In the laboratory, many of the techniques of preparation – removing the bones from the rock – are also classic and have remained unchanged for a century. Today, we do have better power tools, better chemicals, and perhaps better respect for the fossils, so that we try to minimize damage. The study of fossil specimens has been revolutionized, though, by new equipment such as CT scanners and scanning electron microscopes. These have opened up possibilities we would not have dreamed possible a decade or so ago.
Digging and cleaning up dinosaur bones can be great fun, but of course it is only really worthwhile if we can then use those bones to learn something about how the dinosaurs lived. In the next chapter we explore whether dinosaurs were warm-blooded or not, how they breathed, and whether they were as stupid as they are supposed to have been.
Chapter 4
Breathing, Brains and Behaviour
Bringing dinosaurs back to life might seem a vain pursuit, and yet that is what palaeobiologists attempt to do. When I was an undergraduate, back in 1975, I was aware of a huge controversy about dinosaurs – were they warm-blooded or not? This engaged many smart scientists, and the public of course. The reporting often focused on the heated arguments – and the various names the scientists called each other, but then everyone likes a bit of rude behaviour. The scientific community had chosen a core question about dinosaurs, and one that has proved difficult to resolve.
The story goes back long before 1975. About 1840, when Sir Richard Owen began his studies of dinosaurs, and indeed when he was gearing up to name the group in 1842, as we saw in Chapter 2, he was also thinking about their palaeobiology. His core question was ‘where did the dinosaurs fit into the story of life?’ At the time, speculation about evolution was generally regarded as dangerous or improper, perhaps something for the French philosophers to contemplate…but the English elite were aware of what that sort of thinking led to – the French Revolution of 1789 – and didn’t want any of that sort of nonsense on their side of the Channel!
Nonetheless, Owen was a skilled anatomist, and the evidence was there. Plants and animals showed evidence of shared similarities, but often with different functions, what Owen and we call homologies. A homology is an anatomical feature with a fundamental structure that is shared, but adapted, by different creatures – such as the arm of vertebrates. We know that the arm of the bird is modified as a wing, the arm of the whale is modified as a fin, the arm of a horse has a single finger and a single hoof, and the human arm has five fingers, but they are all homologues because the fundamental structure is the same: a single upper arm bone (the humerus), two forearm bones (ulna and radius) and fundamentally five fingers can be identified in the bird wing or whale’s fin.
Owen must have struggled to avoid an evolutionary interpretation – that these arms are homologous because they shared a common ancestor. Had they been created, there would be no need for them to show the identical arrays of bones deep inside; but it all makes sense if they shared an ancestor. Later, Owen was framed as the opponent of evolution after the publication of Charles Darwin’s On the Origin of Species in 1859.
Homology of limbs of six different vertebrates.
For Owen, the dinosaurs were amazing and tricky. He looked for similarities with modern reptiles, and found a few. Still, he could see they were not overgrown crocodiles or lizards, as others had said before, and he had the insight and the courage to identify them as members of an entirely new group: hence the name Dinosauria. Further, and amazingly, he argued that they were mammal-like in many characters, including that they were probably warm-blooded. When he, crusty as he might seem to us now, was commissioned to advise on the London Great Exhibition of 1851, he was frivolous enough to imagine the first serious reconstructions ever of dinosaurs, the famous Crystal Palace models of 1853 – he showed Iguanodon and Megalosaurus as overgrown rhinoceros-like animals.
Owen had his reasons. He wanted to argue against evolution by showing that the ancient reptiles were more advanced than modern reptiles, that reptiles had in some way degenerated over time. Be that as it may, we can be grateful to the old warthog for giving us the name ‘dinosaur’, and for daring to step out of line and do the daringly populist thing of bringing whole-body reconstructions to the public – he triggered the first phase of ‘dinomania’, a term invented later to describe the all-consuming public appetite for dinosaurs.
