Dinosaurs Rediscovered

by Michael J Benton

  We used Bayesian statistical methods, which involve seeding the calculations with a starting model, and then running the data millions or billions of times to assess how well the starting model fits the data, allowing for every possible source of uncertainty, and repeatedly adjusting the model to make it fit better. In this case, Manabu modelled uncertainty about dating the rocks, gaps in the record, accuracy of the phylogenetic tree, and many other issues. The results were unequivocal: for all dinosaurs, and for each of the three major dinosaur groups (theropods, sauropodomorphs, ornithischians), they showed buoyant evolution until 100 million years ago, with speciations exceeding extinctions, and then they all tailed off about 40 million years before the end, turning down into decline. Only two groups, as we had found before, the duck-billed hadrosaurs and the horn-faced ceratopsians, showed increasing diversity.

  All three groups of dinosaurs – sauropodomorphs (top), theropods (middle), and ornithischians (bottom) – show a long-term decline in the last 40 million years of the Cretaceous.

  Our 2016 paper has been controversial – some researchers have got it, but others have misunderstood. Our result does not say that there are not individual locations, such as the Hell Creek Formation, where diversity remained locally high, nor that all groups of dinosaurs were in free-fall. There’s a time-scaling aspect as well – we see a gradual decline over 40 million years, but at regional scales and over short time spans, diversity would continue to rise and fall, depending on local conditions.

  There is one broader implication, and that concerns whether dinosaurs could have continued evolving. It was a common pursuit once to indulge in such imaginative ‘what if ’ thinking: if the asteroid had not hit, what would the world look like today? One Canadian palaeontologist even reconstructed the modern dino-man, a kind of humanoid dinosaur with a big brain, no tail, and human-like posture. However, our work suggests that, even if the asteroid had not hit, the dinosaurs would probably not have survived to the present day. Cooling climates, changing vegetation, and changing configurations of the continents might have done for them anyway by 50 or 40 million years ago.

  The killing worldwide

  The sequence of events following the asteroid impact was outlined at the start of this chapter. We do not yet know exactly how quickly the crisis spread over the world, but the effects differed from place to place. Within a few hundred kilometres of the strike, the impact would have stimulated an immediate blinding flash of light, followed a few seconds later by a massive earthquake, as the asteroid drove into the crust and vaporized. Then the debris cone shot sideways and in an ever-expanding circle. Within 500 kilometres (310 miles) of the impact, the power of the shock wave would have killed everything, including any dinosaurs, such as T. rex, Triceratops, or Ankylosaurus. The rocks carried by the blast would also have pummelled the trees and animals to the ground. Further away, the larger rocks would have already fallen to the ground, but the expanding shock wave continued for thousands of kilometres. Around the crater, the oceans mounted into a huge tsunami that probably did not kill life in the sea, except when it came near the shore. There, the hungry wave sucked water back from the shallows, exposing coral reefs and flapping fishes and marine reptiles, and then crashed onto the shores of the proto-Caribbean, along a broad arc through eastern Mexico, and swinging up through Texas and round the southern United States to Florida. Any animals on the shore, including dinosaurs, would have been drowned and their carcasses thrown into piles with loose rocks and trees.

  The shock wave and following fireball probably shot across most of North America, killing plants and animals as they went. Further afield, for example in Europe, Asia, and Africa, the flash of the impact might have been felt, and then, some hours later, the sky would have changed colour as the dust spread outwards, high in the atmosphere. Perhaps the dinosaurs were not much affected during the first day or two, but then the build-up of dust would have made day into night, and the darkness continued perhaps for weeks on end.

  Without light, the dinosaurs remained in a torpid state, lolling on the ground, dozing as they always had during the night. But this time, the warmth of the rising sun did not come, and they slept on, most of them perhaps passing without knowing to death. The lack of light killed most animals, and plants, unable to photosynthesize, would have shrivelled too. Perhaps, if the dust cleared through rainfall during those first grim months, they did not all succumb, and as light and warmth returned, they became restored. Insects could rest dormant and perhaps fishes underwater escaped some of the worst effects.

