Dinosaurs Rediscovered

by Michael J Benton

  Testable methods: bracketing

  In making his comments about the feeding behaviour of Tyrannosaurus, Professor Osborn referred to modern predators such as lions and hunting dogs. Their behaviour may give us clues about how extinct animals behaved. For example, many hunting dogs today are too small to bring their prey down with a decisive bite to the neck, as a lion or tiger might. So, a small pack of wolves in Canada may follow a moose and snap at its leg tendons trying to break them and so cripple the animal. The wolf could be killed any moment by a kick from the moose, so it has to circle and rush in fast to deliver a bite. After many miles of chase, the exhausted moose may collapse, and the wolves can at last kill it and feast on its flesh. These observations provided ideas for how smaller predatory dinosaurs might have harried their larger prey.

  In these cases, the palaeobiologist is using modern analogues as a way of making believable, and vivid, his or her assumptions about the fossils. In some cases, study of the modern analogues points the palaeobiologist at things to seek in the fossils. Maybe pack hunting cannot be determined in a dinosaur skeleton, but hunting modes may be deduced from the frequency of broken and damaged bones found in the hunters – were they risk-takers, like modern wild cats and dogs, leaping at larger prey and risking injury?

  There is a key question, though: how do you choose your modern analogue? If you are trying to understand the hunting tactics of Tyrannosaurus rex, is the wolf, as a mammal, a good analogue? Would an example of hunting behaviour from a lion or eagle, or even from a shark, be equally useful? This question was left unanswered until 1995.

  In that year, Larry Witmer argued that an insight he had developed would allow us to say a great deal about every unpreserved detail of, say, T. rex. We could describe its eyeball, its tongue, its leg muscles, even its behaviour around egg-laying and hunting. Witmer’s insight is called the extant phylogenetic bracket. (Phylogeny is the evolutionary history of an organism or organisms.) He reasoned that, if the analogues were well chosen, they could tell us a great deal. For example, in the evolutionary tree, birds and crocodiles are close relatives – they are all archosaurs (‘ruling reptiles’), together with the dinosaurs. If crocodiles and birds share some detail of the eyeball or the leg muscles, then dinosaurs had it too. We can’t say dinosaurs had feathers simply because birds have feathers – crocodiles do not have feathers, so dinosaurs are not bracketed as far as that character is concerned. That’s why we can confidently describe the form and function of the eyeball of T. rex – not because of random comparisons with lions or sharks, but because crocodiles and birds, which bracket the dinosaurs in the evolutionary tree, share most features of eye structure and function. Likewise, we can say that T. rex probably showed some minimal parental care after its babies hatched – because crocodiles and birds both share this behaviour.

  The extant phylogenetic bracket of dinosaurs.

  I can give a concrete example that was crucial in our work on the colour of dinosaur feathers. We studied how colour is expressed in modern bird feathers, and how the different kinds of melanin, the black-brown kind and the ginger kind, are associated with different microscopic organelles. Black-brown melanin is packed into sausage melanosomes, ginger melanin into ball melanosomes. We saw this in bird feathers, and it was always the same. It’s also true of all mammals, including humans. In the evolutionary tree, dinosaurs, and most other extinct reptiles, are bracketed by birds and mammals, so this is a universal relationship. Therefore, when we saw the ball melanosomes in Sinosauropteryx, we thought, birds have these, mammals have these, and so this works for the bracketed dinosaurs too. Sinosauropteryx had ginger feathers.

  Testable methods: engineering models

  Another testable method in palaeobiology is the engineering analysis of digital models. A digital model is a perfect 3D rendering of an object inside a computer. The model can be rotated and magnified, and the analyst can fly in through the left eye socket of the digital Tyrannosaurus skull, and out through its mouth, before returning through the right nostril to explore inside the nasal cavity. The secret to testability is to map the correct material properties onto the bone – in other words, the material properties of bone, as calculated from modern bone: what force it takes to smash a 1-centimetre (3/8-inch) cube of bone, and how far a bone of a certain diameter can be bent before it snaps. Then the engineering analysis can begin.

