What killed the dinosaurs?

The asteroid impact was severe. Its timing was more difficult * The extinction of the dinosaurs is one of the greatest mysteries in science. A common theory proposed a few decades ago claims that an asteroid impact with Earth doomed them to death, but some have questioned this hypothesis and asked if other factors also contributed to the downfall of the dinosaurs. New analysis suggests the giant space rock hit just as certain populations of dinosaurs were sensitive and vulnerable / by Steven Brest

The article was published with the approval of Scientific American Israel and the Ort Israel network

The Church of the Savior on Spilled Blood, standing on the edge of a cold water canal in St. Petersburg, Russia, looks like it was taken from a fairy tale. Its forest of onion-shaped domes points to the sky and a mosaic of pastel colors covers every square inch of its walls. It's not the kind of place that usually interests paleontologists, but I was in town to research a new dinosaur and insisted on going to see the church as well. The visit was private. The church was built on the spot where the revolutionaries assassinated Tsar Alexander II in 1881, an event that started a chain of events that eventually led to its existence. The death of the tsar was the opening signal for the rampage of pogroms. The Jews who lived on the fringes of the Russian Empire began to fear, and my family, who lived in Lithuania at the time, was very frightened and sent their youngest son to safety in the USA. He was my great grandfather. If it weren't for this series of dominoes that started rolling more than a century ago in St. Petersburg, I wouldn't be here today.

Every family has such a story, strange twists of fate in the distant past without which the present would be very different. Evolution also works this way. The history of life is one big sequence of thing dependent on thing, a sequence that can be diverted from its course at any moment. Indeed, this is exactly what happened 66 million years ago, at the end of the Cretaceous period. In the 150 million years before that, the dinosaurs roamed the earth. They grew to enormous proportions and thrived in almost every imaginable environment on earth. But then some change took place and the tyrannosaurus, the triceratops and the like disappeared.

The extinction of the dinosaurs is one of the biggest mysteries in all of science and is why I fell in love with science when I was a teenager. For the past ten years, as I have collected dinosaur fossils all over the world, the question has haunted me: How come such successful creatures simply disappeared? A popular theory put forward in the 80s of the 20th century holds that it was an asteroid that wiped them out. But there are those who doubt this and wonder if other forces also contributed to their loss. As researchers discover more types of dinosaurs and learn more about the evolution of this group, we are getting closer to a definite answer.

Not long ago I organized a large international conference of paleontologists who met to formulate exactly what we know about the cause of the extinction of the dinosaurs. We used the most up-to-date list of dinosaur species to examine evolutionary trends over time, reviewed the latest information on the timing of the extinction, and looked at any environmental changes that occurred around it. To our surprise, our team of about a dozen dinosaur experts, a group that is often prone to disagreements, came to a clear consensus: as is commonly thought, the extinction was sudden, and an asteroid was largely to blame. But that's not the whole story: this asteroid hit the Earth at exactly a very bad time for dinosaurs, a time when their ecosystems were sensitive and vulnerable due to a climate change that preceded it. This is a new and unexpected twist on an old story and is incredibly relevant to today's world and our story of evolution.

An ongoing mystery

Like most teenagers, I did some reckless things when I was in high school. Perhaps the most daring of them was to pick up the phone one day in the spring of 1999 and call, without any prior arrangement, Walter Alvarez, a geologist at the University of California at Berkeley. I was a dinosaur-crazy 15-year-old and he was a distinguished member of the US National Academy of Sciences, who had proposed almost 20 years earlier the idea of ​​the large asteroid impact that wiped out the dinosaurs. His hypothesis was born following an intriguing discovery. The geological records include a thin layer of clay that separates the sediments rich in dinosaur remains from the Cretaceous period, from 145 to 66 million years ago, and from the sediments lacking remains from the Paleogene period, from 66 to 23 million years ago. Alvarez discovered that this clay layer is saturated with iridium, an element rare on Earth but common in celestial bodies such as comets and asteroids. The first place where he discovered this unusual phenomenon was a rocky ravine not far from the ancient town of Gubbio in Umbria, Italy. As fate would have it, my family prepared for a trip to Italy for my parents' 20th wedding anniversary. I suggested to my parents that we take a break from the basilicas and museums and visit Gubio one day to see the geological features that gave birth to Alvarez's famous deadly asteroid scenario. But I needed directions, and I decided to get them straight from the source.

The fact that Alvarez not only answered the phone but also gave me detailed instructions on how to get to the exact spot in the ravine where he noticed the iridium belt, is still wonderful to me. I didn't expect such a scientific giant to be so nice, and set aside time for it so generously. His asteroid theory, which he published in the journal Science in 1980 with his physicist father, Nobel laureate Luis Alvarez, and two other colleagues from Berkeley, set off a decade of heated debate. Dinosaurs and mass extinctions were in the headlines all the time; The idea of ​​collision has appeared in countless books and television documentaries; And hundreds of scientific articles have argued against each other around the question of what really killed the dinosaurs. Paleontologists, geologists, chemists, ecologists and astronomers contributed to the hottest scientific issue of those times.

