The key characteristics of dinosaurs are in the hindlimbs.
For the Tübingen meeting, we had each come up with a similar list of unique features of Dinosauria. I stopped at the origin of dinosaurs, but Dave Norman and Paul Sereno went further, and proposed outline trees for Ornithischia, whereas Jacques Gauthier did the same for Saurischia. Nervous as we were – and we did receive some criticism at the meeting – it seems the time had come. Bakker and Galton had taken a great deal of the flak in 1974, and so ten years later, when we presented such knock-down evidence – coming from four of us independently – the world was perhaps ready to listen. We all published more detailed papers later, documenting all the evidence, and this became the textbook norm.
In detail, it turned out I had been wrong about many of the supposedly unique dinosaur characters – in fact, many of them were present also in silesaurids and other close relatives of dinosaurs. My only excuse is that many of these beasts had not yet been discovered in 1984, and when they were later presented to the world, they caused a redistribution of characters up and down the cladogram; but it did not affect the overall hypothesis.
Having established the tree of dinosaurs (see overleaf), we should introduce some of the key players, following the narrative through geological time.
The evolution of dinosaurs, from their origin to their extinction.
The Triassic explosion
As we saw in Chapter 1, dinosaurs originated deep in the Triassic – the period from 252 to 201 million years ago – and then diversified in two or three steps. By the time of the Late Triassic, many key forms had emerged. This is clearly demonstrated in the Trossingen Formation in southern Germany, dated at about 215 million years ago. Fossils have been collected from this 40-metre-thick (130-foot) unit of yellow-coloured sandstones, most notably in some large excavations undertaken in the 1920s near Stuttgart.
Picture the Late Triassic in southern Germany. The landscape is quite flat, with abundant wet-climate plants such as horsetails, ferns, and seed ferns growing around the rivers and lakes, and dry-climate conifers around the hills. A two-legged theropod, Liliensternus, flits past, chasing a small lizard. Liliensternus is 5 metres (16½ feet) long, slenderly built, and with a long, narrow skull. It snaps at the lizard, but its prey darts away. Then, a great thundering is heard, as a pack of much larger dinosaurs bursts onto the scene. Liliensternus crouches among the plants, looking out for juveniles it could pick off.
Genus:
Plateosaurus
Species:
engelhardti
Named by:
Hermann von Meyer, 1837
Age:
Late Triassic, 227–210 million years ago
Fossil location:
Germany
Classification:
Dinosauria: Saurischia: Sauropodomorpha: Plateosauridae
Length:
up to 10 m (33 ft)
Weight:
1 tonne (2,205 lbs)
Little-known fact:
Herds of Plateosaurus skeletons were once thought to have died in arid desert, but they were actually trapped in soft mud.
The new dinosaurs on the scene are a herd of Plateosaurus, some twenty individuals ranging in size from babies, barely a metre long, to juveniles 5 metres (16½ feet) long, and hoary old adults 10 metres (33 feet) in length. The Plateosaurus mainly stand up on their hind legs, but flop back down onto all fours to feed on horsetails by the river bank. The feet have four toes that spread wide to support the body weight. The hands also have four main fingers, and the thumb claw is large and flattened. With its broad curve, the Plateosaurus rake up plant food on the ground, before stooping to snatch it in their jaws. Their skull is long, almost horse-shaped, with a nostril at the front, and a long snout, with the jaws lined with twenty-five strong, leaf-shaped teeth above and below. They snip leaves with their front teeth, and tip their head back to push the leaves down into their throats and swallow the chopped plant fragments. When they stand up to look around, they throw their head and neck back, drop their tail towards the ground, and hoist their heavy front quarters off the ground.
A tree falls, and the herd of twenty shoot away at speed. They mostly lift their hands from the ground, and with neck stretched forward, tail stretched back, and the entire backbone horizontal, they charge off, one of them nearly kicking the Liliensternus aside.
