Most everybody knows that the dinosaurs perished rapidly in a tumultuous extinction, caused by an asteroid strike about 66 million years ago. But it looks like another extinction prior to the appearance of the dinosaurs paved the way for their long reign. That extinction took place about 233 million years ago.
It seems almost certain that an asteroid impact wiped out the dinosaurs. But only almost. Another competing theory won’t completely go away: the extinction-by-volcano theory.
A new study from the UK piles more evidence on the asteroid side of the debate, while adding a new volcanic twist. These researchers say that volcanic activity actually helped life recover from the asteroid strike.
When it comes to the extinction of the dinosaurs, science has whittled it down to those two possibilities. The asteroid strike has been the leading candidate for quite some time now, but those darn volcanoes refuse to stand down.
A new study is presenting even more evidence that it was the impact that wiped out the dinosaurs, and not volcanoes.
The asteroid that struck Earth about 66 million years ago and led to the mass extinction of dinosaurs may have hit one of the worst places possible as far as life on Earth was concerned. When it struck, the resulting cataclysm choked the atmosphere with sulphur, which blocked out the Sun. Without the Sun, the food chain collapsed, and it was bye-bye dinosaurs, and bye-bye most of the other life on Earth, too.
But, as it turns out, if it had struck a few moments earlier or later, it would not have hit the Yucatan, and things may have turned out differently. Why? Because of the concentration of the mineral gypsum in that area.
The place where the asteroid hit Earth is called the Chicxulub Crater, and scientists have been studying that area to try to learn more about the impact event that altered the course of life on Earth. An upcoming BBC documentary called “The Day The Dinosaurs Died,” focuses on what happened when the asteroid struck. Drill-core samples from the Yucatan area help explain the events that followed the impact.
The core samples, which are from as deep as 1300 m beneath the Gulf of Mexico, are from a feature called the peak ring.
When the asteroid struck Earth, it excavated a crater 100 km across and 30 km deep. This crater collapsed into a wider but shallower crater 200 km across and a few km deep. Then the center of the crater rebounded, and collapsed again, leaving the peak ring feature. The Chicxulub crater is now partly under water, and that’s where a drilling rig was set up to take samples.
The core samples revealed rock that has been heavily fractured and altered by immense pressures. The same impact that altered those rocks would have generated an enormous amount of heat, and that heat created an enormous cloud of sulphur from the vaporized gypsum. That cloud persisted, which led to a global winter. Temperatures dropped, plant growth came to a standstill, and the course of events on Earth were altered forever.
“Had the asteroid struck a few moments earlier or later, rather than hitting shallow coastal waters it might have hit deep ocean,” documentary co-presenter Ben Garrod told the BBC.
“This is where we get to the great irony of the story – because in the end it wasn’t the size of the asteroid, the scale of blast, or even its global reach that made dinosaurs extinct – it was where the impact happened,” said Ben Garrod, who presents “The Day The Dinosaurs Died” with Alice Roberts.
“An impact in the nearby Atlantic or Pacific oceans would have meant much less vaporised rock – including the deadly gypsum. The cloud would have been less dense and sunlight could still have reached the planet’s surface, meaning what happened next might have been avoided,” said Garrod.
In the documentary, host Alice Roberts will also visit a quarry in New Jersey, where fossil evidence shows a massive die-off in a very short period of time. In fact, these creatures could have died on the very day that the asteroid struck.
“All these fossils occur in a layer no more than 10cm thick,” palaeontologist Ken Lacovara tells Alice. “They died suddenly and were buried quickly. It tells us this is a moment in geological time. That’s days, weeks, maybe months. But this is not thousands of years; it’s not hundreds of thousands of years. This is essentially an instantaneous event.”
There’s lots of evidence showing that an asteroid struck Earth about 66 million years ago, causing widespread extinction. NASA satellite images clearly show crater features, now obscured by 66 million years of geological activity, but still visible.
There’s also what’s called the K-T Boundary, or Cretaceous-Tertiary Boundary. It’s a geological signature dating to 66 million years ago, which marks the end of the Cretaceous Period. In that boundary is a layer of iridium at very high concentrations, much higher than is normally present in the Earth’s crust. Since iridium is much more abundant in asteroids, the conclusion is that it was probably deposited by an asteroid.