By the end of the 80s there was no longer any doubt that an asteroid or comet had collided with Earth 66 million years ago. The same layer of iridium was found all over the world. And other mysterious geological phenomena attributed to the impact of extraterrestrial bodies were also found by her, including glass clusters called tektites and distorted quartz grains. More than that, geologists have even located a crater that dates back to the exact moment of the extinction of the dinosaurs: the Chicxulub Crater in Mexico, which is 180 kilometers in diameter. Something unexpected and huge, about ten kilometers wide, came from space and triggered a chain of disasters of volcanic eruptions, fires, tsunami waves, acid rain and dust that darkened the sunlight, which sealed the fate of the dinosaurs.

And yet, the scientists had little and precious real information about the changes that occurred in the dinosaurs in the times before the impact and about their exact response, and that of the ecosystems they lived in, to this extraordinary environmental disaster. Therefore the polemic continued on the question of whether this asteroid struck the dinosaurs like a thunderbolt on a clear day, while they were at their peak, or whether it landed a final blow on a degenerating group, which was already dwindling and was expected to become extinct even at the worst. After all, the asteroid did not hit a fossilized planet, but a planet that experienced sharp fluctuations in sea level, large temperature changes and vigorous volcanic activity. Maybe some of these factors influenced the extinction?

New findings

I didn't manage to get to Gobbio on that family trip to Italy. Floods blocked the main railway from Rome, and I was very disappointed. Fate is sometimes cruel (ask the dinosaurs), but sometimes the opposite also happens. So imagine how surprised I was, five years later, when I visited Italy again on a field course of the college I attended. We stayed at a small observatory in the Apennines run by Alessandro Montaneri, one of the many scientists who made a name for themselves in the 80s thanks to the study of the end-Cretaceous extinction. On our introductory tour we went through the library. A single man was standing there looking at a geological map under a flickering light. "I want you to meet my friend and mentor, Walter Alvarez," said Montaneri in a playful Italian accent. "Maybe you've heard of him."

Hidden trends: vegetarians are in trouble

A few days later we were in Gobbio Gorge, the Mediterranean sun was shining, and cars were whizzing by. Alvarez stood in front of a class of students and pointed to the exact spot where the asteroid theory was born. My classmates laughed at me because I couldn't stop smiling after I introduced myself to Alvarez, and he remembered our conversation from five years ago. This day is etched in my memory as one of the most important moments at the beginning of my career. That's when I knew I was a prisoner of the dinosaur extinction puzzle.

Somewhat paradoxically, my graduate studies focused mainly on the rise of the dinosaurs to power, the origin of the birds and their early evolution (the birds are the descendants of the dinosaurs and are therefore the only group of dinosaurs that did not become extinct). But in 2012, when I was nearing the end of my PhD, I finally had the opportunity to contribute to the issue of the extinction of the dinosaurs. My colleague Richard Butler from the University of Birmingham in England, who uses statistics to study evolutionary trends, came up with a nice idea: maybe we combine our areas of expertise in different groups of dinosaurs and different analysis techniques for a renewed look that examines the changes that took place in dinosaurs in the period 10 to 15 million years before they became extinct?

We decided to check the trends of the degree of diversity of dinosaurs according to an estimate called morphological disparity. Distinctness is essentially an anatomical measure of biodiversity: it quantifies differences in body size, shape and anatomy within a group, over time or in different ecosystems. Imagine two ecosystems, one with 15 species of rodents and the other with a bat, doe and elephant. In the first system there are apparently more species, but in the second there are species whose differences in terms of size, shape and behavior are much greater. Distinctness can sometimes provide a more complete picture of the vitality and biodiversity of groups than the number of species alone can provide. We wanted to check if there were clear trends in the dinosaur populations. An increasing or constant divergence in the late Upper Cretaceous age would be a sign that the dinosaurs were doing pretty well when the unnamed asteroid interrupted their glory days, while a decreasing divergence would be a sign that they were in trouble even without the big rock that fell from the sky.

The results we got are very interesting. Most dinosaurs had a relatively stable divergence in the 10 to 15 million years preceding the impact, including the carnivorous theropods (such as the Tyrannosaurus and the Velociraptor), the long-necked sauropods, and the small to medium-sized herbivores (the dome-headed Pachycephalosaurus, for example). But there were two subgroups whose distinctiveness was declining when the asteroid landed: the horned dinosaurs (Triceratops and its family) and the flat-beaked dinosaurs. Both groups included large, herbivorous dinosaurs that consumed vast amounts of vegetation. If we were 66 million years ago, we would immediately notice that these are the most common dinosaurs. They were the cattle of the Cretaceous, the main herbivores in the food web.