  A study in 2018 of the Cretaceous–Palaeogene rock section at El Kef in Tunisia has shown that temperatures rose by 5 degrees for about 100,000 years, immediately after the impact. This was determined by measurements of oxygen isotopes from fish bones buried in the rocks, and the high temperatures persisted for about 3 metres (10 feet) of the section. The increase in temperature was probably driven by additional carbon dioxide in the atmosphere produced by melting of limestones by the impact and by the forest fires that followed soon after.

  Such a modest rise in temperature would have driven some species from tropical areas during the initial stages of recovery from the crisis, but the change was not large enough in itself to contribute to a great deal of further extinction. All the evidence suggests, then, that the crisis was short and brutal, and that there were persistent environmental changes induced by the impact. But the Earth and life recovered amazingly fast, in geological terms, say within 100,000 years of those elevated temperatures, and it is worth looking in more detail at the fates of two of the winning groups: the birds and mammals.

  How did birds survive the mass extinction?

  Birds survived the cataclysm 66 million years ago, and a 2018 study has shown what happened. Like mammals, birds had already evolved for some time in the Mesozoic, but identifiable birds are known only from the Late Jurassic onwards, say from 155 million years ago. These first birds include the famous ‘ancient bird’ Archaeopteryx from the Solnhofen lagoons of southern Germany (see p. 111), as well as other examples from rocks of similar, or even slightly older, provenance from north China. In fact, the crossover from small feathered theropod dinosaurs to birds was subtle, so deciding which was the last dinosaur and which was the first bird on that evolutionary branch is a merely semantic exercise.

  It used to be thought that birds had survived the mass extinction with minimal damage – a few ancient lines had died out, but birds survived and then diversified rapidly to reach eventually the 11,000 living species. This was supposed to confirm their great adaptability, hence their modern success, while emphasizing the contrast between them and their relatives the dinosaurs, which all died out. However, the story seems different now.

  Close study of the bird fossil record, coupled with exploration of modern bird evolution based on genomic analysis, shows that only five species of birds crossed the Cretaceous–Palaeogene boundary. In fact, it seems that the whole fluttering and twittering multitude of birds came close to annihilation. So, what happened?

  As we saw earlier, the Jehol Beds of north China have produced thousands of astonishing specimens of Cretaceous birds, showing us a far higher diversity than we had ever imagined. Four main groups of birds reached the end of the Cretaceous and were killed off by the asteroid strike. These are the enantiornithines, a diverse group of eighty species of flying birds, whose diets included hard-shelled prey caught in the water or by probing in mud, or vertebrates and arthropods, possibly snatched from the trees. The second group, the palintropiforms, comprise only two or three species from Mongolia. The ichthyornithines were fish-eating, flying birds with powerful toothy jaws, known from the shores of the Western Interior Seaway of North America. Finally, the hesperornithiforms, also known mainly from the same localities in North America, were generally large, flightless, diving, fish-hunting birds.

  Evolution of birds through the Cretaceous–Palaeogene interval, showing that mainly ground-dwellers survived.

  The only
survivors were a few species of ground-dwelling ancestors of ducks and chickens. This could be explained simply by saying they were lucky – but a 2018 study suggests something else. Dan Field, an enthusiastic young palaeo-ornithologist, worked with colleagues on a large-scale ecological study of birds. Using a phylogenetic tree of modern species, they annotated which forms are tree-dwellers, such as robins, owls, swifts, and parrots, and which live mainly on the ground, such as ostriches, ducks, chickens, rails, shorebirds, and gulls. Tracking back to the latest Cretaceous, they found that the ancestors of all modern birds had been ground-dwellers at the time of the asteroid hit.

  Phylogeny of modern birds, showing that the ancestors of most were ground-dwellers.

