Dinosaurs Rediscovered

by Michael J Benton

  IX The skulls of Camarasaurus (left) and Diplodocus (right) showing reconstructions of the jaw muscles (above) and the application of different loads (below). In the loading illustrations, warm colours (red, yellow) indicate high stress and strain.

  X Comparison of the skulls and their engineering properties of Coelophysis (left), Allosaurus (middle) and Tyrannosaurus (right), showing photographs of the skulls (top row), the surface mesh models (middle row), and the loaded models (bottom row). Red shows high stress and strain, green low values, and arrows the ways in which forces are distributed.

  XI Cutaway of the skull of Iguanodon with the brain imaged in position.

  XII The skull (above), brain (below) and semicircular canals (in pink) that form the middle ear of T. rex.

  XIII Bone structure (a) of a female Confuciusornis, showing medullary bone (white arrows in b and c).

  XIV A mother Maiasaura (meaning ‘good mother lizard’) looks dotingly at a nest of her babies in the Wyoming Dinosaur Centre, Thermopolis.

  XV Juvenile dinosaurs used to hang out together. A block containing six juvenile Psittacosaurus. Bone histology and growth rings show they are all two years old, except number 1 (in pink) which is three.

  XVI Side view of the skull of a juvenile Massospondylus, based on CT scans.

  XVII Side view of the skull of an adult Massospondylus, based on CT scans.

  XVIII The unique teeth of hadrosaurs. Replacement teeth line up below the set in use (above). The dental tissues (below) show multiple folds, as in a bison or horse’s teeth.

  XIX The asteroid impact that ended it all, 66 million years ago. Artist’s impression of the huge rock from space as it hit the proto-Caribbean Sea over what is now southern Mexico (above), and the resulting double-ringed crater (below).

  Chapter 9

  Mass Extinction

  Sixty-six million years ago, a great rock hit the Earth and wiped out the dinosaurs. The rock was an asteroid, essentially a small planet or a large meteorite. It measured up to 7 kilometres (4 miles) across, the size of Manhattan, and as it drove into the Earth’s crust, just off the coast of the Yucatán peninsula in modern Mexico, it blasted out a deep hole and caused shattering of the crust to an even greater depth, and over a much wider radius, than the crater itself (see pl. xix).

  The impact had a kinetic energy of more than 10 billion megatonnes. This is a thousand times the amount of energy contained in all the world’s nuclear weapons arsenals. During the impact, the asteroid vaporized, sending powerful shockwaves downwards and sideways into the surrounding rocks.

  After a second or two, once the asteroid had driven down as far as it would go, there was a massive reaction. Vast quantities of rocks shot upwards and sideways, creating a conical expanding crater. Larger blocks fell back into the crater and around its rim, but smaller boulders, melt materials, and rock dust formed from a mixture of the Earth’s crust at the crater site and the asteroid itself rose as a huge plume and shot out sideways at high speed.

  At the time, there would have been easterly winds around the equator, as today, caused by the westward rotation of the Earth, and these blew the plume of dust and rock fragments west of the crater. A blanket of rubble and bombs ejected from the crater formed outside the crater rim, and the dust was lofted into the upper parts of the atmosphere and travelled around the world.

  The impact had two further effects. First, the melt rock from the impact site formed into small glassy beads, each about a millimetre across, and billions of these flew through the air to land in great mounds all over the landscape and in the sea. It was the speed of transit through the air that made them form into beads, as the molten rock cooled while twirling in the air.

  The asteroid hit the Earth at the edge of the Caribbean Sea, and so it also produced great tsunamis, or tidal waves. The tsunamis formed walls of water probably tens of metres high, and they travelled at the speed of a jet plane, some 800 kilometres (500 miles) per hour. The Mexican and Texan coasts, only a few hundred kilometres away, were devastated, with the tidal waves ripping up rocks and life along the coast and dumping it all in a chaotic pile. Further away, the tsunamis lost much of their height and power, so dinosaurs in Europe would have seen little more than a slight ripple on the beach.