Most importantly, because of his great seriousness and prestige, Owen could sanction such speculation and be accepted. From 1842, palaeontologists and others have sought ways to get to the heart of dinosaurian physiology – how much they ate, how they powered their bodies, and whether they were warm-blooded or not. The quest has involved experts from many fields, new techniques such as the study of fine-scale bone structure, and remarkable new fossils such as the feathered dinosaurs of China.
Sir Richard Owen posing with a friend.
Megalosaurus (foreground) and Iguanodon as visualized by Richard Owen in 1853, in Crystal Palace Park, London.
Were the dinosaurs warm-blooded?
So, were the dinosaurs warm-blooded or not? The answer is, as ever, yes and no. When I joined the debate, with a cheeky article I wrote as an undergraduate, and which was published in 1979 in Evolution, the leading American journal in the field, palaeontologists saw things as polarized. A dinosaur was either warm-blooded like a bird or mammal, or cold-blooded like a reptile. The term ‘warm-blooded’ is a slight misnomer, because the trick birds and mammals have perfected is to keep their internal temperature constant, not necessarily warm; it just so happens that most biologists operate in temperate climates, so when they grab a dog or a baby or a chicken, it feels hot.
This constant temperature comes at a cost, however – typically a human or dog that weighs the same as a crocodile has to eat ten times as much food, because nine-tenths of what we eat is used simply to regulate our core temperature. This is why the crocodile lazes about so much, grinning sardonically at us – he has a great secret. If warm-bloodedness is so costly, why do it? The reason is that birds and mammals can be active all day and all night (lizards and crocodiles are torpid in cold conditions, such as night time), and they can occupy cold parts of the Earth.
In my 1979 article, I picked up two points that were relevant. First, warm-bloodedness is not always better than cold-bloodedness, and second, that living animals show a gradation between the two states. Some new physiological work in the 1970s had shown that insects and reptiles could generate internal heat – think of the bumblebee flying on a frosty day, shivering like mad to warm up the flight muscles before taking off. Further, small birds and mammals often switch off at night because they can’t eat enough to keep warm all the time; others hibernate, which is the same thing.
The debate was kicked off by maverick palaeontologist Bob Bakker, who, as a grad student at Yale in the 1960s, had first shown that many dinosaurs were fleet and fast, and then followed this through to its logical conclusion. He was a smart writer and skilled artist, and could conjure images of T. rex galloping through the undergrowth and the great sauropods rearing up on their hind legs to snatch leaves from high in the trees. Perhaps most would reject those images as over-fanciful, but Bakker stimulated the modern era of dinosaurian palaeobiology. People had to accept that dinosaurs did not have the physiology of modern crocodiles. First, many might have had feathers, especially those on the evolutionary line to birds, and what about the giant sauropods? How could a 50-tonne sauropod have functioned if it had the physiology of a modern lizard or crocodile? We need to explore these two ideas, and then see how the study of the internal structure of dinosaur bones helped to solve the conundrum.
Are birds living dinosaurs?
In 1984, Bob Bakker and Peter Galton, an English palaeontologist who became established in the United States, published a provocative paper in Nature entitled ‘Dinosaur monophyly and a new class of vertebrates’, in which they stated not only that dinosaurs were a single clade (see Chapter 2), but also that modern bir
ds were dinosaurs: ‘Recently Ostrom has argued forcefully that birds are direct descendants of dinosaurs and inherited high exercise metabolism from dinosaurs.’ They were undoubtedly right, but this paper was designed to enrage the old guard.
John Ostrom was the real revolutionary, and he was Bob Bakker’s doctoral supervisor at Yale. As with any professor from Yale at the time, Ostrom was reserved, polite, and always impeccably dressed, being famed for his brightly coloured plaid jackets. Ostrom had spent much of the 1960s excavating and describing an amazing new dinosaur, Deinonychus (see overleaf) from the Early Cretaceous of Wyoming. Ostrom could not escape the observations that, first, this dinosaur was built for speed and manoeuvrability as a hunter, with its huge slashing claw on the second toe of the foot; and, second, that the skeleton of Deinonychus was pretty well indistinguishable from that of Archaeopteryx (see overleaf), the first bird (see pl. iv).
