the volume of an animal with the area of its skin: as an animal gets bigger, its surface area increases more slowly than its volume. At a certain point, the amount of heat radiated away through the skin drops below the amount of heat produced inside the body, forcing animals to use additional methods to avoid overheating. In the case of elephants, they are hairless, and have large ears which increase their surface area, and have behavioral adaptations as well (such as using the trunk to spray water on themselves and mud wallowing). These behaviors increase cooling through evaporation.
Large dinosaurs would presumably have had to deal with similar issues; their body size suggest they lost heat relatively slowly to the surrounding air, and so could have been what are called inertial homeotherms, animals that are warmer than their environments through sheer size rather than through special adaptations like those of birds or mammals. However, so far this theory fails to account for the vast number of dog- and goat-sized dinosaur species which made up the bulk of the ecosystem during the Mesozoic Era.
Soft tissue and DNA One of the best examples of soft tissue impressions in a fossil dinosaur was discovered in Petraroia, Italy. The discovery was reported in 1998, and described the specimen of a small, very young coelurosaur, Scipionyx samniticus. The fossil includes portions of the intestines, colon, liver, muscles, and windpipe of this immature dinosaur.[26]
In the March 2005 issue of Science, Dr. Mary Higby Schweitzer and her team announced the discovery of flexible material resembling actual soft tissue inside a 68-million-year-old Tyrannosaurus rex leg bone from the Hell Creek Formation in Montana. After recovery, the tissue was rehydrated by the science team.[27]
When the fossilized bone was treated over several weeks to remove mineral content from the fossilized bone marrow cavity (a process called demineralization), Schweitzer found evidence of intact structures such as blood vessels, bone matrix, and connective tissue (bone fibers). Scrutiny under the microscope further revealed that the putative dinosaur soft tissue had retained fine structures (microstructures) even at the cellular level. The exact nature and composition of this material, and the implications of Dr. Schweitzer's discovery, are not yet clear; study and interpretation of the material is ongoing.[27]
The successful extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions, but upon further inspection and peer review, neither of these reports could be confirmed.[48] However, a functional visual peptide of a theoretical dinosaur has been inferred using analytical phylogenetic reconstruction methods on gene sequences of related modern species such as reptiles and birds.[49] In addition, several proteins have putatively been detected in dinosaur fossils,[50] including hemoglobin.[51]
Even if dinosaur DNA could be reconstructed, it would be exceedingly difficult to clone and "grow" dinosaurs using current technology since no closely related species exist to provide zygotes or a suitable environment for embryonic development.
Feathered dinosaurs and the origin of birds Main article: Feathered dinosaurs Main article: Origin of birds Birds and non-avian dinosaurs share many features. Birds share over a hundred distinct anatomical features with theropod dinosaurs, which are generally accepted to have been their closest ancient relatives.[52]
Feathers The famous Berlin Specimen of Archaeopteryx lithographica.Archaeopteryx, the first good example of a "feathered dinosaur", was discovered in 1861. The initial specimen was found in the Solnhofen limestone in southern Germany, which is a lagerstätte, a rare and remarkable geological formation known for its superbly detailed fossils. Archaeopteryx is a transitional fossil, with features clearly intermediate between those of modern reptiles and birds. Brought to light just two years after Darwin's seminal On the Origin of Species, its discovery spurred the nascent debate between proponents of evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was mistaken for Compsognathus.[53]
Since the 1990s, a number of additional feathered dinosaurs have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. Most of these specimens were unearthed in Liaoning province, northeastern China, which was part of an island continent during the Cretaceous period. Though feathers have been found only in the lagerstätte of the Yixian Formation and a few other places, it is possible that non-avian dinosaurs elsewhere in the world were also feathered. The lack of widespread fossil evidence for feathered non-avian dinosaurs may be due to the fact that delicate features like skin and feathers are not often preserved by fossilization and thus are absent from the fossil record.
A recent development in the debate centers around the discovery of impressions of "protofeathers" surrounding many dinosaur fossils. Said protofeathers suggest that the tyrannosauroids may have been feathered.[54] However, others claim that these protofeathers are simply the result of the decomposition of collagenous fiber that underlaid the dinosaurs' integument.[55]
The feathered dinosaurs discovered so far include Beipiaosaurus, Caudipteryx, Dilong, Microraptor, Protarchaeopteryx, Shuvuuia, Sinornithosaurus, Sinosauropteryx, and Jinfengopteryx. Dinosaur-like birds like Confuciusornis, which are anatomically closer to modern avians, have also been discovered. All of these specimens come from the same formation in northern China. The dromaeosauridae family in particular seems to have been heavily feathered, and at least one dromaeosaurid, Cryptovolans, may have been capable of flight.