  For her doctoral studies in Cambridge, UK, Emily Rayfield had to work out the form and function of the skull of Allosaurus, a large predatory dinosaur of the Late Jurassic. She scanned the skull, and repaired it by replacing missing and damaged bones and removing distortion to create the perfect 3D digital model of the skull. She then assigned material properties to different parts of the skull – hard and brittle for the enamel of the teeth, softer and more pliable for parts of bone around the sides of the skull.

  To assign material properties the skull is divided into pyramidal ‘cells’ or elements, and then a classic engineering method can be applied, finite element analysis (FEA). This is the method used by architects and civil engineers to stress-test their designs before beginning construction. Every skyscraper, bridge, or aircraft to which you entrust your life has been pre-tested using FEA.

  The argument is that we know the method works. The digital model of a future skyscraper, bridge, or aircraft is stress-tested to see at what point of applied pressure it breaks. This is the basis of the engineering design of the structure before it is completed, and we live in skyscrapers and fly in aircraft designed this way, trusting that the calculations were correct. Therefore, if we use the same approach to study a dinosaur skull or leg, we should accept the results as true. Inside the computer is a perfect functioning model of the extinct animal. This is a pretty amazing claim – that palaeobiology is testable science. Even Ernest Rutherford might have accepted that we can now turn some parts of palaeobiology into rigorous, hard science.

  The revolution

  I have lived through a revolution. When I started as a student some forty years ago, palaeobiology was a practical subject aimed at solving problems for the oil industry – especially relevant in the town where I grew up, Aberdeen. The granite city was experiencing massive economic growth as a result of the North Sea oil boom. If my professors talked about form and function or evolution, they did so a little apologetically, because they were straying from hard facts.

  Through my scientific career, I have seen dinosaur science (and palaeobiology in general) change from natural history to testable science. New technologies have revealed secrets locked in the bones – we can now work out the colour of dinosaurs, their bite forces, speeds, and levels of parental care. I have taken an active part in the debates about reconstructing the tree of life, the Jurassic Park phenomenon and the viability (or not) of dinosaur DNA, the CT scanning and digital imaging revolution, and new engineering models to test the bite force and running speed of T. rex, as well as the colour of dinosaurs.

  Much of the press coverage of modern palaeobiology focuses on remarkable new fossil finds, such as giant sauropod dinosaur skeletons from Patagonia, dinosaurs with feathers from China, and even a tiny dinosaur tail in Burmese amber. New fossils are the lifeblood of modern palaeobiology, of course, but it is the advances in technologies and methods that have driven the revolution in scope and confidence.

  The skull of T. rex (above) and a digital model that enables the skull to be stress-tested (below). In the lower diagram, the darkness of shading indicates the amount of stress, with light greys indicating high stress.

  The aim of this book is to show all the latest amazing fossils, and to take the reader behind the scenes on the expeditions and in the museum laboratories. The key theme throughout is the transformation of a historical science from its roots in Victorian natural history to a highly technical, computational, and thoroughly scientific field today. These have been exciting times of rapid change and astonishing new discoveries, happening at a rate never seen before.

  Chapter 1 />
  Origin of the Dinosaurs

  One thing is known for sure: the dinosaurs originated during the Triassic period, between 252 and 201 million years ago. Nearly everything else is uncertain. For example, just when did they originate, in the Early or Late Triassic? What was the world like as they emerged on the scene? Did they force their way to dominance of global ecosystems by fighting hard for their place against other beasts, or did they achieve their position by good luck? When I began my career as a palaeontologist, back in the 1980s, these were all hot topics of discussion. Solving the questions has been my life’s work, but I can’t say everything is sorted out: whenever one problem is resolved, further questions are raised. It’s a story of changing ideas about evolution, new fossils, and new analyses.

  As part of my doctoral studies I tried to work out an ecological model for the origin of the dinosaurs. The ‘standard’ model then was a three-step process. First, the synapsids, ancestors of mammals, were the key herbivores and carnivores. Then, the synapsids were replaced by rhynchosaurs as herbivores, and early archosaurs as carnivores. Archosaurs include birds and crocodiles today, and dinosaurs and their ancestors. Finally, the rhynchosaurs and early archosaurs gave way to dinosaurs. We will encounter all these animals shortly, especially the rhynchosaurs and the first dinosaurs.

