Dinosaurs Rediscovered
Page 7
When a mixed herd of these giant sauropods got moving, the thunderous noise and rising dust would have been incredible. The flesh-eating theropods Allosaurus and Ceratosaurus were both large enough, at 8.5 metres (28 feet) long, that smaller dinosaurs would have fled before them. The giant sauropods were vulnerable only when young; once they had reached an age of five years or more they probably would have been immune to predation. Similar Late Jurassic faunas are known also from Tanzania, Portugal, and China, representing a time when sauropods in particular were distributed worldwide and reached their largest sizes. This was not to last, however.
Genus:
Anchiornis
Species:
huxleyi
Named by:
Xu Xing and colleagues, 2009
Age:
Middle Jurassic, 166–164 million years ago
Fossil location:
China
Classification:
Dinosauria: Saurischia: Theropoda: Maniraptora: Anchiornithidae
Length:
40 cm (16 in.)
Weight:
0.7 kg (1½ lbs)
Little-known fact:
Anchiornis had small wings on its hind legs, which would have looked like cowboy trousers when it walked along.
Foodwebs and the Cretaceous heyday of the dinosaurs
There were major changes on the Earth about 145 million years ago, and the rock successions in certain parts of the world show evidence of shifts of continental plates and climate change. The sauropods, which had dominated Late Jurassic faunas, dropped in diversity and the ornithopod dinosaurs became the dominant herbivores. These include the classic ornithopod Iguanodon (see overleaf), named in 1825 – the second dinosaur ever named – from Sussex in southern England.
Iguanodon is one of the most-studied dinosaurs – well, probably not quite as much as T. rex! Nonetheless, dozens of specimens have been found, mainly in Europe, and many publications have been made. Most notably, Dave Norman, one of the four of us who were struggling with the first dinosaur cladograms in 1984, has made Iguanodon his life’s work. The specimens are beautiful, and many skeletons are complete. Iguanodon was typically 10 metres (33 feet) long and weighed about 3 tonnes (6,615 pounds). It was built as a biped, but commonly went down on all fours to feed. The hands had five strong fingers for grasping, but they bore small hooves – perfect evidence that they were sometimes used in walking. The spiky thumb was probably used in defence. The massive hind legs made Iguanodon a powerful bipedal runner, with the tail stretched out straight behind. In fact, the balancing function of the tail is enhanced by thin rods of bone that run down on either side of the backbone, stopping the tail from waving about too much at speed.
Iguanodon had an elongate skull, with a deep snout, and it snipped leaves with the toothless bony plates at the front of its jaws. It passed the fragments back to be roughly chopped between the long, straight tooth rows down each side of the jaw. It was the immense capacity of Iguanodon and its relatives to process plant food that made them so successful. This was the first dinosaur ever to be able to chew its food. Other dinosaurs had simply grabbed and swallowed, but by chewing its food, Iguanodon could extract much more goodness from every mouthful. It did not chew the food as we do, by rotating the lower jaw around the pivot at the back, but more by a mechanism that allowed the lower jaw to chop up into the upper jaws, a little like the blade of a penknife shutting into the handle. As the lower jaw cut upwards, the teeth ground across the upper set of teeth, both sets sharpening each other as they did so. Further, the cheeks could expand a bit outwards, so providing some lateral grinding.
Genus:
Iguanodon
Species:
bernissartensis
Named by:
Gideon Mantell, 1825 (genus); Louis Dollo, 1881 (species)
Age:
Early Cretaceous, 140–125 million years ago
Fossil location:
England, Belgium
Classification:
Dinosauria: Ornithischia: Ornithopoda: Iguanodontidae
Length:
10 m (33 ft)
Weight:
3 tonnes (6,615 lbs)
Little-known fact:
The most complete collections of Iguanodon came from the roof of a coal mine in Belgium.
Since the first find in the 1820s, Iguanodon has been found at numerous localities throughout the Weald area of southeast England, and the Early Cretaceous rocks of this area were soon called Wealden, and were identified widely across Europe. The Wealden was a time of warm, damp climates, and the rocks record life on the lowlands, with rich remains of lush plants, insects, amphibians, lizards, crocodiles, dinosaurs, and even some birds and mammals.
The Wealden sediments accumulated up to 700 metres (2,300 feet) thick, as sands and muds washed down from highland areas around what are now London and Belgium. As the Weald basin sank, the sediments piled up, and they span 15 million years of the Early Cretaceous, from 140 to 125 million years ago. The sediments record ancient rivers in some cases, deltas and shallow marine incursions in others. After years of detailed study, Percy Allen from Reading University was able to make a detailed palaeoenvironmental reconstruction of the Wealden scene, showing lakes, rivers, crevasse splays, trapped logs, and charting how the sediments and fossils accumulated.
The Wealden fossils give us a much clearer picture of Early Cretaceous life. Fossils have been collected from the Weald for over 200 years, and these include logs and tree trunks found at some levels, complete skeletons of Iguanodon and other dinosaurs elsewhere, and bonebeds of microvertebrates. The microvertebrates – tiny bones and teeth, as the name suggests – may seem less spectacular than the huge bones, but they provide a rich record of diversity at the time. If you truly want to understand ancient faunas and floras, you must include everything.