  Map of the proto-Caribbean, showing the end-Cretaceous shoreline, evidence of tsunami beds, and the impact site.

  The tsunami would have killed everything along the shores of the proto-Caribbean, probably for several hundred kilometres inland. Any dinosaurs dipping their toes in the water would have been rudely awakened, flung in the air, and hurled to the ground. Other killing effects of the impact included the rain of huge rocks from the sky – but that probably didn’t kill too many dinosaurs, since most of these rocks would have fallen back into the sea. Yet the smaller gravel-sized particles, including the glass melt beads, did travel over land and they would have peppered the landscape, and any dinosaurs in their path, with scattershot punches as if from a huge shotgun.

  Then came the second pulse, beginning a few seconds after the debris cone. A huge fireball shot upwards from the crater site, composed of vaporized material from the asteroid and taking heat from the huge energy of the impact. The fireball expanded sideways, after the debris of the first phase had settled. It set fire to all plants and animals in its path, leaving a blackened landscape. Again, like the physical blast, the fireball could not have encircled the Earth, but it devastated North America and the Caribbean. So, the fireball was a fourth potential killer (after the rocks, melt beads, and tsunami): bad news if you were in its way, but still not enough to cause worldwide extinction.

  It was the dust, drifting seemingly innocently in the upper reaches of the atmosphere, many kilometres up, that was the real killer. As the tsunami, rockfalls, and wildfires were sweeping out from the crater, the huge black dust cloud blew rather passively with the winds around the northern hemisphere. It probably covered some of the southern hemisphere, maybe all of it. Global wind patterns do not guarantee that the whole globe would have been shrouded.

  It may have taken a few days after the impact for the dust cloud to reach its full extent. It probably showered fine dust particles on the Earth all the time, but would have taken years to dissipate completely. This was no innocent cloud, though. It contained millions of tonnes of dust. As it spread and thickened, the Earth beneath was thrown into total blackness. With the sun’s rays entirely blocked, no light or heat could get through for perhaps a year. Now that might be the real killer.

  In addition to all these details, we know that the impact happened in June…more about that later.

  These cataclysmic events 66 million years ago set the shape of the modern world, including the dominance of modern ecosystems by birds and mammals. And yet the whole scenario was unknown when I learned geology in the 1970s – indeed, if anyone had talked about such a catastrophe they would have been ridiculed. We have thus seen an astonishing switch in our understanding of the mass extinction that finished the dinosaurs, from rejection and speculation to a substantial body of established scientific knowledge. How did this happen?

  The road to accepting mass extinction

  This whole dramatic story of devastation now seems clear and it is supported by a great deal of evidence. When I was a student, though, mass extinction was not considered. We heard that dinosaurs had died out gradually, over millions of years, and so, in some way, the transition from the Cretaceous period to the Palaeogene period 66 million years ago was thought to be gradual. Looking back, this seems amazing – how could geologists and palaeontologists have misunderstood what the rock and fossil records showed?

  I believe there were three reasons, back in the 1970s, why geologists and palaeontologists kept well clear of mass extinctions: fear of catastrophe, fear of numbers, and fear of ridicule.

  Geologists had been taught by Charles Lyell in the 1830s that catastrophes did not occur. In his seminal Principles of Geology, he set out with lawyerly skill, and based on extens
ive field work, the new geological science. The science was what he had seen in his native Scotland, in England, France, and Italy, and his main argument was that geologists must use pure observation and a view of modern processes to interpret the history of the Earth. To add piquancy, he used his training as an advocate to identify an opposing view, termed catastrophism, which he then exaggerated in order to bolster his principle of uniformitarianism: that geological processes occurred at a uniform rate, as we can observe today. Lyell was the supreme rationalist, and he painted his opponents, including Georges Cuvier and others, as dangerous, wild, relying on supernatural explanations – in the case of Cuvier also French, and conveniently deceased as the last volume of Principles went to the press in 1832.