This was the world of the Late Triassic, and it did not last. Even though, in parts of the world, sauropodomorphs such as Plateosaurus were abundant, making up large herds, they and the others died out at the end of the Triassic, 201 million years ago. Great volcanic eruptions began along the sides of rifts opening up between what is now Europe and Africa on the one side and North America on the other. In the Triassic, as we saw, all continents were fused as one, Pangaea, but at the end of the Triassic, the Atlantic Ocean began to unzip. This process was driven by great eruptions of basalt lava along straight-line rifts, the precursor of the mid-ocean ridge of today, which we can observe onshore as it slashes across Iceland.
The basalt lavas belched out continuously over thousands of years, and with them came gases such as sulphur dioxide and carbon dioxide, which mixed with water in the atmosphere to produce acid rain. This killed plants on land, and the landscape was swept clear of forests and soils. The oceans became acidified, and this killed animals with carbonate shells. The volcanoes also poured out other gases, including methane and water vapour, which, with the carbon dioxide, led to sharp greenhouse warming, and this in turn drove life from the tropics and also removed oxygen from the ocean floor. This is the model of one of the big mass extinctions, the so-called ‘end-Triassic event’, which saw the end of many dinosaur groups, but also many of the other tetrapods (four-limbed animals) that, as we saw in Chapter 1, had been part of the landscape. Life recovered, and the extinction marks a major boundary: the end of the Triassic and the beginning of the Jurassic.
The Jurassic world
There was a lot going on in the Jurassic, the time from 201 to 145 million years ago. In the Triassic, as we saw, the three main lines of dinosaurs had become established: the theropods, sauropodomorphs, and ornithischians. All three groups branched out substantially in the Jurassic. The meat-eating theropods, all active predators, and mostly with sharp teeth lining their jaws, diverged into some small forms that headed into the trees, evolved feathered wings, and became flyers, some of which we now call birds. Other theropods evolved to be larger and larger, adapting to the hunting opportunities offered by the larger herbivores.
Among these plant-eaters were the long-necked sauropodomorphs, and these included some Late Jurassic giants, weighing up to 50 tonnes, and clearly so large that they were probably not preyed on by any other dinosaur. The third group, the ornithischians, a group of herbivores, some bipedal and others quadrupedal and armoured, were not so diverse in the Jurassic, but two armoured groups did emerge during this time: the stegosaurs, with plates and spines down their backs; and the ankylosaurs, encased in a rather heavily armoured cuirass over the whole body.
I Late Triassic scene in Arizona, showing four examples of the small flesh-eating dinosaur Coelophysis, and a couple of plant-eating sauropodomorphs behind, being menaced by a rauisuchian (a large archosaur).
II March-past of dinosaurs from a single time, the Morrison Formation (Late Jurassic) of Wyoming, showing the range in size, from tiny Ornitholestes at the front to mid-sized Stegosaurus with plates, and giant Diplodocus at the back.
III Dinosaurs in a strangely modern landscape. Flowering plants began to take over most terrestrial landscapes in the Late Cretaceous, and these dinosaurs of the Hell Creek Formation of Montana enjoyed the sights and smells of magnolias and roses – but they still largely fed on the ferns and conifers they had known for millions of years.
IV Replica of the famous London specimen of Archaeopteryx showing the skeleton and feathers on the wings and tail.
V How melanosomes in dinosaur feathers can indicate their colour – sausag
e-shaped eumelanosomes (a) indicate black and brown colours, as in Anchiornis, and spherical phaeomelanosomes (b) indicate orange colours, as in Sinosauropteryx.
VI A dinosaur tail in amber, with all the bones and dried-up muscles inside, and a rich covering of feathers. The fossil (below) shows an ant and other debris caught in the amber, and the feathers show every detail of barbs and barbules (above).
VII Reconstruction of the little theropod whose tail got caught in amber 125 million years ago. Like many other small theropods of the time, it was heavily covered in feathers, and chased bugs on the ground and in the trees.
VIII Supertree of all the dinosaurs – an evolutionary tree showing dinosaur origins at the centre, and the expansion of the group into ornithischians (red), sauropodomorphs (blue), and theropods (green).
