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Researchers have found that the space rock that hit Earth 65 million years ago and was widely implicated in the end of the dinosaurs was likely a speeding comet.

That is the conclusion of research which suggests the 180 km-wide Chicxulub crater in Mexico was carved out by a smaller object than previously thought.

Many scientists consider a large and relatively slow moving asteroid to have been the likely culprit.

Details were outlined at the 44th Lunar and Planetary Science Conference.

However, other researchers were more cautious about the results.

“The overall aim of our project is to better characterize the impactor that produced the crater in the Yucatan peninsula [in Mexico],” said Dr. Jason Moore, from Dartmouth College in New Hampshire.

The space rock gave rise to a global layer of sediments enriched in the chemical element iridium, in concentrations much higher than naturally occurs; it must have come from outer space.

However, in the first part of their work, the team suggests that frequently quoted iridium values are incorrect. Using a comparison with another extraterrestrial element deposited in the impact – osmium – they were able to deduce that the collision deposited less debris than has previously been supposed.

The recalculated iridium value suggests a smaller body hit the Earth. So for the second part of their work, the researchers took the new figure and attempted to reconcile it with the known physical properties of the Chicxulub impact.

For this smaller space rock to have produced a 180 km-wide crater, it must have been travelling relatively quickly. The team found that a long-period comet fitted the bill much better than other possible candidates.

“You’d need an asteroid of about 5 km diameter to contribute that much iridium and osmium. But an asteroid that size would not make a 200 km-diameter crater,” said Dr. Jason Moore.

“So we said: how do we get something that has enough energy to generate that size of crater, but has much less rocky material? That brings us to comets.”

Researchers have found that the space rock that hit Earth 65 million years ago and was widely implicated in the end of the dinosaurs was likely a speeding comet

Researchers have found that the space rock that hit Earth 65 million years ago and was widely implicated in the end of the dinosaurs was likely a speeding comet

Co-author Mukul Sharma, also from Dartmouth College, said: “You would need some special pleading for an asteroid moving very rapidly – although it is possible. But of the comets and asteroids we have looked at in the skies, the comets are the ones that are moving very rapidly.”

Long-period comets are balls of dust, rock and ice that are on highly eccentric trajectories around the Sun. They may take hundreds, thousands or in some cases even millions of years to complete one orbit.

The extinction event 65 million years ago is now widely associated with the space impact at Chicxulub. It killed off about 70% of all species on Earth in just a short period of time, most notably the non-avian dinosaurs.

The enormous collision would have triggered fires, earthquakes and huge tsunamis. The dust and gas thrown up into the atmosphere would have depressed global temperatures for several years.

Dr. Gareth Collins, who researches impact cratering at Imperial College London, described the research by the Virginia team as “nice work” and “thought-provoking”.

He said: “I don’t think it is possible to accurately determine the impactor size from geochemistry.

“Geochemistry tells you – quite accurately – only the mass of meteoritic material that is distributed globally, not the total mass of the impactor. To estimate the latter, one needs to know what fraction of the impactor was distributed globally, as opposed to being ejected to space or landing close to the crater.”

Dr. Gareth Collins added: “The authors suggest that 75% of the impactor mass is distributed globally, and hence arrive at quite a small-sized impactor, but in reality this fraction could be lower than 20%.”

That would easily keep the door open for a bigger, more slowly moving asteroid.

The authors accept this point, but cite recent studies showing suggesting mass loss for the Chicxulub impact was between 11% and 25%.

In recent years, several space objects have taken astronomers by surprise, serving as a reminder that our cosmic neighborhood remains a busy place.

On February 15, 2013, 2012 DA14 – an asteroid as large as an Olympic swimming pool – raced past the Earth at a distance of just 27,700 km (17,200 miles). It had only been discovered the previous year.

And on the same day, a 17 m space rock exploded over Russia’s Ural mountains with an energy of about 440 kilotonnes of TNT. About 1,000 people were injured as the shockwave blew out windows and rocked buildings.

Some 95% of the near-Earth objects larger than 1km have been discovered. However, only about 10% of the 13,000 – 20,000 asteroids above the size of 140m are being tracked.

There are probably many more comets than near-Earth asteroids, but NASA points out they spend almost all of their lifetimes at great distances from the Sun and Earth, so that they contribute only about 10% to the census of larger objects that have struck the Earth.

International experts have identified the creature that gave rise to all the placental mammals – a huge group that includes whales, elephants, dogs, bats and us.

The international effort mapped out thousands of physical traits and genetic clues to trace the lineage.

Their results indicate that all placental mammals arose from a small, furry, insect-eating animal.

A report in Science resolves the debate as to when the creature lived; it came about after the demise of dinosaurs.

That had been a hotly debated question over years of research.

Placental mammals – as opposed to the kind that lay eggs, such as the platypus, or carry young in pouches, such as the kangaroo – are an extraordinarily diverse group of animals with more than 5,000 species today. They include examples that fly, swim and run, and range in weight from a couple of grams to hundreds of tonnes.

A wealth of fossil evidence had pointed to the notion that the group, or clade, grew in an “explosion” of species shortly after the dinosaurs’ end about 65 million years ago.

But a range of genetic studies that look for fairly regular changes in genetic makeup suggested that the group arose as long as 100 million years ago, with mammals such as early rodents sharing the Earth with the dinosaurs.

Deciphering the very distant past on the basis of fossils and animals that are around today is inherently a subjective business.