Skeleton Because feathers are often associated with birds, feathered dinosaurs are often touted as the missing link between birds and dinosaurs. However, the multiple skeletal features also shared by the two groups represent the more important link for paleontologists. Furthermore, it is increasingly clear that the relationship between birds and dinosaurs, and the evolution of flight, are more complex topics than previously realized. For example, while it was once believed that birds evolved from dinosaurs in one linear progression, some scientists, most notably Gregory S. Paul, conclude that dinosaurs such as the dromaeosaurs may have evolved from birds, losing the power of flight while keeping their feathers in a manner similar to the modern ostrich and other ratites.
Theropods, a diverse group of carnivorous dinosaurs that included Tyrannosaurus rex, are generally accepted to have been birds' closest relatives.Comparison of bird and dinosaur skeletons, as well as cladistic analysis, strengthens the case for the link, particularly for a branch of theropods called maniraptors. Skeletal similarities include the neck, pubis, wrist (semi-lunate carpal), arm and pectoral girdle, shoulder blade, clavicle and breast bone.
Reproductive biology A discovery of features in a Tyrannosaurus rex skeleton recently provided even more evidence that dinosaurs and birds evolved from a common ancestor and, for the first time, allowed paleontologists to establish the sex of a dinosaur. When laying eggs, female birds grow a special type of bone in their limbs. This medullary bone, which is rich in calcium, forms a layer inside the hard outer bone that is used to make eggshells. The presence of endosteally-derived bone tissues lining the interior marrow cavities of portions of the Tyrannosaurus rex specimen's hind limb suggested that T. rex used similar reproductive strategies, and revealed the specimen to be female.[56]
A dinosaur embryo (pertaining to the prosauropod Massospondylus) was found without teeth, indicating that some parental care was required to feed the young dinosaur.[57] It is also possible that the adult dinosaurs regurgitated into a young dinosaur's mouth to provide sustenance, a behavior that is also characteristic of numerous modern bird species.
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Post by Melissa Foxworthy on Dec 2, 2007 18:01:47 GMT -5
Lungs Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to an investigation which was led by Patrick O'Connor of Ohio University. The lungs of theropod dinosaurs (carnivores that walked on two legs and had birdlike feet) likely pumped air into hollow sacs in their skeletons, as is the case in birds. "What was once formally considered unique to birds was present in some form in the ancestors of birds", O'Connor said. The study was funded in part by the National Science Foundation.[58]
Model of Microraptor, a four-winged dinosaur with long pennaceous feathers. Heart and sleeping posture Modern computerized tomography (CT) scans of a dinosaur chest cavity (conducted in 2000) found the apparent remnants of complex four-chambered hearts, much like those found in today's mammals and birds.[59] The idea is controversial within the scientific community, coming under-fire for bad anatomical science[60] or simply wishful thinking.[61] A recently discovered troodont fossil demonstrates that the dinosaurs slept like certain modern birds, with their heads tucked under their arms.[62] This behavior, which may have helped to keep the head warm, is also characteristic of modern birds.
Gizzard Another piece of evidence that birds and dinosaurs are closely related is the use of gizzard stones. These stones are swallowed by animals to aid digestion and break down food and hard fibres once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths.[63]
Extinction Main article: Cretaceous–Tertiary extinction event Main article: K–T boundary Non-avian dinosaurs suddenly became extinct approximately 65 million years ago. Many other groups of animals also became extinct at this time, including ammonites (nautilus-like mollusks), mosasaurs, plesiosaurs, pterosaurs, herbivorous turtles and crocodiles, most birds, and many groups of mammals.[64] This mass extinction is known as the Cretaceous–Tertiary extinction event. The nature of the event that caused this mass extinction has been extensively studied since the 1970s; at present, several related theories are supported by paleontologists. Though the general consensus is that an impact event was the primary cause of dinosaur extinction, some scientists cite other possible causes, or support the idea that a confluence of several factors was responsible for the sudden disappearance of dinosaurs from the fossil record.