I first encountered Jurassic dinosaurs when I was a Junior Research Fellow at the University of Oxford. We were taken on field trips to the local Middle Jurassic by the excellent Curator of Geology at the University’s Museum of Natural History, Phil Powell. Phil was a man of many talents, renowned for his expertise on the bagpipes. He would practise on his chanter (the tubular part of the bagpipes with fingering holes) at lunchtime in the museum. As a Scot, I appreciate the bagpipe, but it is really an outdoor instrument, best played at some distance from the audience.
The first dinosaur bone ever illustrated, from Robert Plot’s 1677 The Natural History of Oxford-shire.
Phil Powell took us to the limestone quarries around Oxford from which numerous members of the university had recovered dinosaur bones. Indeed, the first dinosaur fossil ever recorded came from a small quarry in Cromwell parish, north of Oxford, but it was not at first recognized for what it was. The specimen was the lower end of a femur of the theropod Megalosaurus (see overleaf), showing two bulbous facets, and broken across to reveal the internal structure. It was illustrated by Robert Plot, Keeper of the Ashmolean Museum and Professor of Chemistry at Oxford University in his classic The Natural History of Oxford-shire, published in 1677. Here, he illustrated many genuine fossils, as well as curiously shaped stones, some like horse heads, or human kidneys and feet. He identified the dinosaur bone as a leg bone from a very huge human being.1
Genus:
Megalosaurus
Species:
bucklandii
Named by:
William Buckland, 1824 (genus); Gideon Mantell, 1827 (species)
Age:
Middle Jurassic, 174–164 million years ago
Fossil location:
England
Classification:
Dinosauria: Saurischia: Theropoda: Megalosauridae
Length:
9 m (30 ft)
Weight:
1.4 tonnes (3,086 lbs)
Little-known fact:
The first fossil of Megalosaurus was reported in 1676, and named Scrotum humanum in 1763.
We have never been able to collect a complete specimen of Megalosaurus, but the various skeletal parts found are enough to show that it would have measured about 9 metres (30 feet) long and might have weighed 1.4 tonnes (3,086 pounds). This was one of the first truly large predatory theropod dinosaurs, capable of preying on nearly all the other dinosaurs of its day. It habitually ran on its muscular hind legs, the great three-toed feet spreading wide as it paced along, and leaving extensive trackways that can still be seen in the mud of quarries around Oxford. The arms were powerful, and presumably used in grappling with prey.
The fauna of the Middle Jurassic of central England included the theropod Megalosaurus, the sauropod Cetiosaurus, and the armoured, plant-eating ankylosaur Lexovisaurus. These were accompanied by a rich fauna of smaller animals, including salamanders, lizards, crocodiles, pterosaurs (the flying reptiles of the time), and early mammals, which have been remarkably well documented in ancient mines around the village of Stonesfield, 24 kilometres (15 miles) northwest of Oxford. The Stonesfield mines had been worked for centuries for roofing slates, and the miners followed the appropriate layers deep underground. They hauled out the rough limestone slabs, set them up to freeze and split, and then sold them as a safer, but weightier, roofing material than thatch. Since the 1820s, Stonesfield had yielded numerous fossils, including dinosaur teeth, as well as bones and teeth of the small critters from the age of dinosaurs, attracting attention from palaeontologists very early in the history of the discipline.
New discoveries of Jurassic dinosaurs from China
Dinosaurs are known throughout the Jurassic, with rich finds of Early Jurassic forms from North America, South America, and South Africa. The Middle Jurassic was only patchily known – largely from England – and I was intrigued to follow up the Middle Jurassic in China, where astonishing finds had been made since the 1990s.