  As students, we were taught Lyell’s uniformitarianism, as are all geologists today. It is obvious that we should observe how modern volcanoes, rivers, and beaches work, so we can interpret the ancient rocks laid down by these agencies. Lyell went further, though, and claimed that not only were the processes the same, but so too were the magnitudes or scales of such processes. He argued that volcanoes in the past were no greater than today – but critics now point out that this is perverse. Why? Relying solely on human experience is an unnatural narrowing of the frame of reference – and we now know that there were many huge volcanic eruptions and meteorite impacts in the past, far greater in magnitude than any human has observed, or at least observed and written about. Nonetheless, Lyell’s views of uniformitarianism held a tight grip over geology until the 1980s.

  Palaeontologists have always been afraid of numbers. I remember, as a young lecturer, attending a special meeting of the Royal Society in London in 1988, where the theme was extinction, and we heard talks by twenty world experts, some flown in from the United States. The most notable invitee was David Raup from the University of Chicago, and he gave what I thought was a perfectly reasonable talk about the evidence for extraterrestrial causes of extinction. He was looking at how to interpret species extinctions in the fossil record. He deployed a numerical approach, one that used repeated random sampling of simulated data to show how missing rock layers and missing fossils could give misleading results. At the end of his talk, a British professor stood up and said, ‘We don’t want these kinds of crazy ideas brought in from North America’, and more of the same. I was astounded, and should have defended Raup, but the attack was received with some humorous and perhaps supportive snorts from the audience. Raup, an extremely brilliant and gentle man, who had led the field for decades in showing smart ways to turn palaeontology into science, swore never again to visit Britain.

  What was the logic behind this mindless attack? We had a bit of nationalism (‘we don’t want foreign ideas here’), a bit of protectionism (‘these are my fossils and I’m the expert on them; you can’t use my data’), and of course fear of numbers. It’s clear, though, that Raup was right and his critic was wrong. Palaeontology ought to be as much of a science as any other, and mysticism or unfounded claims of authority have no place.

  The third fear was fear of ridicule, which relates to catastrophism, but also to the long history of theorizing about the death of dinosaurs. I have counted more than 100 ‘theories’ for dinosaur extinction that have appeared in scientific journals since the 1920s (see Appendix). These ranged from environmental catastrophes (it got too hot or too cold, too wet or too dry), to dietary issues (caterpillars ate all the plants, mammals ate all the dinosaur eggs, or new plants gave dinosaurs constipation), to mystical assumptions (dinosaurs were too big, they got arthritis, their brains shrank, their horns and headshields were too unwieldy, they were too weird to evolve, they got AIDS). Impact by meteorites or comets seemed just as daft, so it was thought to be safer to keep your head down and say that the study of extinctions is too dangerous for any sane researcher.

  The concept of mass extinctions is now much better understood by scientists and the public. In fact, mass extinctions can be claimed as one of the most important discoveries in the Earth sciences. Geologists and palaeontologists have unique access to the body of data about them, and mass extinctions cannot be predicted from any study of modern organisms. They were events of profound scale and importance in evolution. When one thinks about it, they have a positive side too – we always attribute the success of ‘modern’ groups, such as birds and mammals, including ourselves, to the events that saw the end of the dinosaurs, and freed the world for major ecological restructuring.

  The change in viewpoint, from a fear of talking about mass extinctions to acceptance, happened about 1980, and it took a Nobel-prize-winning physicist to shake the palaeontologists out of their cocoon of complacency.

  The impact of the impact theory in 1980

  The hammer blow fell on 6 June 1980, eight years before the London meeting at which Raup was so snidely dismissed. I was then a doctoral student in Newcastle, reading as widely as I could about dinosaur evolution and extinction. On that day, a paper entitled ‘Extraterrestrial cause for the Cretaceous–Tertiary extinction’ was published. It said:

  A hypothesis is suggested which accounts for the extinctions and the iridium observations. Impact of a large earth-crossing asteroid would inject about 60 times the object’s mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record.