At the peak of the dinosaur era, there were no polar ice caps, and sea levels are estimated to have been from 100 meters (328 ft) to 250 meters (820 ft) higher than they are today. The planet's temperature was also much more uniform, with only +25 °C (77 °F) separating average polar temperatures from those at the equator. On average, atmospheric temperatures were also much warmer; the poles, for example, were +50 °C (122 °F) warmer than today.[65][66]
The atmosphere's composition during the dinosaur era was vastly different as well. Carbon dioxide levels were up to 12 times higher than today's levels, and oxygen formed 32 to 35% of the atmosphere, as compared to 21% today. However, by the late Cretaceous, the environment was changing dramatically. Volcanic activity was decreasing, which led to a cooling trend as levels of atmospheric carbon dioxide dropped. Oxygen levels in the atmosphere also started to fluctuate and would ultimately fall considerably. Some scientists hypothesize that climate change, combined with lower oxygen levels, might have led directly to the demise of many species. If the dinosaurs had respiratory systems similar to those commonly found in modern birds, it may have been particularly difficult for them to cope with reduced respiratory efficiency, given the enormous oxygen demands of their very large bodies.[64]
Impact event The Chicxulub Crater at the tip of the Yucatán Peninsula, the impact of which may have caused the dinosaur extinction.The asteroid collision theory, which was first proposed by Walter Alvarez in the late 1970s, links the extinction event at the end of the Cretaceous period to a bolide impact approximately 65.5 million years ago. Alvarez proposed that a sudden increase in iridium levels, recorded around the world in the period's rock stratum, was direct evidence of the impact. The bulk of the evidence now suggests that a 5 kilometers (3 mi) to 15 kilometers (9 mi) wide bolide hit in the vicinity of the Yucatán Peninsula, creating the 170 kilometers (106 mi) wide Chicxulub Crater and triggering the mass extinction. Scientists are not certain whether dinosaurs were thriving or declining before the impact event. Some scientists propose that the meteorite caused a long and unnatural drop in Earth's atmospheric temperature, while others claim that it would have instead created an unusual heat wave.
Although the speed of extinction cannot be deduced from the fossil record alone, various models suggest that the extinction was extremely rapid. The consensus among scientists who support this theory is that the impact caused extinctions both directly (by heat from the meteorite impact) and also indirectly (via a worldwide cooling brought about when matter ejected from the impact crater reflected thermal radiation from the sun).
In September of 2007, U.S. researchers led by William Bottke of the Southwest Research Institute in Boulder, Colorado, and Czech scientists used computer simulations to identify the probable source of the Chicxulub impact. They calculated a 90% probability that a giant asteroid named Baptistina (about 160 kilometres (100 miles) in diameter) orbiting in the asteroid belt which lies between Mars and Jupiter, was struck by a smaller unnamed asteroid about 55 kilometers (34 mi) in diameter about 160 million years ago. The impact shattered Baptistina, creating a cluster which still exists today as the Baptistina family. Calculations indicate that some of the fragments were sent hurtling into earth-crossing orbits, one of which was the 10 kilometers (6 mi) wide meteorite which struck Mexico's Yucatan peninsula 65 million years ago, creating the Chicxulub crater (175 kilometers (109 mi)). The researchers also calculated a 70% probability that an earlier-arriving fragment (108 million years BP) struck the Moon, creating the Tycho crater. Philippe Claeys of Vrije Universiteit Brussel in Belgium stated that the findings were "clear evidence that the solar system is a violent environment and that collisions taking place in the asteroid belt can have major repercussions for the evolution of life on Earth."[67]
While similar to Alvarez's impact theory (which involved a single asteroid or comet), this theory proposes that "passages of the solar companion star Nemesis through the Oort comet cloud would trigger comet showers."[68] One or more of these objects then collided with the Earth at approximately the same time, causing the worldwide extinction. As with the impact of a single asteroid, the end result of this comet bombardment would have been a sudden drop in global temperatures, followed by a protracted cool period.[68]
Deccan Traps Main article: Deccan Traps Before 2000, arguments that the Deccan Traps flood basalts caused the extinction were usually linked to the view that the extinction was gradual, as the flood basalt events were thought to have started around 68 mya and lasted for over 2 million years. However, there is evidence that two-thirds of the Deccan Traps were created in 1 million years about 65.5 mya, so these eruptions would have caused a fairly rapid extinction, possibly a period of thousands of years, but still a longer period than what would be expected from a single impact event.[69][70]
