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RE: Dinosaur extinction
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We can scratch a couple of suspects from the list of possible dinosaur killers. Too few dinosaur fossils show evidence of bone cancer to conclude that a lethal dose of ionising radiation did them in - or that they took up smoking, as Gary Larson once whimsically suggested in a Far Side cartoon.
Adrian Melott, an astrobiologist at the University of Kansas in Lawrence, has suggested that periodic overdoses of cosmic rays or bursts of gamma rays might have caused mass extinctions. Past periods of heavy radiation leave little direct trace, but cancers induced by cosmic rays could have spread to bones.
So Melott turned to data collected by his colleague Bruce Rothschild on signs of cancer in the fossilised bones of 708 dinosaurs. When they compared the incidence of bone cancer with that in today's birds and reptiles, they found no evidence for elevated cancer ..

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Title: Bone Cancer Rates in Dinosaurs Compared with Modern Vertebrates
Authors: L.C. Natarajan, A.L. Melott, B.M. Rothschild, L.D. Martin (University of Kansas)
(Version v3)

Data on the prevalence of bone cancer in dinosaurs is available from past radiological examination of preserved bones. We statistically test this data for consistency with rates extrapolated from information on bone cancer in modern vertebrates, and find that there is no evidence of a different rate. Thus, this test provides no support for a possible role of ionising radiation in the K-T extinction event.

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A collision 160 million years ago of two asteroids orbiting between Mars and Jupiter sent many big rock chunks hurtling toward Earth, including the one that zapped the dinosaurs, scientists said on Wednesday.
Their research offered an explanation for the cause of one of the most momentous events in the history of life on Earth -- a six-mile-wide (10-km-wide) meteorite striking Mexico's Yucatan peninsula 65 million years ago.
That catastrophe eliminated the dinosaurs, which had flourished for about 165 million years, and many other life forms, and paved the way for mammals to dominate the Earth and the eventual rise of humankind, many scientists believe.

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Tyrannosaurus rex wasnt the only killer walking the Earth 100 million years ago.
Bugs infected with diseases and parasites could have devastated dinosaur populations, and insects also impacted plant food sources, according to an upcoming book by Corvallis scientists George Poinar Jr. and Roberta Poinar, whose research served as the inspiration for Jurassic Park.

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Jay Melosh, Regents Professor of Planetary Science at the Lunar and Planetary Lab at the University of Arizona, in Tucson, will be in Canada on June 6th to examine past history and present day research into asteroid impacts on earth.
He will be providing a Perimeter Institute Public Lecture on the subject to a sold out audience in Waterloo, Ontario, just outside Toronto.
Sixty-five million years ago dinosaurs ruled the warm Cretaceous Earth. Without warning, this world was swept away forever by the impact of an asteroid about 15 km in diameter, leaving a huge scar now called the Chicxulub crater in Yucatan, Mexico. This catastrophe set the stage for the ascendance of our own biological group, the mammals. Although the fact of this impact is now established beyond doubt, the precise means by which an impact could wipe out such a large fraction of the Earths inhabitants is not fully understood. Recent study of the physical consequences of a large impact on the Earth have revealed a plethora of potentially disastrous effects, ranging from an immediate firestorm that ignited global wildfires to sulphuric acid aerosols, acid rain, and ozone depletion lasting decades. The extinctions caused by these physical traumas changed the way that the Earths biosphere recycles carbon, leading to climatic changes lasting nearly a million years longer. Concerns over the future possibility of such large impacts have lead to a worldwide program to identify potentially threatening asteroids and has generated discussion of what humans might do to deflect such an asteroid, should one be discovered.

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Dinosaurs dominated our planet for 160 million years. Now, all that remains of these huge creatures are bones. Fossil records show that 65 million years ago, something devastated the Earth's entire ecosystem and dinosaurs suddenly died out along with around 50% of all other species. But what caused their extinction? For many years, this question has remained a mystery shrouded in speculation.
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Title: Bone Cancer in Dinosaurs
Authors: L.C. Natarajan, A.L. Melott, B.M. Rothschild, L.D. Martin (University of Kansas)

We test the prevalence of bone cancer in dinosaurs as determined from radiological examination of fossils against the rate extrapolated from modern vertebrates, and find that there is no evidence of a different rate, and therefore no supporting evidence for any role for extraterrestrial radiation sources.