But this is the first evidence that shows how critical the actual location of the event may have been. If it had not struck where it had, dinosaurs may never have gone extinct, you and I would not be here, and things on Earth could look much different.
It might sound like the stuff of science fiction, but who knows? Maybe a race of intelligent lizards might already have mastered interstellar travel.
To kids, there are only two kinds of dinosaurs: meat-eaters and plant-eaters. But to paleontologists, those are just diet distinctions. Paleontologists divide dinos into two different groups based largely on pelvic structure: reptile-hipped saurischians, and bird-hipped ornithischians.
Those two categories are called ‘clades’, and they’re fundamental to the study of dinosaurs. But a new study is casting doubt on those two groups, as well as moving the infamous Tyrannosaurus Rex to a new spot on the dinosaur family tree.
The study, by Matthew G. Baron, David B. Norman & Paul M. Barrett, was published in the journal Nature. If the findings in this study are accepted by paleontologists, then it will upset our understanding of the family tree that was first established in Victorian times.
The T. Rex is the most famous member of the reptile-hipped saurischians. Many other carnivorous theropods are saurischians too, like Giganotosaurus and Spinosaurus. Other famous dinosaurs, like Stegosaurus, are bird-hipped ornithischians. The distinction between the saurischians and the ornithischians has been workable for a long time. But there were always problems with the two clades of dinosaurs.
Some of the earliest ornithischian dinosaurs in the Triassic period had some theropod qualities: they were bipedal and probably meat-eaters. This clouded the separation between ornithischians and saurischians. There are also the herrerasaurids, small dinosaurs not larger than 4 meters long. They were some of the earliest dinosaurs, carnivores that look like both sauropods and theropods, and even though they appear early in the fossil record, they are not considered ancestors to any other group of dinosaurs. They show a mixture of both primitive and derived traits.
Huge plant-eating sauropods like the Brontosaurus and the Diplodocus are included in the reptile-hipped saurischians with the meat-eating theropods, even though there are some key skeletal differences between the two groups.
Another problem centers around birds. Believe it or not, birds have theropods as ancestors, even though theropods are in the reptile-hipped clade, rather than the bird-hipped clade.
Are You Confused Yet?
If this all seems kind of confusing, let’s back up for a minute.
When we think of dinosaurs, we tend to think of full-scale rebuilt skeletons of the type on display in museums around the world. But for paleontologists, the reality is much different. Many dinosaur species are known only by a few bones or teeth. These samples are studied in great detail. Any groove in a bone or slightly different shape in a tooth is analyzed, and out of this a dinosaur family tree is constructed.
It’s hard work, and our fossil record is spotty at best. Some new dinosaur taxa are proposed based only on the discovery of isolated teeth in the fossil record. With all of this in mind, you can see that the dinosaur family tree is an ongoing work in progress.
The authors of the study say that many ornithischian dinosaurs were overlooked in the past, because paleontologists didn’t really know what to do with them. Many of the ornithischians had weird traits like extra chin bones and molar-like teeth in their cheeks. These ornithischian dinos were thought of as oddities, early offshoots from other species.
The authors studied 457 traits in 74 taxa, looking at details like the shapes of tiny eye-socket bones and grooves on femurs. They found that Theropods, even though they have reptile-like hips, don’t belong in the saurischian clade. They’re suggesting that Theropods are a sister clade to the ornithischians. The revised grouping of Ornithischia and Theropoda has been named the Ornithoscelida. The authors are also proposing that the herrerasaurids did not branch off as early as previously thought, and should form a sister clade with the sauropods.
But this study does even more. It’s been long understood by paleontologists that dinosaurs appeared in the southern hemisphere first. That’s where the herrerasaurids were found, dating back to 240 million years ago. The authors remind us that there are very few Herrerasaurus skeletons and bones, and there are uncertainties in the age of the Triassic fossil beds where herrerasaurids are found. A nearly complete skeleton was found in Argentina, and less complete ones have been found in North America.