  All the tree-dwellers, together with the unfortunate ichthyornithines and hesperornithiforms, died out. Field and colleagues coupled this finding with evidence from the fossil records of plants and pollen, which showed that forests had been devastated for about a thousand years by the consequences of the asteroid strike. The blotting out of the sun would have halted photosynthesis and generally killed plants, while acid rain and other dire consequences of the impact probably flattened forests. Birds and other animals that lived in the complex ecosystems in and around trees would have been left homeless.

  Then, in the Palaeogene, Field and colleagues noted that the first birds of modern aspect to emerge were largely ground-dwellers, scampering around after insects or feeding on marine life on the shore. They could tell the broad mode of life of the fossils by looking at the relative lengths of portions of the leg – tree-dwellers tend to have quite long femurs, whereas ground runners have shorter femurs, and the parts of the leg below the knee are longer. True tree-dwelling took some time to re-emerge, and many different lines of birds independently adapted back to tree life when the opportunity arose.

  The survival of birds through the mass extinction was, one might say, a close-run thing then. What about the mammals?

  How did mammals take over from dinosaurs?

  The classic story is that after the mass extinction 66 million years ago, the mammals took over – which indeed they did. The asteroid impact was tough for the dinosaurs, as well as for the pterosaurs, marine reptiles, ammonites, belemnites, and many other groups, but it was good for our ancestors, and therefore ultimately for us. Mammals had, in fact, originated at the same time as the dinosaurs, in the Triassic, and yet most of them remained small and nocturnal throughout the Mesozoic. How can we be sure that it was the removal of dinosaurs from ecosystems that enabled mammals to diversify in the Palaeogene?

  The answer comes from numerical modelling. Graham Slater of the University of Chicago ran tests in a 2013 paper. First, he constructed a dated phylogenetic tree of the evolution of early mammals through the Mesozoic and Palaeogene. He then tried fitting all kinds of evolutionary models to the data, and found that the best one in terms of its ability to explain the data was what he called the ‘release and radiate’ model – this model described the evolution of mammals as having been held back by the dinosaurs, and then, when the dinosaurs disappeared, they were released from the negative pressure and radiated explosively. The other models, including various random patterns, a driven trend, or simply extinction of the dinosaurs with no ecological reaction from mammals.

  So, the old assumption that mammals were held back by the dinosaurs has been confirmed by a smart computational model – when competition from the large, daytime dinosaurs had disappeared, mammals could evolve diurnal forms and larger forms, covering the whole range of habitats. Indeed, their evolution was particularly rapid in the early Palaeogene, which is striking because they had been around for 170 million years of the Mesozoic. Ten million years after the mass extinction, mammals had evolved all modern groups, ranging from small shrew-like creatures to flying bats and monster whales, from large, armoured herbivores to massive-skulled predators, and even monkey-like tree-dwellers, including our direct ancestors.

  The explosive evolution of mammals after the Cretaceous–Palaeogene event, 66 million years ago.

  The evolution of mammals in the past 66 million years has seen continuing expansion worldwide. As temperatures zig-zagged up and down, but mainly down, habitats changed profoundly. Most notable was the spread of grasslands about 30 million years ago. As climates cooled, rainfall patterns became less tropical and more temperate, and the centres of continents became dry. The forests drew back, and great prairies spread over much of North America and South America, as well as central Africa and Asia. Whereas mammals until then had been adapted to living somewhat secretive lives in the forests, the new grasslands provided opportunities. The ancestors of horses and rhinos on the one hand and cattle and deer on the other crept out of the edges of the forests, and some began eating the burgeoning new grasses.

  This was tough food, because most grasses contain silica to protect themselves from grazing, so the herbivores had to evolve deeper and tougher teeth that could grind the grit. Further, the early grassland herbivores were vulnerable out in the open. Their small size, which had worked to protect them in the forests, was no longer an advantage. Successful species evolved to be taller, with long thin legs so they could peer around for predators, but also run off at speed when threatened. Famously, horses evolved from animals the size of a Labrador dog to the modern size at this time, and the same goes for most other successful herbivores. The predators, too, adapted to running rather than climbing.