When Ostrom published his monograph on Deinonychus in 1969, it was an instant hit – a very careful piece of anatomical description, with beautiful illustrations of an astonishing dinosaur. The frontispiece was an inspired pencil drawing of Deinonychus at speed, reflecting precisely John Ostrom’s vision of the dinosaur, and a revolutionary depiction of dinosaurs as fast and active. The artist? Bob Bakker.
John Ostrom, affable as ever, but not wearing his signature plaid jacket.
The amazing image of Deinonychus drawn by Bob Bakker in his student days.
What is the evidence that birds are dinosaurs? Evidence was first noted back in 1870, in fact, by Thomas Henry Huxley, when he wrote about the newly discovered fossil bird Archaeopteryx. The skeleton had been unearthed in a limestone quarry at Solnhofen in southern Germany in 1861, igniting a bidding war among the museums of Europe. The fossil ended up in the British Museum in London, purchased thanks to the drive of Richard Owen, director of the natural history portion of the museum, for the enormous sum of £700 (equivalent to £80,000 today). Owen wanted to have the specimen so he could publish the first description, which he did. But it was an embarrassment to him in many ways. He noted the close similarity of all its bones to those of dinosaurs, and indeed modern birds. He also noted the clear impressions of feathers on the wings and over the body.
Genus:
Deinonychus
Species:
antirrhopus
Named by:
John Ostrom, 1969
Age:
Early Cretaceous, 115–108 million years ago
Fossil location:
United States
Classification:
Dinosauria: Saurischia: Theropoda: Maniraptora: Dromaeosauridae
Length:
3.4 m (11 ft)
Weight:
97 kg (214 lbs)
Little-known fact:
Deinonychus preyed on the much larger Tenontosaurus either by slashing or biting into its flesh until it bled to death.
Genus:
Archaeopteryx
Species:
lithographica
Named by:
Hermann von Meyer, 1861
Age:
Late Jurassic, 152–148 million years ago
Fossil location:
Germany
Classification:
Dinosauria: Saurischia: Theropoda: Maniraptora: Avialae (birds)
Length:
0.5 m (1¾ ft)
Weight:
0.9 kg (2 lbs)
Little-known fact:
The first Archaeopteryx fossil to be found was an isolated feather, in 1860. The first complete skeleton was discovered a year later.
Owen was loath to call Archaeopteryx a ‘missing link’, as he had opposed Darwin’s dangerous new ideas of evolution, published two years earlier in 1859. Thomas Henry Huxley, by contrast, had no such qualms. As skilled an anatomist as Owen, he got sight of the specimen and used Owen’s description to write his own paper about dinosaurs and birds. He pointed out all the similarities, and that Archaeopteryx was key evidence for evolution – the perfect intermediate between dinosaurs and birds, with its primitive long bony tail and teeth and its advanced feathers and wings.
For nearly a century, everything had seemed settled – additional specimens of Archaeopteryx and of small theropod dinosaurs continued to confirm Huxley’s insights – but then the research field veered wildly off track. For all sorts of reasons, palaeontologists stopped seeing birds as dinosaurs – maybe they couldn’t believe such amazing flying machines as birds could have evolved in as little as 20–30 million years, or they were afraid to admit that they had some great evidence of evolution in their hands. Whatever the reasons, it took a century for palaeontologists to emerge from their state of denial and to accept that Huxley was right in 1870, just as Ostrom was right in 1970: birds really are dinosaurs.
Ostrom noted everything Huxley had seen, and especially the fact that Deinonychus had bucked the trend of evolution of the other theropods – it was relatively small and it had long arms. Other theropods, such as T. rex, became huge and their arms dwindled. What Ostrom didn’t know, but guessed, was that Deinonychus had had feathers, and indeed its arms were long just so that they could carry the specialist flight feathers seen along its arm and that of Archaeopteryx and other birds. Confirmation had to await the discovery of the remarkable birds and dinosaurs from China in the mid-1990s, as we shall see.