But this shuffling of the family tree moves the herrerasaurids further away from the base of the tree. Remember, the herrerasaurids look like both sauropods and theropods, and they show both derived and primitive traits. If it’s accepted that the herrerasaurids did not appear as early as thought, that might mean that dinos did not appear first in the southern hemisphere. The authors say that some enigmatic fossils found in the northern hemisphere should be re-examined in case they are earlier than the ones found in the south.
Enter the Saltopus
A fossil of a cat-like creature found in Scotland, called the Saltopus, is a part of the shake-up of the dinosaur family tree. It was considered a pre-cursor to dinosaurs, rather than a true dinosaur. As part of their analysis, the Saltopus has been re-positioned in the earliest part of the dinosaur lineage, as the first true dinosaur. This supports the idea that dinosaurs appeared first in the northern hemisphere rather than the south.
If this new family tree for dinosaurs is accepted, it will change our understanding of the way dinosaurs evolved. We’ve relied on similarity in hip shape to ascertain ancestry, but that may be a little simplistic.
Our understanding of dinosaurs changes frequently. Remember when dinosaurs were slow, dim-witted creatures with tiny brains and huge bodies? Now we think of dinosaurs as feathered and fast, using cunning and perhaps teamwork to hunt in packs. Remember when the prevailing wisdom was that some dinosaurs got so large and spiny that they were doomed to extinction? That was proven false as well.
If it does stick, this new family tree will be a huge change in paleontology, a field where knowledge is overturned on a regular basis, sometimes by little more than a few teeth.
It’s an apocryphal image. The ignorant faces of the dinosaurs, roaring helplessly at their fate, and looking skyward as an asteroid plunged to Earth. And the sneaky, clever little mammals coming out of their hiding holes to take their rightful place. If you grew up reading about this version of things, you’re not alone.
The line of reasoning says that mammals were present during the dinosaur’s reign, but their potential to thrive was suppressed by the dinosaurs, which were supremely evolved to dominate conditions on Earth at the time. It took the extinction of the dinosaurs to allow mammals to flourish. But according to new studies, that might not have been the case. As it turns out, mammals may have been well on their way to displacing the dinos long before the Chicxulub meteor hastened the dinosaur’s demise.
One such study, from researchers at the Universities of Southampton and Chicago, focused on hundreds of fossilized mammal teeth. As you know if you’ve been paying attention to how you eat, different teeth have different purposes. Carnivores have sharp teeth designed to rip and shred flesh, while herbivores have duller teeth for grinding up vegetation. Omnivores, like us, have a bit of both. That’s a simplification, of course, but its generally true.
What this study showed is that mammals with varied diets began to appear 10 to 20 million years before the dinosaurs were extinguished. It focused on early therian mammals, which are the ones that gave rise to the modern marsupials (ones with pouches) and placentals (ones where a fetus is carried inside the uterus). The third class of mammal, monotremes, were egg-laying mammals like the platypus.
In recent years, more and more early mammal fossils have been discovered, and they show that mammals were well on their way to diversifying long before the dinosaurs disappeared. The mammal fossil record also shows that mammal diversity suffered from the meteor strike, but mammals recovered and diversified into a greater number of species in the new conditions.
Another study, by Manabu Sakamoto and Chris Venditti from the University of Reading, and by Michael Benton from the University of Briston, shows that the opposite is true for dinosaurs. For tens of millions of years before their extinction, dinosaur species were becoming extinct and new species were not taking their place. This made the dinosaurs more vulnerable to extinction, whereas the diversifying mammals were in a better position to thrive, regardless of dinosaur extinction.
The main threat posed by the asteroid strike was the climate change that followed it. With greater species diversity in place immediately preceding the strike, mammals had a greater probability to survive the changing climate than did their dinosaur counterparts.
Evolutionary biologist and co-author of the study, Dr. Chris Venditti, told BBC News, “The current widespread view is that dinosaurs were reigning strong right up to the impact that hit the Earth – and it’s the impact that drove their final extinction,” he said. “And while that’s certainly true, what we found was that they were on the decline long before that.”
“If they were reigning strong perhaps they would have fared much better than they did,” said Venditti. Dinosaurs had been around for 160 million years and had faced pressures and had dips in their diversity before.
This begs the question, why were dinosaurs in decline?