  The classic model for dinosaur origins by progressive competitive replacements in the Triassic.

  These three steps were said to form an ecological relay, in which one group gives way to another, which in turn gives way to another. This ecological-relay model for dinosaur origins had been presented by the two great American palaeontologists at the time, Al Romer and Ned Colbert, who were the authors of all the standard textbooks, so their ideas were widely distributed and widely read. Importantly, the Romer–Colbert relay model assumes competition between all these animals, and that the dinosaurs in some way fought their way through to dominance. How did they do it? Possibly because they had erect or upright posture, and so could run faster than their unsuccessful neighbours. In broader evolutionary terms, the Romer–Colbert ecological-relay model was firmly framed within an assumption that large-scale evolution was progressive.

  I presented an entirely opposing view in a 1983 publication, as a cheeky young research fellow. I argued that the dinosaurs had exploded onto the scene about 230 million years ago, not after a long competitive struggle, but following an extinction event. The rhynchosaurs and early archosaurs had been killed off by climate change, which had led to drying conditions and the prevalence of different kinds of plants, notably conifers. The rhynchosaurs chomped unhappily at the unforgiving needles and cones of the new arid-land conifers; they were, in fact, adapted to feeding on equally tough, but more nutritious, vegetation such as seed ferns, but these plants required damper climates. Perhaps the drying climate and spread of conifers led to the rapid demise of the seed ferns, and then of the rhynchosaurs. In their heyday, the rhynchosaurs had been hugely abundant, making up as much as 80 per cent of the entire fauna. After their extinction, the dinosaurs took their chance and expanded into empty ecospace – this, I argued back in 1983, is opportunism, rather than progress.

  This new idea of mine was probably quite annoying for the established palaeontologists. Indeed, I had a somewhat heated, and unexpected, discussion with the doyen of Triassic dinosaur studies in Britain, Dr Alan Charig, head of the dinosaur section at the Natural History Museum in London. He buttonholed me at a conference in Manchester in 1985 and we had a serious discussion – in the showers. (In those days conferences were typically housed in university halls of residence with communal shower facilities.) I was trying to convince Charig that we should use numerical, phylogenetic methods to resolve big questions in macroevolution, but he could not agree; we agreed to differ, and parted on good, if slightly damp, terms.

  This is a story, then, of evolution on the large scale, but it depends also on a good knowledge of the fossils, the rocks, and the models of large-scale evolution. We shall look at the ecology of Triassic beasts, then the rhynchosaurs (an odd, but endearing, group of Triassic animals that are key in many ways), then the question of the very first dinosaurs, and how we can put together the story of fossils, changing climates, and mass extinctions to see how dinosaurs rose to dominate the Earth.

  Ecology and the origin of dinosaurs

  So why did Romer, Colbert, and Charig argue that dinosaurs outcompeted their rivals? It was partly that progress was assumed in evolution – dinosaurs replaced their inferior competitors (the synapsids, rhynchosaurs, and early archosaurs), and were in turn replaced by mammals, 180 million years later. Each step along the way marked an improvement of some sort, by which the animals became faster, smarter, or at least better competitors.

  This is in some ways pure Darwin – survival of the fittest, constant improvement. We have learned since 1980, however, that evolution is not unidirectional or relentless. In fact, the physical environment keeps changing, as, for example, climates become warmer or cooler, continents move positions, mountain ranges emerge, and sea levels rise and fall. As conditions change, the plants and animals embedded in them keep adapting in a purely Darwinian evolutionary way, but they never quite attain perfection. Environmental changes are unpredictable, and somewhat random, so species are on the whole good at what they do, but probably never perfect.