Block diagram showing the different environments of the Wealden in the Early Cretaceous.
One researcher who sampled the Wealden microvertebrates, Steve Sweetman, discovered a remarkable range of creatures including sharks and bony fishes, salamanders, frogs, the most diverse lizard fauna yet discovered from the Early Cretaceous, turtles, crocodiles, pterosaurs, ornithischian and saurischian dinosaurs, birds, and mammals. The tiny teeth may look rather bewildering, even if startlingly beautiful, but they all show diagnostic characters that, after a while, palaeontologists learn to recognize, and which show in some detail how many species were present, and what they were all up to.
How to put this all together? The food web is a classic way to represent all the species in an assemblage, and especially who ate whom. Steve Sweetman has kindly shared his lifelong knowledge of the Wealden ecosystem by preparing the food web shown opposite. This kind of diagram makes the past come back to life, and the Wealden ecosystem was similar to those today in some ways, profoundly different in others. The main difference, of course, is the role of diverse dinosaurs, and the absence of birds and large mammals.
The Wealden shows us a very typical picture of life in the times of the dinosaurs, but here and there were hints of changes to come. Among the ferns, seed ferns, conifers, and other vegetation were some unusual incomers – the world’s first flowers.
Food web, showing who ate whom, for the Early Cretaceous Wealden of southern England.
Constructing the dinosaur supertree
This is the place to come back to evolutionary trees. We have reviewed some of the early stages in the history of dinosaurs, and the effort to establish the broad outlines of dinosaurian evolution. By the year 2000, some 500 species of dinosaurs had been named from the Triassic, Jurassic, and Cretaceous of nearly all countries of the world. They could all be assigned quite easily to theropods, sauropodomorphs, or ornithischians, but what about the finer-scale patterns of relationships? When we thought about this question, around 2000, biologists were already well advanced in drawing up ever more detailed trees of their favourite groups.
We felt it was important to constr
uct a complete tree of all dinosaur species using as much information as we could. This had not been attempted before. In this case, rather than looking at all the dinosaur specimens, coding the characters of their skeletons and skulls, we thought we would try a new set of computational procedures and construct a so-called supertree. A supertree, as the name suggests, is built from many regular trees. So, my then PhD student, Davide Pisani, now a professor at Bristol, took the lead. We scanned every paper written between 1980 and 2000 about the relationships of dinosaurs, and we collected together 150 such trees.
The theory of building a supertree is quite simple, although in practice there are many headaches in achieving an agreed result. If you have two evolutionary trees, each of ten dinosaurs, and two of the species are shared in both trees, you fix the trees together using these common species, and make a tree of eighteen species. Of course, among the 150 trees, there were lots of proposed relationships that disagreed, so the computational exercise Davide set in train had to crunch through these disagreements and try to identify the most likely solution. After weeks and weeks of computing, we had come up with a complete tree of 277 dinosaurs. We couldn’t include all 500 known dinosaur species because, at that time, many had not yet been included in any kind of cladistic analysis.
The tree was pretty well resolved, meaning that most dinosaurs were in a specific position, but there were quite a few places where a bunch of five or six species all branched off from a single spot. These were places where disagreements existed, and we could not make the program give us a more precise answer. It was frustrating, but realistic – often there just is not enough information to decide definitively one way or the other. These places where relationships are unresolved are a useful marker for researchers to go hunting for more information.
Six years later, we did it again. This time, the project was led by Graeme Lloyd, another Bristol doctoral student who is now on the staff at the University of Leeds. Graeme was able to identify 550 papers with dinosaur trees of one kind or another, and including 420 species. The number-crunching was even more brutal, but we were very proud of our much-improved dinosaur supertree, now in full colour, and drawn as an attractive circle (see pl. viii). When we submitted the work for publication, we were crestfallen that the journal declined to print our lovely diagram – they said it was too big to fit on the page.2 Nonetheless, with 420 species, this was one of the largest supertrees available at the time. Now, it is commonplace to produce massive trees – the biggest perhaps comprising all 11,000 species of birds.
Is this just a child’s game – my supertree is bigger than yours? Well, yes, in a way. There is the technical side and the desire to get it right, and to push the software and computing hardware to the limits. Significantly, though, we could use the supertree for studies of macroevolution. In particular, we asked a simple question, which was, when were dinosaurs evolving fast? Graeme Lloyd looked at every one of the 423 branching points in the whole tree and calculated whether it showed an unusual rate of evolution, in terms of the number of species that had branched from that point over a known amount of time.
He identified only 11 of the 423 branching points as showing evolutionary rates that were statistically faster than expected. Seven occurred in the Late Triassic, and another two in the Middle Jurassic and two in the Late Cretaceous. This bottom-heavy aspect of the dinosaur tree was unexpected – it shows that, in a broad sense, dinosaurs had done much of their evolving in the first half of their history, and not so much after that.
We took this a step further, and compared dinosaur evolution in the Cretaceous with the evolution of other groups of land-living plants and animals. The slow rate of dinosaur evolution in the Cretaceous stood out as unusual – something pretty major was going on at that time, and we named it the Cretaceous Terrestrial Revolution. Dinosaurs, unexpectedly, were not part of this dramatic evolutionary step.