The Deccan Traps could have caused extinction through several mechanisms, including the release of dust and sulphuric aerosols into the air which might have blocked sunlight and thereby reducing photosynthesis in plants. In addition, Deccan Trap volcanism might have resulted in carbon dioxide emissions which would have increased the greenhouse effect when the dust and aerosols cleared from the atmosphere.[70] Before the mass extinction of the dinosaurs, the release of volcanic gasses during the formation of the Deccan traps "contributed to an apparently massive global warming. Some data point to an average rise in temperature of +8 °C (46.4 °F) in the last half million years before the impact [at Chicxulub]."[71]
In the years when the Deccan Traps theory was linked to a slower extinction, Luis Alvarez (who died in 1988) replied that paleontologists were being misled by sparse data. While his assertion was not initially well-received, later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely or at least partly due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level and massive volcanic eruptions that produced the Indian Deccan Traps, and these may have contributed to the extinctions.[72]
Possible Paleocene dinosaurs Nonavian dinosaur remains are occasionally found above the K-T boundary In 2002, paleontologists Zielinski and Budahn reported the discovery of a single hadrosaur leg bone fossil in the San Juan Basin, New Mexico and described it as evidence of Paleocene dinosaurs. The formation in which the bone was discovered has been dated to the early Paleocene epoch approximately 64.5 million years ago. If the bone was not re-deposited into that stratum by weathering action, it would provide evidence that some dinosaur populations may have survived at least a half million years into the Cenozoic Era.[73] Other evidence includes the finding of dinosaur remains in the Hell Creek Formation up to 1.3 metres above (40,000 years later than) the K-T boundary. Similar reports have come from other parts of the world, including China.[74] Many scientists, however, dismiss the "Paleocene dinosaurs" as re-worked, i.e. washed out of their original locations and then re-buried in much later sediments,[75][76] or find that, if correct, the presence of a handful of dinosaurs in the early Paleocene would not change the underlying facts of the extinction.[75]
History of discovery Dinosaur fossils have been known for millennia, although their true nature was not recognized. The Chinese, whose modern word for dinosaur is konglong (恐龍, or "terrible dragon"), considered them to be dragon bones and documented them as such. For example, Hua Yang Guo Zhi, a book written by Zhang Qu during the Western Jin Dynasty, reported the discovery of dragon bones at Wucheng in Sichuan Province.[77] Villagers in central China have been digging up dinosaur bones for decades, thinking they were from dragons, to make traditional medicine.[78] In Europe, dinosaur fossils were generally believed to be the remains of giants and other creatures killed by the Great Flood.
William Buckland.Megalosaurus was the first dinosaur to be formally described, in 1677, when part of a bone was recovered from a limestone quarry at Cornwell near Oxford, England. This bone fragment was identified correctly as the lower extremity of the femur of an animal larger than anything living in modern times. The second dinosaur genus to be identified, Iguanodon, was discovered in 1822 by the English geologist Gideon Mantell, who recognized similarities between his fossils and the bones of modern iguanas. Two years later, the Rev William Buckland, a professor of geology at Oxford University, unearthed more fossilized bones of Megalosaurus and became the first person to describe dinosaurs in a scientific journal.
The study of these "great fossil lizards" soon became of great interest to European and American scientists, and in 1842 the English paleontologist Richard Owen coined the term "dinosaur". He recognized that the remains that had been found so far, Iguanodon, Megalosaurus and Hylaeosaurus, shared a number of distinctive features, and so decided to present them as a distinct taxonomic group. With the backing of Prince Albert of Saxe-Coburg-Gotha, the husband of Queen Victoria, Owen established the Natural History Museum in South Kensington, London, to display the national collection of dinosaur fossils and other biological and geological exhibits.
In 1858, the first known American dinosaur was discovered, in marl pits in the small town of Haddonfield, New Jersey (although fossils had been found before, their nature had not been correctly discerned). The creature was named Hadrosaurus foulkii. It was an extremely important find; Hadrosaurus was the first nearly complete dinosaur skeleton found and it was clearly a bipedal creature. This was a revolutionary discovery as, until that point, most scientists had believed dinosaurs walked on four feet, like other lizards. Foulke's discoveries sparked a wave of dinosaur mania in the United States.