  The focus in the 1980s was on posture. Today, reptiles such as turtles, lizards, and crocodiles are sprawlers: that is, they hold their arms and legs quite a bit out to the side. When they walk, if you view them from above, each arm and leg describes a wide arc, and the backbone bends from side to side. Sprawling reptiles keep their belly close to the ground, and can generally only scuttle fast for short distances. Mammals, on the other hand, have erect gait, meaning their arms and legs are tucked right under their body. When they walk they use the whole length of the arm and leg in making a stride, and there isn’t much lateral movement of the limbs or the body. Famously, many mammals, such as horses or wolves, can run fast for very long distances, which generally sprawlers cannot do.

  Key stages in the origin of dinosaurs through the Triassic.

  The argument, then, was that there had been a major transition in the posture of reptiles during the Triassic. The synapsids and rhynchosaurs were mainly sprawlers, so it was argued, and the dinosaurs were erect, and this gave them the competitive edge. The dinosaurs lived life at a faster pace than their synapsid and rhynchosaur precursors, and they won out in a kind of biological arms race that lasted through the entire 50 million years of the Triassic.

  This theory seemed clear and it explained the data. However, I found it unsatisfactory, and this came from my realization that the fossils and rocks told a different story. The dinosaur takeover was rapid, not gradual, and there was no evidence for direct competition. This grew out of my doctoral studies on rhynchosaurs, a group of reptiles that were ecologically dominant worldwide just before the dinosaur explosion.

  Rhynchosaurs

  Starting my doctoral studies in 1978, I was assigned the topic of working on Hyperodapedon (see overleaf), a rhynchosaur from the Late Triassic of Elgin in Scotland, by my supervisor, Alick D. Walker, at the University of Newcastle-upon-Tyne. My job was to examine the twenty or so specimens of this rather odd, four-legged, bulky herbivorous reptile. The specimens had been collected since the 1850s in yellow sandstones around Elgin, an attractive market town in northeast Scotland.

  The fossils were annoying because they were holes in the rock. At some point in the 230-million-year history of that corner of Scotland, the rocks had been buried, squeezed, baked, and then uplifted. The bone material was still there, but awkwardly putty-like. In Victorian times, the museum preparators had laboured hard with hammer and chisel to remove the fine-grained sandstone from these squishy bones, but the results were generally disappointing.

  Alick Walker had had the insight in the 1950s, when he began his life’s work on the Triassic fauna near Elgin, to remove any remaining bone scrap and
then make high-fidelity casts from the natural rock moulds. By some means that I never discovered, he selected PVC as his moulding material of choice. This is the stuff of rubber gloves, starting out as a thick liquid that can carry colour, which is poured into a mould, baked to cure, and then pulled out again. The extreme flexibility and strength of a PVC rubber glove was what we wanted – after pouring and baking, the PVC had infiltrated deep into every cavity and crack within the rock.

  Sometimes, I had to round up three or four fellow students to help me haul a PVC cast of a leg bone or a skull out of the rock. It was worth it, though, because the sandstone retained very fine details, revealing, for example, the tear duct of the eye, major blood vessels, and bone sutures in the skull of Hyperodapedon.

  Now, rhynchosaurs could be up to 1.5 metres (5 feet) long, and they have a very recognizable skull, with a hooked snout, a sort of grin when viewed in side view, and a very broad skull at the back. The breadth of the skull created a huge space between the (small) braincase in the middle of the skull and the jaws, which in life was filled with several powerful jaw muscles. The diameter of a muscle gives a measure of its power, and there was no doubt that rhynchosaurs had amazingly powerful jaws. Their dentition bore this out, consisting of several rows of teeth, being emplaced at the back of each jawbone, and expanding the tooth row as the animal grew larger. Teeth near the front were worn flat by close wear against those of the opposite jaw. Indeed, one of the first palaeontologists to describe rhynchosaurs, the famous Victorian supporter of Darwin, Thomas Henry Huxley, compared their jaw action to the closing of a penknife – the lower jaw is the blade, and it fits snugly into the deeply grooved upper jaw. This shows that the only jaw action they were capable of was to cut the food precisely, as if with a strong pair of fabric scissors, an action technically termed ‘shearing’. The jaws could not move sideways, so they could not chew their food.