My chance came in 2016, when I was invited by Professor Baoyu Jiang from the University of Nanjing to accompany him on a field trip to Inner Mongolia, in the north of China. We were to visit various sites in the Tiaojishan Formation, which extends over much of the south of Inner Mongolia, and neighbouring Hebei and Liaoning provinces. Our special focus was a unit within this formation, the Daohugou Bed, the source of an amazing assemblage of plants and animals. I was excitedly anticipating finding dinosaurs and pterosaurs, but in fact we spent several weeks splitting rocks and finding insects. These were impressive enough, however, including hand-sized cockroaches, beetles, flies, and water boatmen. Jiang had employed teams of local farmers to help, and we squatted in the burning heat under tarpaulin awnings, splitting rocks. I chipped away, and inevitably did not find much, while one of the local Mongol farmers amassed a huge pile of excellent specimens. Professor Bo Wang, the palaeoentomologist from the Nanjing Institute of Geology and Palaeontology, was excited, and he could see opportunities for new work. I was fretting for dinosaurs.
Up to 2016, eleven dinosaurs had been reported from the Daohugou Bed, as well as pterosaurs, lizards, and other reptiles. The dinosaurs were all astonishing, being mainly small tree-dwellers, most of them with feathers, and equipped for gliding in one way or another. One, which rejoices in the shortest name ever given to a dinosaur, is Yi qi, a real oddball, with all the normal limbs of a small dinosaur, and feathers, but also special struts along the arm that strongly suggest it had bat-like membranes as well to aid gliding and capture of insects.
The most famous Daohugou dinosaur is Anchiornis (see overleaf), now known from dozens of skeletons, and one of the first dinosaurs to have had its colour determined (Chapter 4), reconstructed as a proud turkey-like animal, with long, black tail, legs sporting feathered banners behind like some crazy cowboy trousers, wings with long black and white striped feathers, and a ginger tuft of feathers on its head. I wanted to find one.
I failed.
Each day, my haul of insects was respectable, but far short of those being unearthed by the farmers and students. Dr Jiang was keen to acquire specimens of dinosaurs and pterosaurs, so we arranged to meet various dealers. On several occasions, I was brought along to comment or evaluate (which I could not really do), and we would meet a dealer, often quite expert in the local fossils, and acting as an intermediary for the farmers. Slabs were brought out of the boots of cars, or we visited workshops in anonymous tower blocks to see the wares. This is the way palaeontology works in China, because the hours that scientists and their students can spend in the field are limited, and the local farmers have the time and the eye to make amazing discoveries.
We were shown some wonderful Anchiornis specimens, but Dr Jiang wanted something really special, something new. In the end, in an unlit room in an industrial town in neighbouring Hebei Province, we were shown a beautiful pterosaur specimen, which had been cleaned up but not coated in glue or preservative. This is important when you want to run the specimen under the scanning electron microscope to identify tiny structures or run chemical analyses. This specimen proved to be important in determining something about the evolution of feathers in dinosaurs and their relatives, as we shall see.
Th
e dinosaur world becomes busier and more familiar in the Late Jurassic, with the classic Morrison Formation dinosaurs of the Midwestern United States, including all the favourite sauropods such as Diplodocus, Brontosaurus, and Brachiosaurus, theropods like Allosaurus and Ceratosaurus, the plate-backed Stegosaurus, and many more (see pl. ii). The Morrison dinosaurs had first been excavated in the 1870s, when railroad crews were cutting through the mountains and plains of Wyoming, Colorado, and Utah. Great crates full of these dinosaur bones were sent back to the east coast, where they can still be seen in the museums of Philadelphia, New Haven, New York, and Washington. The density of fossil finds at some localities tells us these dinosaurs were abundant. It’s nearly impossible to imagine the giants of the Late Jurassic world. The largest sauropod, Brachiosaurus, would walk over you without noticing – its belly was 2.5 metres (8 feet) above the ground, and it towered to a height of 9 metres (30 feet), reaching around in the tree tops for leafy branches to eat. The other sauropods of the Morrison (such as Diplodocus, with their long, horizontally held necks and whip-like tails, and Brontosaurus and Camarasaurus) were nearly as large, but more heavily built.
Dinosaurs Rediscovered Page 6