Ostrom, though, saw that theropods shared hollow bones with birds, as well as the fused clavicle (commonly called the wishbone) in the chest region, the semilunate carpal in the wrist (allowing Deinonychus and birds to fold the hand back, as birds do when they tuck their wings back along the side of the body), expanded eyes capable of 3D vision, an appropriately expanded brain (needed for leaping or flying from tree to tree), and many more characters.
Thomas Henry Huxley – did he perhaps know how smart he was?
Nonetheless, since these early papers by Ostrom, Bakker, and Galton, there has been a remarkably vocal crew of nay-sayers who continue to express a counter view until well into the twenty-first century, and will doubtless carry on doing so. They have survived on the ‘balanced’ airtime given by scientific documentaries – ‘here’s one view; and here’s the other’. Never mind that the bird-dinosaur view is supported by hundreds of independent bits of evidence and the ‘birds are not dinosaurs’ view lacks an alternative theory and lacks evidence. This might be the one negative aspect of the great public interest in advances in dinosaur science: the fact that proponents of rejected views can promote their ideas directly to the public even if the scientific journals, with their systems of scrupulous peer review, no longer accept their papers.
Bone histology and being huge
Ostrom’s evidence that Deinonychus was a dinosaur close to the origin of birds in the evolutionary tree gave credence to Bakker’s (and Richard Owen’s) view that dinosaurs were warm-blooded. However, those early debates around 1970 were quite unsophisticated. Many of the lines of evidence brought forward at the time were suggestive, but not decisive, and so the debate meandered inconclusively. One, though, has proved to be fruitful.
This is bone histology, the study of the internal microscopic structure of bones. Since the 1800s, biologists had used the light microscope to study cells and microscopic life. Sections of bones showed their complex internal structure, with dense bone on the outside, and often more open bone tissue near the centre. In life there are no spaces, and bone is full of fat, blood vessels, and nerves. Bone histologists noted that modern cold-blooded animals in particular, such as fishes and reptiles, have distinctly layered bone – this tracks their fast growth in summer and slow growth in winter, and the growth layers build up more or less like growth rings in a tree. As we shall see later (Chapter 6), palaeontologists can use the growth lines to age dinosaur skeletons and to build up growth curves for individual species, showing how the rate of growth can vary over the years, from hatchling to adult.
Warm-blooded animals such as birds or mammals, on the other hand, tend to have bone w
ithout obvious layering, because they grow evenly all the time (a consequence of having internal temperature control) and the bone often shows evidence of remodelling. Bone remodelling is represented by tubular structures that cut through the background structure, and is a result of the high metabolic rates of birds and mammals, which mean that they lay down calcium and phosphorus in their bones, but also need to remobilize those elements at times for other purposes such as producing eggs or surviving a tough winter.
It turns out that dinosaur bone structure is more bird- or mammal-like than reptile-like. In the microscopic section illustrated opposite, there is a background of regular so-called fibrolamellar bone that has some layering, but this does not represent annual increments. The black spots are cavities in which the cells that build up bone and the cells that break down bone would have resided. Scattered over the field of view are secondary remodelled canals, some highlighted by orange iron staining – these cut across the regular structure. So, dinosaur bones show evidence of extensive internal remodelling, and Bakker rightly interpreted this to mean that dinosaurs were warm-blooded. As many pointed out at the time, though, there are ways and ways of being warm-blooded, and size is one of them. Some large crocodiles and snakes today show gigantothermy, a great word that says it all: they are huge and their size helps regulate their internal temperature. It’s simple physics that if a cylinder is heated up, it cools fast if it is small, and takes much longer to cool down if it is large.
Dinosaurs Rediscovered Page 10
