Dinosaurs Rediscovered
Page 11
Bone histology of the dwarf sauropod dinosaur Europasaurus, showing growth lines (white arrows).
So, in a series of wonderful experiments in the 1940s, Ned Colbert and colleagues showed that the core temperature of small alligators is more or less in tune with air temperature, but as the alligators got larger and larger, the hot-cold-hot cycles of day-night-day temperature did not directly drive the animal’s core temperature; rather, it was damped, meaning that it adjusted up and down more slowly. The experimenters predicted that at a certain size the alligator would keep a constant core temperature, even though it was living in air temperatures that rose and fell by 20 or 30 degrees each day and night. I have always wondered how they determined the core temperature of those over-heated alligators – presumably using a thermometer on a very long broomstick!
Birds and crocodiles have a second, related, characteristic, almost certainly also shared with dinosaurs: they breathe air through their lungs in a one-way manner. Humans and other mammals breathe in and out using a tidal system, meaning that there is always some foul air in our lungs even when we puff out as hard as we can. Birds and crocodiles, on the other hand, breathe oxygenated air into the lungs, where oxygen passes into the bloodstream, and the air passes also into extensive air sacs around the backbone and guts. Then, when birds breathe out, everything is cleared from the air sacs and lungs. Dinosaurs, including sauropods, did the same, and this gave them a more efficient way to keep their metabolism high without eating huge amounts.
These, then, are two of the characteristics that perhaps allowed dinosaurs to be truly gigantic, and we will explore these further in Chapter 6. One-way respiration increased their ability to acquire oxygen and so to power a high metabolic rate with less energy than we have to use; and gigantothermy meant they could be warm-blooded just by being huge.
Mesozoic birds from China
We left the story of bird evolution with the recalcitrant dinosaur-bird deniers. In fact, and as if Huxley and Ostrom required any vindication, the discovery of abundant feathered dinosaurs from China since 1990 has been an amazing confirmation that birds are dinosaurs. I remember seeing the first images of feathered dinosaurs to reach the West at the annual meeting of the Society of Vertebrate Paleontology in New York in 1994. Two Chinese professors were there, wearing smart suits, and they created quite a stir – this was early in the days of the political opening up of China, and we remembered how China had been a closed country. The professors had astounding photographs of feathered dinosaur fossils – there was the skeleton laid out, complete with traces of internal organs such as the liver inside the rib cage, and – there could be no doubt – a fuzz of bristly feathers round the edges.
Skeleton of the first of the Chinese feathered dinosaurs to be announced: Sinosauropteryx.
Skeleton of the four-winged gliding dinosaur Microraptor.
These rare visitors from China – rara aves, one might say – were buttonholed by all the great and the good. Shortly after, John Ostrom, Phil Currie and colleagues visited China for the first time, and they were convinced of the fossil’s authenticity. The beast was named in 1996 by Drs Ji and Ji, and the images made available for all to see two years later in a fuller description by Pei-ji Chen and colleagues, in the world’s leading scientific journal, Nature. This was Sinosauropteryx prima (see overleaf); little did I realize at the time that I would one day have a chance to study it.
The critics declared it was a fake, artfully put together from bits and pieces of several skeletons, and with feathers glued on. Those who had seen it knew it was real. Professor Chen and colleagues were cautious in their Nature paper, however, and called the feathers ‘protofeathers’, saying that ‘much more work needs to be done to prove that the integumentary structures of Sinosauropteryx have any structural relationship to feathers’. This caution was understandable, but soon the specimens piled up and the feathers were unequivocal. Whereas in Sinosauropteryx, the feathers were just bristles, in Caudipteryx (see overleaf), named in 1998, there were branching feathers, like the down feathers of a modern bird. Then Microraptor, named in 2000, showed all the flight feathers you could wish for – primaries and secondaries arrayed along the wing. And, not only that, they were arrayed along the hind wings. This was a four-winged beast, similar to a postulated ‘tetrapteryx’ or four-winged flyer that some experts on the origins of flight had earlier speculated must have existed.
