I want to start by clearing up some common misconceptions about dinosaurs. If you’re a dino buff then you’ll probably know all of this already and want to skip forward a few pages.
1 What is a dinosaur?
Dinosaurs are a group of reptiles defined by many features of their skeletons – most particularly the fact they hold their limbs erect beneath them rather than out to their sides in a sprawling posture as lizards and crocodiles do. Dinosaurs are made up of two major subgroups: the ornithischian or ‘bird-hipped’ dinosaurs, which included the giant long-necked sauropods (such as Diplodocus) and all the bipedal, predatory theropods (such as Tyrannosaurus and Velociraptor); and the saurischian or ‘lizard-hipped’ dinosaurs, which included heavy-set and armoured species (such as Triceratops and Ankylosaurus) and herd-living herbivores (such as Hadrosaurus and Pachycephalosaurus). Officially dinosaurs are deemed to be all the animals that descended from the last shared ancestor of the ornithischian and saurischian groups. Confusingly, birds are theropods and are therefore part of the ‘lizard-hipped’ group.
2 Dinosaurs and people didn’t coexist
Modern humans that look like you and me evolved around 200 000 years ago, while our more distant ancestors diverged from chimpanzees 6–7 million years ago. The last of the non-avian dinosaurs (that is dinosaurs that weren’t birds) disappeared in a mass extinction around 66 million years ago at the end of the Cretaceous period. At around the time the dinosaurs winked out, the lineage of mammals that eventually gave rise to humans and all other primates included a small squirrel-like animal called Purgatorius.
The earth formed around 4.5 billion (4500 million) years ago. A common analogy is to imagine the earth’s history as a 24-hour clock. On this scale, the earth formed at midnight. Simple bacterial life appeared at 6 am, more complex plants and animals didn’t make it onto land until 10 pm and humans appear at one minute to midnight. Dinosaurs enter the stage at around 11.20 pm, in the Triassic period (200–250 million years ago), enjoy great success through the Jurassic (145–200 million years ago) and make their exit at 11.50 pm at the end of the Cretaceous period (66–145 million years ago).
4 Birds are dinosaurs
Birds evolved from within a group of fast two-legged predatory dinosaurs called theropods, which includes such creatures as Velociraptor and T. rex. This means that dinosaurs are in fact more successful today than they have been at any other point in their history. There are nearly 10 000 living species and perhaps as many as 400 billion individual birds flitting about on the planet at any one time. As British writer Colin Tudge says, ‘The dinosaurs did not disappear when the putative asteroid struck, 65 million years ago. They are calling to us from every hedgerow’. One estimate of total dinosaur diversity (including those we haven’t yet found as fossils) puts it at around 1850 species – so, by sheer weight of numbers, the small, feathered, flying dinosaur model really has been fabulously successful. Flight opened up a whole new range of lifestyles for dinosaurs and it was a leap into the skies from which they’ve rarely looked back. Living mammal species number only about 5500, and birds outnumber them and all other groups of terrestrial vertebrates, including amphibians and reptiles.
5 Ancient marine reptiles and flying pterosaurs are not dinosaurs
The flying pterosaurs and the aquatic ichthyosaurs, plesiosaurs, pliosaurs and mosasaurs were large prehistoric reptiles that lived at the same time as the dinosaurs, but they were not dinosaurs. Pterosaurs were the sister group to dinosaurs and birds, and in turn all of these animals are close relatives of crocodiles (which are therefore the closest living relatives of birds). The other ancient marine reptiles are more distantly related members of the reptile group, which also includes turtles, snakes and lizards.
6 What do I mean when I say ‘dinosaur’?
Since birds are simply small, specialised, mostly flight-capable forms of dinosaur, this presents a problem for how to define the rest of the dinosaur group. Officially, everything we would traditionally have thought of as a dinosaur is a ‘non-avian dinosaur’, but that’s one ugly mouthful, so in this book you can assume that when I say ‘dinosaur’ that’s usually what I mean.
7 What defines a bird?
It used to be easy to define a bird: birds had feathers and beaks, they flew, they were bipedal, they laid eggs, they were warm-blooded and quick-witted, and they had wishbones. The problem is that most of the 30 or so once-defining features have quite literally gone the way of the dinosaurs. Of course, birds still have all these things, but now we know that many dinosaurs had them too. So what now defines a bird? In some ways the distinction between birds and dinosaurs is arbitrary. Mike Benton, a palaeontologist at the University of Bristol in the United Kingdom, says that powered flight and a wing that was large enough to achieve it, is the one remaining thing that separates Archaeopteryx, the ‘first bird’, from its dinosaur relatives, but it may be only a matter of time until other dinosaurs are found that had powered flight too.
8 Dinosaurs didn’t all live at the same time
Compared to the very brief tenure of humans on this planet, dinosaurs ruled the roost for an extremely long time. The earliest recognised dinosaur is a bipedal labrador-sized animal called Nyasasaurus, which was named after Lake Nyasa in Tanzania (today called Lake Malawi). The fossil was collected in the 1930s but languished for many decades in a drawer at London’s Natural History Museum. A reappraisal of it in 2012 found it to be up to 247 million years old, making it significantly older than any other known dinosaur (assuming it really is a dinosaur – the remains are very incomplete). From these somewhat diminutive beginnings in the early Triassic, dinosaurs went on to great peaks and troughs of diversity in the late Triassic and through the Jurassic, before slowly declining in species numbers towards the end of the Cretaceous, when something cataclysmic finished them off 66 million years ago. A 180-million-year run really isn’t a bad innings. During this period, individual dinosaur species were continuously going extinct while other new ones were evolving.
9 Birds and dinosaurs did live at the same time
The first bird, or at least one of its fairly close relatives, was an animal called Archaeopteryx, which lived in the Late Jurassic around 150 million years ago. Non-avian dinosaurs didn’t disappear until 66 million years ago, so we know that birds and dinosaurs coexisted for at least 85 million years. For a long time we had very few early bird fossils, but large numbers of exquisitely preserved fossils from China are now showing that a diverse assemblage of dinosaurs and birds shared the same habitat there during the Early Cretaceous period.