Dinosaurs: Their Rise & Fall

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Edmontonia was an "armored dinosaur" of the group Ankylosauria.

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Stegosaurus skeleton, Field Museum, Chicago.

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Hip joints and hindlimb postures.

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Origins and early evolution

For a long time many scientists thought dinosaurs were polyphyletic with multiple groups of unrelated "dinosaurs" evolving due to similar pressures,[25][26][27] but dinosaurs are now known to have formed a single group.[17][28]

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.[29][30] 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;[31] if this is true, its traits suggest that the first dinosaurs were small, bipedal predators.[32] 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 rhynchosaurs, 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.[17]

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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 a long period of competition. This now appears unlikely, for several reasons. 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 been 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.[17] Crurotarsans, today represented only by crocodilians but in the Late Triassic also encompassing such now-extinct groups as aetosaurs, phytosaurs, ornithosuchians, and rauisuchians, were actually more diverse in the Late Triassic than dinosaurs, indicating that the survival of dinosaurs had more to do with luck than superiority.[33]

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Eoraptor, an early dinosaur

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Low diversification in the Cretaceous

Statistical analyses based on raw data suggest that dinosaurs diversified, i.e. the number of species increased, in the Late Cretaceous. However in July 2008 Graeme T. Lloyd et al. argued that this apparent diversification was an illusion caused by sampling bias, because Late Cretaceous rocks have been very heavily studied. Instead, they wrote, dinosaurs underwent only two significant diversifications in the Late Cretaceous, the initial radiations of the euhadrosaurs and ceratopsians. In the Mid Cretaceous, the flowering angiosperm plants became a major part of terrestrial ecosystems, which had previous been dominated by gymnosperms such as conifers. Dinosaur coprolites (fossilized dung) indicate that, while some ate angiosperms, most herbivorous dinosaurs mainly ate gymnosperms. Meanwhile herbivorous insects and mammals diversified rapidly to take advantage of the new type of plant food, while lizards, snakes, crocodilians and birds also diversified at the same time. Lloyd et al. suggest that dinosaurs' failure to diversify as ecosystems were changing doomed them to extinction.[34]

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Evolution of dinosaurs



http://upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Evolution_of_dinosaurs_EN.svg/800px-Evolution_of_dinosaurs_EN.svg.png

Dinosaurs evolved from the archosaurs 232-234 Ma (million years ago) in the Ladinian age, the latter part of the middle Triassic. Dinosauria is a well-supported clade, present in 98% of bootstraps. It is diagnosed by many features including loss of the postfrontal on the skull and an elongate deltopectoral crest on the humerus.[1]

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From archosaurs to dinosaurs

The process leading up the first dinosaurs can be followed through fossils of the early Archosaurs such as the Proterosuchidae, Erythrosuchidae and Euparkeria which have fossils dating back to 250 Ma, through mid-Triassic archosaurs such as Ticinosuchus 232-236 Ma. Crocodiles are also descendants of mid-Triassic archosaurs.[1]

The Dinosaurs can be defined as the last common ancestor of birds (Saurischia) and Triceratops (Ornithischia) and all the descendants of that ancestor. With that definition, the pterosaurs and several species of archosaurs narrowly miss out on being classed as Dinosaurs. The pterosaurs are famous for flying through the Mesozoic skies on leathery wings. Archosaur species that narrowly miss out on being classed as dinosaurs include Schleromochlus 220-225 Ma, Lagerpeton 230-232 Ma and Marasuchus 230-232 Ma.

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Earliest dinosaurs

The first known dinosaurs were bipedal predators that were one to two metres long.

Spondylosoma may or may not be a dinosaur, the fossils (all postcranial) are tentatively dated at 230-232 Ma.[1]

The earliest confirmed dinosaur fossils include the Saurischia Saturnalia 225-232 Ma, Herrerasaurus 220-230 Ma, Staurikosaurus possibly 225-230 Ma, Eoraptor 220-230 Ma and Alwalkeria 220-230 Ma. Saturnalia may be a basal saurischian or a prosauropod. The others are basal Saurischians.

The earliest Ornithischia includes Pisanosaurus 220-230 Ma. Although Lesothosaurus comes from 195-206 Ma, skeletal features suggest that it branched from the main Ornithischia line at least as early as Pisanosaurus.

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http://en.wikipedia.org/wiki/File:Dino_evol_1_modificated_ES.svg

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A. Eoraptor, an early saurischian, B Lesothosaurus, a primitive ornithischian, C Staurikosaurus (Saurischia) pelvis, D Lesothosaurus pelvis

It is clear from this figure that early saurischian resembled early ornithischia, but not modern crocodiles. Saurischians ('lizard-hipped') are distinguished from the ornithischians ('bird-hipped') by retaining the ancestral configuration of bones in the pelvis. Another difference is in the skull, the upper skull of the Ornithischia is more solid and the joint connecting the lower jaw is more flexible; both are adaptations to herbivory and both can already be seen in Lesothosaurus.

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Saurischia

Setting aside the basal Saurischia, the rest of the Saurischia are split into the Sauropodomorpha and Theropoda. The Sauropodomorpha is split into Prosauropoda and Sauropoda. The evolutionary paths taken by the Theropoda are very complicated. Dinosauria (2004)[1] splits the Theropoda into groups Ceratosauria, Basal Tetanurae, Tyrannosauroidea, Ornithomimosauria, Therizinosauroidea, Oviraptorosauria, Troodontidae, Dromaeosauridae and Basal Avialae in turn. Each group branches off the main trunk at a later date. See Dinosaur_classification for the detailed interrelationships between these.

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Sauropodomorpha

The first sauropodomorphs were prosauropods. Prosauropod fossils are known from the late Triassic to early Jurassic 227-180 Ma.[1] One of the earliest possible prosauropods is Azendohsaurus from 222-225 Ma. They could be bipedal or quadrupedal and had developed long necks and tails and relatively small heads. They had lengths of 2.5 to 10 m and were primarily herbivorous. The earliest prosauropods, such as Thecodontosaurus from 205-220 Ma, still retained the ancestral bipedal stance and large head to body ratio.

These evolved into the sauropods which became gigantic quadrupedal herbivores, some of which reached lengths of at least 26 m. Features defining this clade include a forelimb length to hindlimb length greater than 0.6. Most sauropods still had hindlimbs larger than forelimbs, one notable exception is Brachiosaurus whose long forelimbs suggest that it had evolved to feed from tall trees like a modern-day giraffe.