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Disappearing Dinosaurs didn't clear the way for mammals
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The extinction of the dinosaurs had little impact on the evolution of today's mammals, say researchers. After building a family tree of nearly every living mammal, they show that the main groups arose millions of years before the dinosaurs went extinct, and did not become dominant until millions of years after they disappeared.

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Clock in the rock
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The mass extinction of dinosaurs occurred about 66 million years ago, at the end of the Cretaceous Period (K) and the beginning of the Tertiary Period, known as the K-T boundary. A massive asteroid slammed into what is now the Gulf of Mexico about that time.
"About" is the key word here. The correlation is close, but not close enough for Samuel A. Bowring, a professor of geology in MIT's Department of Earth, Atmospheric and Planetary Sciences.

"With the K-T boundary and the death of the dinosaurs, everyone is happy with it being caused by an impact. To me, that's a reason not to be happy" - Samuel A. Bowring.

Bowring thinks there is a correlation between the two events. But he firmly believes that it may be possible to more precisely date the sequence of events before, during and after the extinction.

"There are questions that need to be addressed that are preserved in the rock record -- questions about rates of evolution, and sudden climate change in the past"

The challenge is in making accurate time scales.
In 2003 Bowring launched the Earthtime initiative to bring together scientists from all over the world to work together to calibrate and sequence Earth history through the integration of high-precision geochronology and quantitative chronostratigraphy.
Now, Earthtime (www.earth-time.org) has more than 200 members, hailing from a variety of disciplines.

"It's creating a lot of new interactions that weren't there before, palaeontologists talking to geochronologists. It brings together evolutionary biology, developmental biology, palaeontology, oceanography" - Samuel A. Bowring.

Earthtime's official goal is to bring dating accuracy to better than 0.1 percent. That means in dating a 250-million-year-old fossil, results would be plus or minus 250,000 years. Bowring thinks it is possible to improve precision to 0.05 percent.
Earthtime utilises two main dating techniques, which are applied to the volcanic ash layers that contain rock fossil records. One technique measures the clocklike decay of uranium to lead inside zircon crystals, which are frequently found in volcanic ash beds. Another measures the amount of argon gas produced in minerals like feldspar by the decay of potassium. The two methods produce slightly different results, but with correct correlation and using data from fossil record from all over the world, they are helping to generate a more accurate time scale.

"We are working on the age of the K-T boundary right now using zircon and we think we can constrain it to within about 50,000 years" - Samuel A. Bowring.

Other issues that a more precise Earth time scale could address include the mystery behind a massive extinction about 252 million years ago at the end of the Permian period, in which 90 percent of all sea life became extinct. The cause might be a gigantic outpouring of lava from volcanoes in Siberia, the so-called Siberian Traps, which occurred about 251 to 252 million years ago. But what if the eruption occurred after the extinction began?

"We have the time of the extinction very well constrained but we don't have the time of the Siberian Traps very well constrained" Samuel A. Bowring.

Another mystery is what caused the Cambrian explosion, a huge proliferation of life about 530 million years ago; every order and class of animals we see today appears in the fossil record in a relatively short time.

"(Was this a )record of sudden appearance or a record of sudden preservation, because it is also when animals developed hard parts?" - Samuel A. Bowring.

Indeed, in a conversation with Bowring, the earth seems to be a giant yo-yo, with climate, temperatures and geography changing drastically over the eons. Such fluctuations have significant implications for the current rise in planetary temperature, he noted.

"If you can move past the last million years and start looking deeper in time -- looking at these huge fluctuations in climate, what caused them and how we came out of them -- that will certainly be very beneficial for understanding what is happening to us now" - Samuel A. Bowring.

Dealing with mind-boggling units of time has made Bowring regard humanity's period on our planet as little more than a blip.

"I do think a lot about how inconsequential humans are in the big scheme of things. We have not yet even proved we're a viable species compared to dinosaurs, for example. Dinosaurs were around from 230 million years ago to 66 million years ago. That's a long time. They were very successful. Humans have been around for maybe a million years. It doesn't look like we're going to have that long of a legacy but we'll see" - Samuel A. Bowring.

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