It likely all revolves around the environmental conditions. At the dawn of the dinosaurs 230 million years ago, Earth was a warm, lush place. Not just near the equators, but all the way to the poles. And there was one single continent, called Pangaea. But it’s the nature of things to change, and change it did.
The climate cooled, the sea level changed, and the dinosaurs were facing new environmental pressures. And as the record shows, the dinosaurs were losing species faster than they could replace them. Chicxulub was more than they could recover from.
Study co-author Mike Benton also talked to the BBC about this study. He said, “World climates were getting cooler all the time. Dinosaurs rely on quite warm climates and mammals are better adapted to the cold.”
“So there might have been a switch over in any case without the asteroid impact.”
Looking back on the older narrative, that the asteroid strike wiped out the dinosaurs, and mammals took their place and became dominant, it looks a little simplistic. But it has a nice narrative hook, and there is the matter of the cataclysmic asteroid strike, which no doubt had a huge effect on life on Earth, any way you want to slice it.
It’s possible that had the asteroid not struck, or had struck a few million years earlier or later, Earth would be a much different place. Perhaps we would not be here, and maybe intelligent dinosaurs would be in our place.
We’ll never know, of course, but it’s a fun narrative.
About sixty five and a half million years ago, the Earth suffered its largest known cosmic impact. An asteroid or comet nucleus about 10 km in diameter slammed into what is now the Yucatan peninsula of Mexico. It gouged out a crater 180 to 200 km in diameter: nearly twice as large as the prominent crater Copernicus on Earth’s moon. But did this impact really cause the extinction of the dinosaurs and many other forms of life? Many earth scientists are convinced that it did, but some harbor nagging doubts. The doubters have marshaled a growing body of evidence for another culprit; the enormous volcanic eruptions that produced the Deccan Traps formation in India. The skeptics recently presented their case at a meeting of the Geological Society of America in Vancouver, Canada, on October 19.
The dinosaurs are the most well-known victims of the mass extinction event that ended the Cretaceous period. The extinction claimed almost all large vertebrates on land, at sea, or in the air, as well as numerous species of insects, plants, and aquatic invertebrates. At least 75% of all species then existing on Earth vanished in a short span in relation to the geological timescale of millions of years. The disaster is one of five global mass extinction events that paleontologists have identified over the tenure of complex life on Earth.
The hypothesis that the terminal Cretaceous extinction was caused by a cosmic impact has been the most popular explanation of this catastrophe among earth scientists and the public for several decades. It was proposed in 1980 by the father and son team of Luis and Walter Alvarez and their collaborators. The Alvarez team’s main line of evidence that an impact happened was an enrichment of the metal iridium in sediments dating roughly to the end of the Cretaceous. Iridium is rare in Earth’s crust, but common in meteorites. The link between iridium and impacts was first established by studies of the samples returned by the Apollo astronauts from the Moon.
Over the ensuing decades, evidence of an impact accumulated. In 1991, a team of scientists led by Dr. Alan Hildebrand of the Department of Planetary Sciences at Arizona University, published evidence of a gigantic buried impact crater, called Chicxulub, in Mexico. Other investigators found evidence of materials ejected by the impact, including glass spherules in Haiti and Mexico. Supporters of the impact hypothesis believe that vast amounts of dust hurtled into the stratosphere would have plunged the surface of the planet into the darkness and bitter cold of an “impact winter” lasting for at least months, and perhaps decades. Global ecosystems would have collapsed and mass extinction ensued. But, they’ve had a harder time finding evidence for these consequences than for the impact itself.
Doubters of the Alvarez hypothesis don’t question the ‘smoking gun’ evidence that an impact happened near the end of the Cretaceous, but they don’t think it was the main cause of the extinctions. For one thing, inferring the exact time of the impact from its putative geological traces has proved difficult. Dr. Gerta Keller of the Department of Geosciences of Princeton University, a prominent skeptic of the Alvarez hypothesis, has questioned estimates that make the impact and the extinctions simultaneous. Analyzing core samples taken from the Chicxulub crater, and glass spherule containing deposits in northeastern Mexico, she concludes that the Chicxulub impact preceded the mass extinction by 120,000 years and had little consequence for the fossil record of life in the geological formations which she studied. Of the five major mass extinction events in Earth’s history, she noted in a 2011 paper, none other than the terminal Cretaceous event has ever been even approximately associated with an impact. Several other large impact craters besides Chicxulub have been well studied by geologists and none is associated with fossil evidence of extinctions. On the other hand, four of the five major mass extinctions appear to have some connection with volcanic eruptions.