Here was a dinosaur, with a wingspan of less than 1 metre (3 feet), that could fly, but not exactly like a bird – well, not at all like a bird. Microraptor was in fact a close relative of Ostrom’s Deinonychus, a member of the Dromaeosauridae, which were close to the origin of birds. Microraptor has been reconstructed and modelled by aerodynamics experts. It might have flown like a kite, with both sets of wings on the same plane, or like a World War I biplane, with the front wings above the hind wings. Either way, it almost certainly used its wings in leaping and gliding from tree to tree, not in flapping flight. The area of the wings is not sufficient to support the body mass during prolonged flights.
Therefore, in evolutionary terms, the new Chinese fossils have shown that the origin of birds, far from being the sudden event people had speculated, was a long and complex process. The early palaeontologists may have rejected the bird-dinosaur model because they thought a flying bird could not have evolved rapidly from a great lumbering theropod dinosaur like Allosaurus or Tyrannosaurus. And they were right. The creationists love to pick on Archaeopteryx as the great ‘missing link’ fossil – if you can ridicule Archaeopteryx (little fluffy bird hatches out of crocodile egg), then you can claim to destroy evolution.
Genus:
Sinosauropteryx
Species:
prima
Named by:
Qiang Ji and Shu’an Ji, 1996
Age:
Early Cretaceous, 125 million years ago
Fossil location:
China
Classification:
Dinosauria: Saurischia: Theropoda: Compsognathidae
Length:
1 m (3 ft 3 in.)
Weight:
1 kg (2 lbs 3 oz)
Little-known fact:
This was the first dinosaur to have its feather colour determined, in early 2010.
Genus:
Caudipteryx
Species:
zhoui
Named by:
Qiang Ji and colleagues, 1998
Age:
Early Cretaceous, 125 million years ago
Fossil location:
China
Classification:
Dinosauria: Saurischia: Theropoda: Oviraptorosauria
Length:
1 m (3¼ ft)
Weight:
1 kg (2 lbs 3 oz)
Little-known fact:
At one time, it was suggested that Caudipteryx was a flightless bird, but it is clearly a theropod and not a bird.
We now know, thanks to the Chinese fossils from the Jurassic and Cretaceous, that there were dozens of feathered, flying dinosaurs, all experimenting with different styles of gliding and parachuting. Then one lineage, of which Archaeopteryx is an early representative, cracked true powered flight, in which the wings beat up and down. That gave them the breakthrough to success, with hundreds of bird species flourishing in the Cretaceous, and nearly 11,000 species today.
Can we tell the colour of dinosaurs?
I discussed the topic of dinosaur colour in the Introduction, and this has been one of the most exciting and unexpected recent discoveries in dinosaurian palaeobiology. Unexpected because we used to lament, ‘We’ll never know what colour they were’. It might be plausible to reconstruct feeding and locomotion from the bones, but surely colour would require a time machine?
As I noted in the Introduction, however, the secret is that much of the colour in bird feathers and mammal hairs comes from variants of the pigment melanin. One form of melanin, called eumelanin, gives all the black, brown, and grey colours, and another,
phaeomelanin, gives ginger colours. This is all that mammals have, whereas birds have two other pigments in their feathers, porphyrins to give purple and green colours, and carotenoids to give red and pink colours. The key is that melanin is a very tough chemical that survives a great deal of heat or compression, and so it survives in fossils. Further, the two key types of melanin are contained within differently shaped capsules, called melanosomes – sausage-shaped ones for eumelanin, spherical for phaeomelanin – and this is constant in birds and mammals. Therefore, applying the extant phylogenetic bracket idea (in evolutionary terms, mammals and birds ‘bracket’ the dinosaurs), it is likely that the same shape–colour relationship applies to all included groups, including dinosaurs. Melanin is produced in the skin and passes into melanosomes in the developing hair or feather through the follicle.
I first had the chance to go to China in 2007, with colleagues Paddy Orr and Stuart Kearns. We spent two weeks in the field, exploring all the localities in northeast China’s Jehol Beds, a major set of formations of Early Cretaceous age, that had yielded specimens of feathered birds and dinosaurs, and a further two weeks in the laboratories of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing. There, we peered down our microscopes at samples of feather and skin and spotted some promising examples.
