Theories why dinosaurs became extinct

American researchers have a new theory about how an object struck the Earth and caused the dinosaurs to die off. Scientists mostly agree on where the impact happened about 65 million years ago. They say a huge object struck an area off the coast of what is now Mexico. Astronomers have said the most likely cause of the strike was either an asteroid or a comet. In recent years, researchers have presented evidence that the impact was caused by an asteroid.

The theory suggests the asteroid came from an area between the orbits of Mars and Jupiter. But a study by two astronomers from Harvard University presents a new theory: that the crash was caused by a comet.

The researchers say the comet came from an area containing icy debris on the edge of the solar system. The area is known as the Oort cloud. The comet then came very close to the sun, whose tidal force caused it to break up into pieces. The researchers believe one of the pieces crashed into the place that scientists have identified in Mexico.

The team based its theory on a model created to predict the probability that a long-period comet from the Oort cloud would hit Earth.

Long-period comets take more than years to orbit the Sun. Because comets come from frozen areas of the outer solar system, they are icier than asteroids. They are known for leaving long trails of gas and dust as they melt.

The new study was recently published in Scientific Reports. No surprise, then, that in ophthalmologist L. Croft suggested that bad eyesight undid the dinosaurs. Since exposure to heat can make cataracts form more quickly, Croft surmised that dinosaurs with weird horns or crests developed these bizarre ornaments to shield their eyes from the relentless Mesozoic sun.

In the world warmed by harsh sunlight, though, Croft expected that even these attempts to shade dinosaur eyes failed and that the creatures started to go blind before they hit sexual maturity. Supernova Before the asteroid impact hypothesis gained widespread credibility, in physicist Wallace Tucker and paleontologist Dale Russell suggested another kind of death from above.

Although the researchers lacked any direct evidence for their idea, they proposed that a nearby supernova could have had catastrophic consequences for life at the end of the Cretaceous. The explosion of a neighboring star, Tucker and Russell proposed, would bombard the upper atmosphere with X-rays and other forms of radiation that would quickly alter the climate, causing temperatures on Earth to plummet. No evidence of such a nearby event 66 million years ago has ever been uncovered.

Apatosaurus only ever faced down aliens in comic books and movies. Dinosaur Farts Much like death by aliens, the idea that dinosaurs farted themselves into extinction was never a scientific hypothesis. The notion was a misconstrued conclusion drawn from some recent dinosaur research. Last year, paleontologist David Wilkinson and collaborators tried to calculate how much gas the long-necked, hefty sauropod dinosaurs could have produced. After all, a variety of sauropods existed for tens of millions of years without showing any sign of gassing themselves out of existence.

Ignoring the actual research by Wilkinson and colleagues,, various news sites jumped on the study to suggest that dinosaurs gassed themselves into oblivion. Such sites were only blowing hot air. Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American. Silhouette of the Tyrannosaurus called Stan. This rare peek inside the guts of the crater showed that the impact would have been powerful enough to send deadly amounts of vaporized rock and gases into the atmosphere, and that the effects would have persisted for years.

And in , paleontologists digging in North Dakota found a treasure trove of fossils extremely close to the K-Pg boundary , essentially capturing the remains of an entire ecosystem that existed shortly before the mass extinction. Tellingly, the fossil-bearing layers contain loads of tiny glass bits called tektites—likely blobs of melted rock kicked up by the impact that solidified in the atmosphere and then rained down over Earth.

However, other scientists maintain that the evidence for a massive meteor impact event is inconclusive, and that the more likely culprit may be Earth itself.

Ancient lava flows in India known as the Deccan Traps also seem to match nicely in time with the end of the Cretaceous, with massive outpourings of lava spewing forth between 60 and 65 million years ago. Today, the resulting volcanic rock covers nearly , square miles in layers that are in places more than 6, feet thick.

Proponents of this theory point to multiple clues that suggest volcanism is a better fit. Other research has found evidence for mass die-offs much earlier than 66 million years ago, with some signs that dinosaurs in particular were already in a slow decline in the late Cretaceous. This all makes sense, supporters say, if ongoing volcanic eruptions were the root cause of the world-wide K-Pg extinctions. Increasingly, scientists trying to unravel this prehistoric mystery are seeing room for a combination of these ideas.

This nearly whole, deep-black skull belongs to the most complete specimen of Tyrannosaurus rex on display in Europe, an individual nicknamed Tristan Otto. But that notion depends a lot on more precise dating of the Deccan Traps and the Chicxulub crater. This debate may rage for years, as scientists dig up new clues and develop new techniques for understanding the past.

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