Around the time we published the results, other researchers were looking at the extinction of the dinosaurs from other angles. Groups of researchers led by Paul Upchurch from University College London and Paul Barrett from the Science Museum in London conducted a census of a variety of dinosaur species over time, and found that the species diversity of the dinosaurs as a whole was still very large at the time of the asteroid impact, but the number of species in the group that included the beaked platypus and The horned owner was on the decline. Their findings clearly matched our variance calculations.

How could the decline in species richness and diversity in the large herbivores affect the rest of the dinosaurs? Research using an innovative computer model conducted by Jonathan Mitchell, who was then a doctoral student at the University of Chicago, provided insights into the matter. Mitchell and his team built food webs in several dinosaur ecosystems during the Cretaceous period and simulated a scenario where several species are removed from the web. The result was amazing: the food webs that existed when the asteroid hit, which included few large herbivores due to their reduced diversity, collapsed more easily than the more diverse webs that existed a million years earlier.

Bad timing

In light of the amount of new information about the extinction of the dinosaurs that appeared in the journals, Butler and I came up with a somewhat risky idea: we might be able to recruit a team of dinosaur experts who would agree to sit down and discuss everything we now know about the extinction and try to reach an agreement on the question of why the dinosaurs disappeared from the world. At first it was just for fun. Paleontologists have been debating this for decades and have not solved the problem. The chances are therefore that our small and subversive plot will hit a dead end, or worse: a shouting match. In practice, the opposite happened. The group we formed, which included 11 scientists from the USA, Canada and Great Britain, actually came to an agreement. We published our research in the journal Biological Reviews in May 2015.

Here's what we found after looking at the data: Dinosaurs were probably in pretty good shape at the end of the Cretaceous. There are no signs that their overall diversity (in terms of number of species and distinctness) has gradually declined in the millions of years preceding the asteroid impact. The large groups of dinosaurs all survived right up to the end of the Cretaceous, and at least in North America, where the fossil record of dinosaurs from the last age of the Upper Cretaceous is the most complete, we know that the tyrannosaurus, the triceratops and their tribes witnessed the impact of the asteroid. This finding refutes the once prevalent assumption that the dinosaurs gradually became extinct, apparently due to long-term fluctuations in sea level and temperature, which changed the land area and the types of food available to them. The extinction of the dinosaurs was sudden, on a geological time scale. It is therefore likely that the impact of the asteroid, a sudden and unexpected event, is to blame for this.

However, as we hypothesized based on our previous studies, the asteroid is not the whole story. The large herbivorous dinosaurs were indeed in a certain decline right at the end of the Cretaceous. It is not clear what exactly caused this decrease, but there may have been a connection between it and the short-term decrease in sea level that sharply changed the land areas accessible to dinosaurs in the last few million years of their existence, at least in North America, where the fossils from this period have been preserved much better than in other places. The flat-billed dinosaurs and the horned dinosaurs, being the most common herbivores, were certainly the first to be affected by these changes in habitat and vegetation. Their deterioration had consequences: it made the ecosystem more prone to collapse by undermining the foundations of the food web, which increased the chance that the extinction of a few species would trigger an ecosystem-wide collapse.

All in all, it seems that the asteroid hit the dinosaurs at a very bad time. If this had happened a few million years earlier, before the decline in the diversity of the large herbivore species, the dinosaur ecosystem would have been more robust and better able to withstand the damage. If this had happened a few million years later, the diversity of vegetarians might already have returned to us, as happened countless times in the 150 million years of dinosaur evolution. There is no good time for an asteroid with a diameter of 10 kilometers to fall from the sky, but for the dinosaurs, 66 million years ago, it might have been the worst possible time. If the timing of events had been slightly different, the dinosaurs might still be here today.

What happened 66 million years ago, when this block of rock and ice from space was slammed into Mexico at the most inappropriate time for dinosaurs gives its signals to this day. Mass extinctions are a tragedy, but they also make room for new plants and animals and allow them to evolve and take over. The death of the dinosaurs opened an opportunity for mammals, who lived in the shadows for more than 100 million years and now got a chance to develop undisturbed. Mammals began to flourish almost immediately after the extinction of the dinosaurs. They grew to large sizes, developed a multitude of sources of nutrition and behaviors and spread throughout the world. This prosperity eventually led to the emergence of primates, including us. The removal of any link in this chain of reactions means, with high probability, that there were no humans.

But there is an important lesson from this in the story of the extinction of the dinosaurs. This is not just a deceptive act of evolutionary alternatives, one of those events in the distant past that allow us to play "what if..." In simple words, what happened at the end of the Cretaceous era teaches us that even the most dominant group of living creatures can become extinct, and quite suddenly. The dinosaurs own the dome more than 150 million years before their doomsday: a split second of collision between the Earth and the space rock. The depletion of their biological diversity in the period preceding the asteroid impact facilitated their extinction, and perhaps even enabled it. Modern man has existed for at most a few hundred thousand years, and we are changing our environment at such a rate that a "sixth extinction" occurs, in which the biological diversity of the world decreases rapidly. Who knows how vulnerable we become because of this process?