Dinosaur Extinction by Gyan.com

Dinosaurs were a group of land animals that lived from about 230 million years ago until about 60 million years ago. This spans the era of the Earth’s history known as the Mesozoic era, which includes, from most ancient to most recent, the Triassic, Jurassic and Cretaceous periods. Dinosaurs grew in population and diversity during their time on Earth before becoming extinct at the end of the Cretaceous period.

No one knows exactly how many types of dinosaurs inhabited the planet. There are currently about 700 named species, but this probably represents a fraction of the dinosaurs that ever existed.

Dinosaurs ranged in size from immense to tiny, and they came in a range of shapes. Today’s dinosaur classifications come from these differences in shape and size. Carnivorous dinosaurs were all theropods, bipedal animals with three-toed feet. Carnosaurs were a small, agile type of theropod. One of the most widely-known carnosaurs was Velociraptor, which is considerably smaller than depicted in the “Jurassic Park” films. Sauropods, on the other hand, were enormous, four-legged herbivores like Brachiosaurus, Apatosaurus and Diplodocus. Dinosaurs with armored bodies and spiny tails were ankylosaurs. Ceratopians — like Triceratops — had frills and horns on their heads.But not every reptile that lived during the Mesozoic era was a dinosaur. In fact, a lot of extinct animals that people think of as dinosaurs aren’t classified as dinosaurs. This is because they don’t share one or more of dinosaurs’ basic traits:

* Dinosaurs were animals with four limbs, although not all walked on all four legs.
* Although they may have ventured into the water, they were terrestrial, or land-dwelling, animals.
* Their muscles and bones had several specific features. For example, all dinosaurs had cheek muscles that extended from their jaws to the tops of their skulls.
* Their hip girdles comprised three bones — the ilium, ischium and pubis. These bones fit together in one of two configurations: ornithischian (bird-hipped) or saurischian (lizard-hipped).
* They had an upright gait. Dinosaurs held their bodies over their legs like rhinoceroses do rather than using the sprawling gait that crocodiles do.
These traits keep some well-known prehistoric animals from being considered dinosaurs:

* Plesiosaurs were aquatic creatures with long bodies and flipper-like fins.
* Another aquatic reptile group, ichthyosaurs, had a more dolphin-like body structure.
* Pterosaurs, like Pteranodon and the Pterodactyl subgroup, were flying reptiles.
* Synapsids had an opening behind their eye socket that also occurs in mammals. One of the most well-known synapsids is Dimetrodon, a lizard-like animal with a large sail on its back.

So because of their bone structure, habitat or other traits, these animals weren’t technically dinosaurs. But they did leave behind same evidence that dinosaurs did fossils.

The World of Dinosaurs
The earliest dinosaurs lived in a world that looked very different from the Earth we know today. Rather than being separated by expansions of ocean, the continents were packed together in a mass known as Pangea.

Dinosaurs lived for about 170 million years, and during that time, the continents gradually spread to form the shapes we recognize today. Dinosaurs continued to live on every continent — there are even fossils buried under the ice in Antarctica.

Dinosaur Extinction
Dinosaurs became extinct at the K-T boundary — the dividing line between the Cretaceous and Tertiary periods. The end of the Cretaceous marks the end of the dinosaurs, while the beginning of the Tertiary marks the rise of mammal life on Earth.

Asteroid
One compelling theory about the extinction of the dinosaurs is a massive asteroid impact.Dinosaurs aren’t the only things that died out at the K-T boundary. About 50 percent of the species on Earth became extinct. This included many other large reptiles, like pterosaurs and plesiosarus, as well as lots of plant species and marine animals. Other life forms, such as ferns, flourished by taking advantage of the sudden abundance of natural resources.

Scientists have proposed a number of theories about exactly what happened at the K-T boundary. There’s not much physical evidence for some of them. For example, one theory is that dinosaurs were allergic to flower pollen — flowering plants and bees evolved together during the late Cretaceous period. However, flowering plants existed for millions of years before the dinosaurs died out. Another theory is that mammals, which began to proliferate at the end of the Cretaceous period, ate dinosaurs’ eggs. But considering the number of whole fossil egg specimens, this seems somewhat unlikely.Then there’s the Alvarez hypothesis. In 1980, Luis and Walter Alvarez proposed that comets or asteroids had hit the Earth, causing massive shock waves, debris clouds and other devastation. There’s a lot of evidence to support this hypothesis. One is a layer of a mineral called iridium, which exists in many locations on the planet at depths equated with the end of the Cretaceous period. Iridium is more common in space debris than on Earth, so the huge impact of an object from space could have caused this effect.

Perhaps the biggest support for this hypothesis is the Chicxulub crater. This is a massive asteroid crater off the coast of the Yucatan peninsula. Based on measurements of sediments and analysis of the surrounding rock, scientists estimate that the asteroid that caused the crater was between 145 and 180 kilometers in diameter. This would have caused exactly the kind of devastation described in the Alvarez hypothesis. A team of three researchers even believes to have discovered the identity of the asteroid itself. Using mathematical models, the group narrowed the field to the Baptistina cluster, a group of asteroids created by a large impact beyond the orbit of Mars.
According to the Alvarez theory, the extinction of the dinosaurs was extrinsic — it came from outside of the Earth — and catastrophic. However, other theories suggest that the mass extinction was intrinsic and gradual. One idea is that volcanoes in what is now India experienced massive eruptions just before the end of the Cretaceous. These eruptions filled the air with carbon dioxide and sulfur, changing the climate and damaging plant and animal life.