  The paper was led by Luis Alvarez, who had won the Nobel prize in 1968 for his invention of a means to image interactions between particles in the newly developed hydrogen bubble chamber. He was well respected for his brilliance and skill at building impossible pieces of equipment in the laboratory. He also had a brusque approach to scientists when he thought they were poor thinkers. For example, in a telephone interview reported by a New York Times writer in 1988, he said: ‘I don’t like to say bad things about paleontologists, but they’re really not very good scientists. They’re more like stamp collectors.’ Understandably, remarks such as these, whether true or false, did not endear him to the dinosaur community.

  The Alvarez team also included his son, geologist Walter Alvarez, as well as geochemists Frank Asaro and Helen Michel. The discovery hinged on Luis Alvarez’s invention of a means to measure vanishingly tiny amounts of the element iridium, chemically related to platinum. Iridium occurs in minute quantities in soil and rocks, and it can be a small component of some volcanic lavas, but is much more abundant in space, by a factor of 720 times. Therefore, most of the tiny quantities of iridium found on the Earth’s surface comes from extraterrestrial sources, primarily the continuing shower of small meteorites (tektites) over the surface of the Earth.

  The idea of Alvarez père et fils was to use the steady (but minute) rain of iridium as a chronometer against which rocks could be dated. For a long time, geologists had realized that rock thickness does not equal time, for two reasons. First, some rocks are deposited very fast and some very slowly – an extreme example comes from the deep ocean, where muds may typically accumulate at rates of only a few centimetres per century, but can be interrupted by catastrophic turbidity flows, sometimes triggered by earthquakes, in which hundreds of metres of sand and rock may be dumped in a single day. Second, we have the gaps between rock layers, and we have no idea how long a gap might be. If there were a yardstick of time, such as a measurable influx of iridium dust, then geologists could at least tackle the first issue.

  Luis (left) and Walter Alvarez with his hand on the Cretaceous–Palaeogene boundary in the Gubbio section, Italy.

  Walter Alvarez chose a rock section near Gubbio in central Italy, near Perugia, where he knew there were hundreds of metres of marine limestones of latest Cretaceous and earliest Palaeogene age, which were well dated by microfossils. The sampling showed similar levels of iridium near the bottom and top of the section, suggesting in fact that the limestones had been deposited in
a steady manner. But in the middle, at the Cretaceous–Palaeogene boundary, now dated at 66 million years ago, there was a spike where values shot up to ten times normal – that is from 0.6 parts per billion to 6 parts per billion (these tiny quantities show the need for a sensitive measuring instrument). Now, this is where the first smart thing happened – the Alvarezes took a sideways leap. If they had stuck to their working hypothesis, they would have said this boundary layer was highly condensed, meaning that it took ten times longer to deposit that particular 1 centimetre (3/8 inch) of sediment than above or below, and so the iridium level shot up ten times. Instead, they daringly said that this indicated the sudden and rapid arrival of a huge amount of iridium from outer space: thus a vast meteorite.

  The iridium spike.

  In their paper, Luis Alvarez and colleagues then used this one observation as a basis to erect their hypothesis. (They did cross-check it against one other section, at Stevns Klint in Denmark.) The reasoning was that if a large meteorite, an asteroid, killed the dinosaurs, it must have thrown up a sufficiently large cloud of dust to encircle the Earth, and they did a back-calculation from that assumption. Here is their formula:

  M = 0.22f/sA

  where ‘M’ is the mass of the asteroid, to be worked out from the other factors, which are known: ‘s’ is the surface density of iridium just after impact (8 × 10−9 grams per square centimetre); ‘A’ is the surface area of the Earth; ‘f’ is the fractional abundance of iridium in meteorites (0.5 × 10−6, known from modern meteorites); and 0.22 is the proportion of material from the 1883 eruption of Krakatoa that entered the stratosphere. So, they calculated M = 34 billion tonnes, equivalent to an asteroid diameter of 7 kilometres (remarkably, precisely the size estimated when the actual impact crater was eventually found), and predicting a crater twenty times as large, say 150 kilometres (over 90 miles) across.