“Comparative anatomy” – in which, for example, the forelimbs of a number of fossils are compared to establish which are most closely related – was the entire toolbox for the earliest palaeontologists. The era of genetics ushered in a more incisive tool to compare similarities across species.

But the new work tackles the question of placental mammals in unprecedented detail, developing a database of physical and genetic data some 10 times larger than any used previously – and taking a decidedly modern take on it.

“Anatomy and research in palaeontology had a very 19th Century veneer to it – that we would sit in small groups in a lab with a fossil describing it,” said lead author of the study Maureen O’Leary of Stony Brook University in New York, US.

“That is a very effective and important part of what we do, but by trying to bring this into the 21st Century and using new software, we were able to really band together as a group of experts and tackle a much larger problem,” she said.

International experts have identified the creature that gave rise to all the placental mammals

International experts have identified the creature that gave rise to all the placental mammals

To build the database, the team gathered more than 4,500 details of phenotype – diet, lengths of limbs, shapes of teeth, length of fur if any, and so on – from 86 different species that are around today, and from 40 fossils of extinct animals.

To that they added some 12,000 detailed images and genetic information for all of the current species, putting all the data into what Dr. Maureen O’Leary called “a supermatrix – essentially like a spreadsheet, filled with observations and images, to create a really rich description of mammals we’d sampled”.

That, in essence, allows not just one or a few researchers to log details and make comparisons of, for example, fossil or genetic data; it becomes a problem shared – in this case, among 23 co-authors.

“That really wasn’t possible until we developed this software called Morphobank. Our experts in China or Brazil or Canada or the US or just across the hall could all be working in one place at the same time,” Dr. Maureen O’Leary said.

By noting which traits have been preserved down the lineage and how they are expressed, the team was able to feed their phenotypic and genetic data into standard software that makes relationship and age estimates – suggesting the ancestral animal lived just 200,000 years after the extinction event that saw the end of the dinosaurs.

It also yields informed guesses as to the traits of the ancestral animal that gave rise to them all – incorporating those traits into an artist’s conception of what it would have looked like.

The result resolves a long-standing debate, but Dr. Maureen O’Leary said it could also contribute to a new way of tackling such debates.

“I think that it will go a long way toward showing people a way forward for using all the data… both DNA and anatomy. I think that had been an intimidating kind of project because of its scale,” she said.

“Now that we can do that…we are sort of iteratively working and refining the <<Tree of Life>> in that way.”

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One of the strongest lines of evidence that dinosaurs were cold-blooded, like modern reptiles, has been knocked down.

Prior studies of dinosaur bones uncovered what are known as “lines of arrested growth”.

The creatures were presumed to be cold-blooded because modern cold-blooded animals show these same lines.

But scientists reporting in Nature have studied the bones of 41 modern mammal species from around the world, finding everyone had these lines as well.

A number of discoveries in recent years have challenged the 40-year-old notion that dinosaurs were cold-blooded.

But because soft tissues such as organs and skin are not preserved, much of what is known about dinosaurs must be inferred from their bones, and comparisons made with modern animals that can be studied in greater detail.

Lines of arrested growth, or Lags, occur because organisms tend to suspend their growth and rally their resources during seasonal periods of environmental stress such as cold or dry conditions.

One of the strongest lines of evidence that dinosaurs were cold-blooded, like modern reptiles, has been knocked down

One of the strongest lines of evidence that dinosaurs were cold-blooded, like modern reptiles, has been knocked down

This forms a boundary from one season to the next as growth resumes when conditions are more favorable.

They are familiar in creatures such as molluscs, whose slow annual accumulations can be seen as ridges in their shells.

Lags have also been found in the bones of reptiles and amphibians and have until now been assumed to be limited to ectotherms – cold-blooded animals – that are more subject to the whims of harsh environments.

Meike Koehler of the Catalan Institute of Palaeontology in Barcelona and her colleagues were therefore surprised by what they found.

“Originally this was not a paper that we aimed to do,” Dr. Meike Koehler.

“We were very curious to know how environmental conditions and changes affect bone growth in fossil and extant mammals, to get a good idea about… how they may have coped with these changes in the past.”

As the team studied the thigh bones of animals from all over the world – ranging from the Svalbard reindeer in the Arctic to muntjac deer species from South Asia – Lags showed up in every one.

“These lines of arrested growth have been used a lot in dinosaurs, but nobody has ever had a really deep look at mammals,” Dr. Meike Koehler explained.

David Weishampel, a palaeontologist at the Center for Functional Anatomy and Evolution at Johns Hopkins University School of Medicine in Maryland called the new work “a wonderful paper” and said it was a welcome addition to the debate.

“I think most (palaeontologists) regard dinosaurs as being [warm-blooded] but there’s a lot of waffling in the data that appeared before that wasn’t conclusive,” he said.

“It’s about time we have a connection between the modern bone histology and fossil bone histology, through a very nice ecological and metabolic comparison.”

While Prof. David Weishampel considers it a closed case, Dr. Meike Koehler herself is more reserved about the result.

“I don’t think that this debate is really settled,” she said.

“But this is the first time that you can say that Lags do not say anything about warm- or cold-bloodedness.”

She and her team will go on and put the Lags to use in studies of modern animals instead.

“It’s like dendrochronology – the rings in trees. You can do skeletal chronology in bones and infer things like longevity, age at maturity, juvenile states – traits which are very, very important to get an idea about the health of a population and whether it is vulnerable.

“It is very good to know now that mammals do show these Lags and we can use them in the same way that we do in amphibians and reptiles to understand the situation of a population.”