Othniel Charles Marsh, 19th century photograph. Edward Drinker Cope, 19th century photograph.Dinosaur mania was exemplified by the fierce rivalry between Edward Drinker Cope and Othniel Charles Marsh, both of whom raced to be the first to find new dinosaurs in what came to be known as the Bone Wars. The feud probably originated when Marsh publicly pointed out that Cope's reconstruction of an Elasmosaurus skeleton was flawed; Cope had inadvertently placed the plesiosaur's head at what should have been the animal's tail end. The fight between the two scientists lasted for over 30 years, ending in 1897 when Cope died after spending his entire fortune on the dinosaur hunt. Marsh 'won' the contest primarily because he was better funded through a relationship with the US Geological Survey. Unfortunately, many valuable dinosaur specimens were damaged or destroyed due to the pair's rough methods; for example, their diggers often used dynamite to unearth bones (a method modern paleontologists would find appalling). Despite their unrefined methods, the contributions of Cope and Marsh to paleontology were vast; Marsh unearthed 86 new species of dinosaur and Cope discovered 56, for a total of 142 new species. Cope's collection is now at the American Museum of Natural History in New York, while Marsh's is on display at the Peabody Museum of Natural History at Yale University.[79]
Since 1897, the search for dinosaur fossils has extended to every continent, including Antarctica. The first Antarctic dinosaur to be discovered, the ankylosaurid Antarctopelta oliveroi, was found on Ross Island in 1986, although it was 1994 before an Antarctic species, the theropod Cryolophosaurus ellioti, was formally named and described in a scientific journal.
Current dinosaur "hot spots" include southern South America (especially Argentina) and China. China in particular has produced many exceptional feathered dinosaur specimens due to the unique geology of its dinosaur beds, as well as an ancient arid climate particularly conducive to fossilization.
The "dinosaur renaissance" Main article: Dinosaur renaissance The field of dinosaur research has enjoyed a surge in activity that began in the 1970s and is ongoing. This was triggered, in part, by John Ostrom's discovery of Deinonychus, an active, vicious predator that may have been warm-blooded, in marked contrast to the then-prevailing image of dinosaurs as sluggish and cold-blooded. Vertebrate paleontology, arguably the primary scientific discipline involved in dinosaur research, has become a global science. Major new dinosaur discoveries have been made by paleontologists working in previously unexploited regions, including India, South America, Madagascar, Antarctica, and most significantly in China (the amazingly well-preserved feathered dinosaurs in China have further consolidated the link between dinosaurs and their conjectured living descendants, modern birds). The widespread application of cladistics, which rigorously analyzes the relationships between biological organisms, has also proved tremendously useful in classifying dinosaurs. Cladistic analysis, among other modern techniques, helps to compensate for an often incomplete and fragmentary fossil record.
Dinosaurs in culture Main article: Cultural depictions of dinosaurs A Megalosaurus stalks Crystal Palace Park in London.By human standards, dinosaurs were creatures of fantastic appearance and often enormous size. As such, they have captured the public imagination and become an enduring part of human culture. Only three decades after the first scientific descriptions of dinosaur remains, the famous dinosaur sculptures were erected in Crystal Palace Park in London. These sculptures excited the public so strongly that smaller replicas were sold, one of the first examples of tie-in merchandising. Since Crystal Palace, dinosaur exhibitions have opened at parks and museums around the world, both catering to, and reinforcing, the public interest.[80] Dinosaur popularity has long had a reciprocal effect on dinosaur science, as well. The competition between museums for public attention led directly to the Bone Wars waged between Marsh and Cope, each striving to return with more spectacular fossil remains than the other, and the resulting contribution to dinosaur science was enormous.[81]
Dinosaurs hold an integral place in modern culture. The word "dinosaur" itself has entered the English lexicon as an expression describing anything that is impractically large, slow-moving, or obsolete, bound for extinction.[82] The public preoccupation with dinosaurs led to their inevitable entrance into worldwide popular culture. Beginning with a passing mention of Megalosaurus in the first paragraph of Charles thingyens' Bleak House in 1852,[83] dinosaurs have been featured in a broad array of fictional works. Sir Arthur Conan Doyle's 1912 book The Lost World, the iconic 1933 film King Kong, the 1954 introduction of Godzilla and its many subsequent sequels, the best-selling 1990 novel Jurassic Park by Michael Crichton and its 1993 film version, briefly the highest-grossing film of all time, are just a few prominent examples of the long tradition of dinosaurs in fiction. Non-fiction authors, including some prominent paleontologists, have also sought to take advantage of dinosaur popularity, especially among children, to educate readers about dinosaurs in particular and science in general. Dinosaurs are ubiquitous in advertising, with numerous companies seeking to utilize dinosaurs to sell their own products or to characterize their rivals as slow-moving or obsolete.[84]
Religious views Main article: Young Earth Creationism#Palaeontology and dinosaurs Various religious groups have views about dinosaurs that differ from those held by scientists, usually due to conflicts with creation stories in their scriptures. However, the scientific community rejects these religiously-inspired interpretations of dinosaurs.[85][86]
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