Keller and other Alvarez skeptics look to a major volcanic event that occurred towards the end of the Cretaceous as an alternate primary cause of the extinction. The Deccan Traps formation in central India is a plateau consisting of multiple layers of solidified lava 3500 m thick. Today, it extends over an area larger than all of France. It was once three times that large. It was formed in a series of three volcanic outbursts that may have been among the largest in Earth’s history. At the October conference, Dr. Theirry Adatte of the Institute of Earth Sciences at the University of Lausanne in France presented evidence that the second of these outbursts was by far the largest, and occurred over a period of 250,000 years prior to the end of the Cretaceous. During this period, 80% of the total lava thickness of the Deccan formation was deposited. The eruptions produced lava flows that may be the longest on Earth, extending more than 1500 km.
To illustrate the likely environmental consequences of such a super-eruption, Adatte invoked the worst volcanic catastrophe in human history. Over eight months from 1783-84 a major eruption in Laki, Iceland, deposited 14.3 square kilometers of lava and emitted an estimated 122 megatons of toxic sulfur dioxide into the atmosphere. About a quarter of the people and half of the livestock in Iceland died. Across Europe the sky was darkened by a pall of haze, and acid rain fell. Europe and America experienced the most severe winter in history and global climate was disrupted for a decade. Millions of people died from the resulting drought and famine. The Laki incident was nonetheless miniscule by comparison with the second Deccan Traps outburst, which produced 1.5 million square kilometers of lava and an estimated 6,500- 17,000 gigatons of sulfur dioxide.
The Deccan Traps eruptions would also have emitted immense quantities of carbon dioxide. Carbon dioxide is a heat trapping greenhouse gas responsible for the oven-like temperatures of the planet Venus. It is released by the burning of fossil fuels and plays a major role in human-caused global warming on Earth. Thus Geller surmised that the Deccan Traps eruptions could have produced both periods of intense cold due to sulfur dioxide haze, and intense heat due to carbon dioxide induced global warming.
At the October conference she presented the results of her studies of geological formations in Tunisia that preserved a high resolution record of climate change during the time of the main pulse of Deccan Traps volcanic activity. Her evidence shows that near the onset of the 250,000 year pulse, there was a ‘hyperthermal’ period of rapid warming that increased ocean temperatures by 3-4 degrees Celsius. She claimed that temperatures remained elevated through the pulse culminating with a second ‘hyperthermal’ warming of the oceans by an additional 4-5 degrees Celsius. This second hyperthermal warming occurred within a 10,000 year period of mega-eruptions, which corresponded with the terminal Cretaceous extinction. The Chicxulub impact occurred during the 250,000 year pulse, but well prior to the extinctions and the hyperthermal event.
The debate over the relative importance of the Chicxulub impact and the Deccan Trap volcanoes in producing the terminal Cretaceous extinction isn’t over. In May of this year, a team headed by Dr. Johan Vellekoop at the Department of Earth Sciences at Ulrecht University in the Netherlands published evidence of a geologically brief episode of cooling which they claim as the first direct evidence of an “impact winter”. Whatever the outcome of the debate, it seems clear that the end of the Cretaceous, with its super-volcanoes and giant impacts, was not a good time for life on Earth.
Hey, remember that one time when 90% of all life on Earth got wiped out?
I don’t either. But it’s a good thing it happened because otherwise none of us would be here to… not remember it. Still, the end-Permian Extinction — a.k.a. the Great Dying — was very much a real crisis for life on Earth 252 million years ago. It makes the K-T extinction event of the dinosaurs look like a rather nice day by comparison, and is literally the most catastrophic event known to have ever befallen Earthly life. Luckily for us (and pretty much all of the species that have arisen since) the situation eventually sorted itself out. But how long did that take?