  One view of early human evolution posits a similar shift from forest to plain. Our nearest relatives, chimpanzees and gorillas, stayed in the African forests where they still reside, but our ancestors apparently stepped out onto the hot grasslands of central and eastern Africa 5 million years ago. We owe our upright posture to the need of these early ape-persons to peep about over the tall grass in search of lions and cheetahs, and then to carry food and tools in their free hands.

  Study of the end-Cretaceous mass extinction has been a helter-skelter of new ideas. It’s also a great example of how speculation has turned into science, and then told us things we never expected. The brilliance of the insights by Luis Alvarez and his team, when they proposed their model for asteroid impact and global devastation back in 1980, has been borne out by thousands of investigations since. What they predicted has been found, and few now doubt the strength of the evidence in favour of impact – the Chicxulub crater in Mexico, the tsunami beds and evidence of melt beads all round the proto-Caribbean, and the worldwide spread of fine dust with iridium enrichment.

  Palaeobiological work has also massively refined our picture of the sequence of events of the mass extinction. It seems that dinosaurs were already in decline, slowly, over their last 40 million years on Earth, but in the end the impact and subsequent cold and darkness finished them off at a stroke. Birds and mammals also suffered, and only certain species struggled through the grim environmental conditions. These surviving birds and mammals then evolved explosively when the world settled down after the crisis, and they form the basis of modern terrestrial ecosystems.

  There is much more to be done in focusing ever closer on the event, to work out the step-by-step deterioration of environments. Also, we cannot yet fully integrate the devastation caused by the asteroid strike with that caused by the longer-running Deccan Traps eruptions in India. Then, the nature of evolution before, during, and after the crisis is yet to be explored fully. Recent studies of dinosaur decline and selective extinction and then diversification of birds and mammals are resolving long-running questions. The fact that these events 66 million years ago still resonate in the structure of modern ecosystems adds urgency to these further studies.

  Afterword

  During the past forty years, we have seen the transformation of palaeontology into science. This has involved expansion of the area of testable science, and reduction of the area of speculation. Arguably, the area of speculation is infinite, and probably grows as people ask more questions. Nonetheless, I have documented how major questions about function (W
hat did that dinosaur eat? How fast could it run? What colour was it? How fast did it grow?) can now all be answered by testable means.

  What is science?

  Philosophers have long debated the definition of science. One thing for sure is that there is more to science than simply mathematics, where proof is possible. In all other science, proof is never possible, merely disproof. This leads to massive misunderstanding, especially by flat-earthers, climate-change deniers, creationists and others – they like to argue that science is about facts, and think that if they can dislodge a fact, the whole science fails.

  In reality, natural science is about hypotheses and theories. I can have many hypotheses about why the dinosaurs died out (see Appendix) or why sauropods were so large, and some of these modify into theories when there is a sufficient body of coherent evidence. For example, the Alvarez model for catastrophic impact at the end of the Cretaceous and its role in killing the dinosaurs is a theory with very good evidence. It cannot be proved, but it could easily be disproved. Especially when Luis Alvarez and his team established the theory in 1980, the actual evidence was modest – essentially the iridium spike from two localities, Gubbio and Stevns Klint. Year by year, evidence accumulated that confirmed their suggestion, and nothing was found to refute it. If it had been shown that the iridium spikes were all of different geological ages, or the iridium spike only occurred in those two localities, the theory would have been refuted. Then, in 1991, scientists found the impact crater itself, which gave good, clinching evidence for the Alvarez theory. The exact role of the asteroid impact in killing the dinosaurs, and whether it did so alone or with additional pressure from changing climates and the eruption of the Deccan Traps, are still matters of some debate. But the theory is the most convincing one yet.