The changing face of the planet may have also played a role. As the continents moved, ocean currents would have changed the weather patterns in different parts of the world. Various forms of life may have been unable to survive these changes.

The best explanation for what happened to the dinosaurs may be a combination of intrinsic and extrinsic theories — an asteroid impact combined with geological changes and volcanic eruptions. There are also indications that dinosaurs were becoming less diverse before the end of the Cretaceous period. But regardless of the cause, not everything on Earth died at the K-T boundary. Frogs, mollusks and crocodilians survived, and so did birds.

Cloning the Dinasaurs DNA 

In the 1993 movie “Jurassic Park” wasn’t the first film to show dinosaurs in the modern world. But it did a good job of bringing the idea of cloning dinosaurs into popular culture. It portrayed dinosaur cloning in a way that made sense to a lot of people, and it was a blockbuster, making more than $900 million worldwide.”Jurassic Park” built on the idea of extracting DNA from the bellies of mosquitoes preserved in amber. While this might seem possible at first glance, it’s highly unlikely that scientists could find usable dinosaur DNA in mosquito fossils. Scientists would need a very specific specimen — a female mosquito that had consumed lots of dinosaur blood immediately before landing in tree resin. Since fossilization in amber is a relatively rare event, the chances of this happening are pretty small.

The lack of possible specimens isn’t the only problem. Most insect fossils found in amber are also too young to contain dinosaur blood — dinosaurs were extinct by the time the insects became trapped. Many insects decay from the inside out after they’re trapped, leaving nothing inside for scientists to try to extract. Finally, the sample would have to be very dry, since DNA can break down quickly in the presence of water.

But if researchers did find a perfectly preserved mosquito with a body full of dinosaur blood, retrieving its DNA would still be difficult. The blood with the dinosaur DNA would be surrounded by the body of an insect, which has its own DNA. There could also be DNA from other cells trapped in the amber, which could contaminate the sample. Then, of course, there’s the DNA in the laboratory itself — and in the body of the scientist doing the extraction.

                                                                DNA in Amber
[The idea of extracting DNA from amber isn't confined to the movies. The first reported success in the real world came in 1992, when scientists reported finding DNA from an extinct bee encased in Dominican amber. Other reports of successful extraction followed. However, there's some controversy surrounding these discoveries. In some cases, other research has disproved the original findings. In others, researchers simply can't replicate the original DNA extraction.]

The fictional scientists in “Jurassic Park” try to get around these difficulties by combining dinosaur DNA with frog DNA. But this would be like trying to assemble a jigsaw puzzle using billions of pieces that come from two different puzzles. Plus, frogs might not be the best candidates for providing replacement DNA. Today, one of the most prevalent theories about the fate of the dinosaurs is that some evolved into birds, not frogs.

On top of all that, the most common form of cloning used on animals today involves nuclear transfer. Scientists put the nucleus of one cell into a second cell of the same species after destroying the second cell’s nucleus. There are no dinosaur cells or dinosaur eggs that could host new set of DNA. Researchers would have to find a different way to let the DNA grow into a living dinosaur.

So the “Jurassic Park” method is out — but are there other ways to bring dinosaurs to life?

Besides using DNA from insects trapped in amber, there are several theories about how scientists might clone dinosaurs. One involves finding DNA specimens in fossilized bones instead of in the bodies of insects. The problem with this idea is that DNA is a complex, delicate structure. The process of fossilization involves replacing the organic tissue in an animal’s bones with minerals. This effectively destroys the cells that may contain DNA.
A frozen mammoth discovered in the permafrost zone in Siberia is displayed at Global House March 18, 2005 in Nagakute, Japan.

However, one team of paleontologists has discovered what appears to be soft tissue in the bones of a Tyrannosaurus rex. It was a discovery no one could have predicted — until that point, scientists thought all soft tissue was destroyed in the fossil process. However, the research team has not yet isolated any DNA from the soft-tissue samples. While the discovery doesn’t necessarily guarantee that researchers will ever find intact DNA in fossils, it makes the idea more likely than it was just a few years ago.

Another appealing idea is that researchers could sequence the DNA of dinosaurs and recreate the necessary DNA strands. At this point, this isn’t possible. The sequencing of the human genome, for example, took 13 years to complete, and the final product wasn’t something researchers could use to clone people. Reconstructing complete strands of dinosaur DNA would require technology far beyond what exists today.

That leaves what may be the next best thing to cloning dinosaurs — cloning extinct mammals, like mammoths. Mammoth fossils are significantly younger than dinosaur fossils. They’re only about 30,000 years old. This difference in age gives the DNA much less time to decompose. But a mammoth cloning project would still require a perfectly preserved specimen. The mammoth’s tissue couldn’t have gone through cycles of freezing and thawing or been preserved at extremely low temperatures that could damage the DNA. Yet, the idea of piecing together this evidence to get an idea of mammoth genes isn’t completely unreasonable. In 2005, one research team reported that it had sequenced part of the mammoth genome.

A second option for bringing mammals to life could be to use fossilized sperm to inseminate the eggs of a related mammal. The resulting animal would be a hybrid, with only half its genetic code from its male mammoth parent. A project to do exactly that was proposed by a Japanese research team in 2006.
These advances in paleontology and genetic technology have made the idea of cloning dinosaurs just a little more probable sometime in the far future. But it’s still not at all likely, especially not in our lifetimes.

Gyan.com

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