The Cretaceous Terrestrial Revolution: the trigger for modern life
About 125 million years ago, flowers revolutionized the Earth; indeed, they triggered what has come to be known as the Cretaceous Terrestrial Revolution, the time when terrestrial ecosystems, like that of the Wealden, were remodelled radically. The key questions are to determine the scope and impact of the ecological revolution, and to determine the extent to which the huge changes in plants and animals did, or did not, affect dinosaur evolution.
The Cretaceous Terrestrial Revolution marked the point in the entire history of life when life on land first became hugely diverse. It’s been worked out that in the Early Cretaceous there were about equal numbers of species on land and in the sea, whereas now life on land is five to ten times as diverse as life in the sea. Most of this huge diversity of modern life is made up from insects, but other very rich groups include spiders, lizards, birds, and the flowering plants themselves.
There may be as many as a million species of beetles on Earth. The great twentieth-century English biologist J. B. S. Haldane was once asked what he had learned about Creation from his long studies of nature. He replied that evidently ‘God has an inordinate fondness for beetles’. Indeed. We don’t even know how many beetle species there are – some 400,000 have been named, and whenever a beetle expert is let loose in a new bit of jungle, he or she comes back with fifty new species each day. There is a limit to how fast any individual can work and publish the descriptions and names…so it will take a few centuries to get on top of this pile of unfinished work.
Anyway, in total, there may be 15 million species of all kinds on Earth, and a good 80–90 per cent of these are on land, with only 10–20 per cent in the sea.
When biologists produce evolutionary trees of modern organisms, most of these highly species-rich groups (flowering plants, beetles, butterflies, bees, bugs, spiders, lizards, mammals) seem to have radiated explosively in the mid-Cretaceous, about 100 million years ago. What was going on?
The driver was the explosion in numbers of flowering plants, or angiosperms as they are properly called. Angiosperms include nearly all familiar plants, from fruits to vegetables, oaks to palms, and all the grasses. They are economically crucial to humans as virtually all grains and pulses we eat are angiosperms. It is said that the angiosperms diversified in the Cretaceous because of their unique fertilization system, which involves flowers, seeds contained within nutritious fruits. This fertilization system gave them immediate advantages over other plants, and they were able to adapt to new settings and survive crises better than, for example, the conifers or ferns.
From the first, angiosperms engaged in mutual relationships with pollinators such as birds and bees…but also butterflies, moths, and wasps. Bugs and other insects adapted to feed on the succulent new leaves, flowers, and stems. Thus, some 300,000 species of angiosperms today support 2 million or more species of rather specialist insects, and the angiosperms make lush, complex forests, much, much richer in species than comparable conifer-dominated forests, which today tend to occur in colder climates.
Did the Cretaceous Terrestrial Revolution affect the dinosaurs? The new food sources could have given them opportunities to adapt and diversify. However, the consensus is that they were not much affected. The dinosaurs stomped around as ever, probably spurning the flowering plants, trampling over their perfumed flowers to get a good mouthful of crunchy ferns or spiky conifer leaves.
There were new dinosaur groups at the time, but they did not seem to depend on flowering plants, or any of the new insect groups. As we move from the Early to the Late Cretaceous, the iguanodons and other dinosaurs of the Wealden were replaced by hugely abundant duck-billed hadrosaur dinosaurs, some such as Parasaurolophus (see overleaf) with remarkable head crests. The function of these crests has been debated. The crests were hollow, made from the bones of the snout, so they could not have been for fighting or defence. The best explanation is that they were for species recognition – at any time, a hadrosaur herd might include five or six species, each characterized by its own headgear – meaning that any indivi
dual would want to associate in herds with members of its own species. Like birds today, the dinosaurs were probably visual animals, and so used the same kinds of cues to identify species as we do.
Equally common in some places were the ceratopsians, the horned-faced dinosaurs, like huge rhinos. Other plant-eaters included the tank-like, armoured ankylosaurs, some with great whacking clubs on their tails, as well as a few long-necked sauropods, especially in southern continents. The predatory theropods included a huge array, from tiny feathered insect-catchers, through the long-limbed ostrich-like ornithomimosaurs, to the greatest Jaws of the lot, Tyrannosaurus rex, apex killer of the latest Cretaceous of North America.
Genus:
Parasaurolophus
Species:
walkeri
Named by:
William Parks, 1922
Age:
Late Cretaceous, 76–73 million years ago
Fossil location:
United States, Canada
Classification:
Dinosauria: Ornithischia: Ornithopoda: Hadrosauridae
Length:
9.5 m (31 ft)
Weight:
5.1 tonnes (11,244 lbs)
Little-known fact:
The crest used to be misinterpreted by some as a breathing snorkel, but the top end is sealed.
After this survey of dinosaurs through time, everything seemed to be more or less settled. Palaeontologists believed they had wrestled most of the dinosaurs into their correct positions in the great tree of life. Then came a bombshell in March 2017 – it seemed that the consensus on fundamental dinosaurian relationships had been completely wrong.