The Permian Extinction was a perfect storm of geological events that resulted in the disappearance of over 90% of life on Earth — both on land and in the oceans. (Or ocean, as I should say, since at that time the land mass of Earth had gathered into one enormous continent — called Pangaea — and thus there was one ocean, referred to as Panthalassa.) A combination of increased volcanism, global warming, acid rain, ocean acidification and anoxia, and the loss of shallow sea habitats (due to the single large continent) set up a series of extinctions that nearly wiped our planet’s biological slate clean.
Exactly why the event occurred and how Earth returned to a state in which live could once again thrive is still debated by scientists, but it’s now been estimated that the recovery process took about 10 million years.
Research by Dr. Zhong-Qiang Chen from the China University of Geosciences in Wuhan, and Professor Michael Benton from the University of Bristol, UK, show that repeated setbacks in conditions on Earth continued for 5 to 6 million years after the initial wave of extinctions. It appears that every time life would begin to recover within an ecological niche, another wave of environmental calamities would break.
“Life seemed to be getting back to normal when another crisis hit and set it back again,” said Prof. Benton. “The carbon crises were repeated many times, and then finally conditions became normal again after five million years or so.”
“The causes of the killing – global warming, acid rain, ocean acidification – sound eerily familiar to us today. Perhaps we can learn something from these ancient events.”
– Michael Benton, Professor of Vertebrate Palaeontology at the University of Bristol
It wasn’t until the severity of the crises abated that life could gradually begin reclaiming and rebuilding Earth’s ecosystems. New forms of life appeared, taking advantage of open niches to grab a foothold in a new world. It was then that many of the ecosystems we see today made their start, and opened the door for the rise of Earth’s most famous prehistoric critters: the dinosaurs.
“The event had re-set evolution,” said Benton. “However, the causes of the killing – global warming, acid rain, ocean acidification – sound eerily familiar to us today. Perhaps we can learn something from these ancient events.”
The team’s research was published in the May 27 issue of Nature Geoscience. Read more on the University of Bristol’s website here.
[/caption] What killed the dinosaurs? That’s a question that has puzzled paleontologists since dinosaurs were first discovered. Maybe the global climate changed, maybe they were killed by disease, volcanoes, or the rise of mammals. But in the last few decades, a new theory has arisen; an asteroid strike millions of years ago drastically changed the Earth’s environment. It was this event that pushed the dinosaurs over the edge into extinction. What’s the evidence for this asteroid impact? A thin dark line found in layers of sediment around the world; evidence that something devastating happened to the planet 65 million years ago. This line is known as the K-T boundary.
What is the K-T boundary? K is actually the traditional abbreviation for the Cretaceous period, and T is the abbreviation for the Tertiary period. So the K-T boundary is the point in between the Cretaceous and Tertiary periods. Geologists have dated this period to about 65.5 million years ago.
When physicist Luis Alvarez and geologist Walter Alvarez studied the K-T boundary around the world, they found that it had a much higher concentration of iridium than normal – between 30-130 times the amount of iridium you would expect. Iridium is rare on Earth because it sank down into the center of the planet as it formed, but iridium can still be found in large concentrations in asteroids. When they compared the concentrations of iridium in the K-T boundary, they found it matched the levels found in meteorites.
The researchers were even able to estimate what kind of asteroid must have impacted the Earth 65.5 million years ago to throw up such a consistent layer of debris around the entire planet. They estimated that the impactor must have been about 10 km in diameter, and release the energy equivalent of 100 trillion tons of TNT.
When that asteroid struck the Earth 65.5 million years ago, it destroyed a region thousands of kilometers across, but also threw up a dust cloud that obscured sunlight for years. That blocked photosynthesis in plants – the base of the food chain – and eventually starved out the dinosaurs.
Researchers now think that the asteroid strike that created the K-T boundary was probably the Chicxulub Crater. This is a massive impact crater buried under Chicxulub on the coast of Yucatan, Mexico. The crater measures 180 kilometers across, and occurred about 65 million years ago.
Geologists aren’t completely in agreement about the connection between the Chicxulub impact and the extinction of the dinosaurs. Some believe that other catastrophic events might have helped push the dinosaurs over the edge, such as massive volcanism, or a series of impact events.