When we read the important paper in 2008 by Jakob Vinther, then a doctoral student at Yale, who had identified melanosomes preserved in fossil bird feathers from Brazil and Denmark, we immediately thought, ‘let’s see if we can find these in dinosaur feathers’. We spoke to Fucheng Zhang at IVPP, who had visited Bristol in 2005 to work on fossil bird specimens, and arranged to borrow some samples, including small slivers of fossilized feathers from different parts of the body of Sinosauropteryx, and he visited Bristol for a second time in 2008. That was when we found the melanosomes.
We wrote the paper and sent it to Nature in early 2009. As is the way of things, it took forever to convince all the reviewers. Indeed, the paper was reviewed twelve times – three cycles of four reviewers each time – and one was absolutely not to be convinced. ‘They’re not melanosomes, these aren’t feathers, and they aren’t dinosaurs…’ I was able to talk to Vinther and colleagues while on sabbatical at Yale in early 2009, and eventually our paper came out in February 2010. We showed that Sinosauropteryx had phaeomelanosomes, the capsules that contain the ginger version of melanin, and lots of them. It was ginger! The tail was striped, with equal bands of white and ginger along its length. So, we published our reconstruction (see pl. v) with a confident statement: ‘The reconstruction shows the correct colours of a dinosaur for the first time.’ This is important: we were not giving an opinion, however informed, but stating an objective fact, and our statement could be refuted if someone showed our observations of melanosomes to be false.
At the same time, the Yale team, led by Jakob Vinther, published their reconstruction of an even more gaudy dinosaur, Anchiornis from the Jurassic of China, which sported black and white stripes on its wings and tail, and a lovely ginger crest on top of its head, as well as specklings of black and ginger feathers on its cheeks. What does this all mean, though? Determining the colour of a dinosaur may be a bit of smart lateral thinking, perhaps a clever parlour trick, but can it tell us anything useful?
Did dinosaurs indulge in sexual selection?
Identifying the colour of dinosaur feathers revolutionized our appreciation of the complexity of dinosaurian behaviour. Birds today have feathers for three main reasons – insulation, signalling, and flight. It was obvious that insulation came before flight; the downy feathers over a bird’s breast are to keep in the warmth and help its thermoregulation, and these feathers are much simpler than the flight feathers. So, it was assumed that if dinosaurs had had feathers, as Bakker suggested, they would likely have been for insulation. However, in our 2010 papers, both our team and the Vinther team declared that feathers were clearly for signalling from an early point in their evolution. We could not, however, go so far as to say that had been their initial purpose – but it might have been.
The striped tail of Sinosauropteryx and the barred wings and coloured crest of Anchiornis could have had no other function than signalling. No such patterns would be needed for insulation or flight. Further, it does not look as if the colours are there for camouflage – the barred tail might suggest it, but animals today that use bars for camouflage, such as tigers and zebras, have bars all along the body, not just on the tail.
So, signalling means sexual signalling. We can now imagine male dinosaurs, particularly the small theropods, as hopping about and showing off their wares to the females, just as so many birds do today. One reason that birds are so diverse, with nearly 11,000 species known, is that sexual selection helps to maintain and drive splitting of species, each with its own colourful feather patterns. Strip off the feathers, and most perching birds have nearly identical skeletons, for example, but when the males are equipped with their plumage, they are gloriously different, and the species do not interbreed because their pre-mating dances and displays are intensely interesting only to females of that species.
The realization that many dinosaurs might have been sexually selected has posed a conundrum: not many of them show evidence of sexual dimorphism (differences in form between male and female). Today, many reptiles, birds, and mammals show sexual dimorphism – think of the sleek lioness and the larger, maned male lion, or many examples among primates in which the male is much larger and equipped with bigger teeth. Maybe, though, birds give part of the answer – although the male and female peacock are indisputably different, it’s all in the feathers. Their skeletons are very similar, perhaps differing in some small details of size. The same might have been true of displaying theropods too.
This has been part of a recent heated debate between those who see many horns and crests in dinosaurs as evidence for sexual dimorphism or sexual signalling, versus those who would interpret all such structures as having different functions, for example in feeding, defence, or species recognition. Kevin Padian and Jack Horner made a strong case for the ‘species recognition hypothesis’ in a paper in 2011 – they argued that all ‘bizarre structures’ in dinosaurs were to allow individuals to spot other members of their own species, perhaps across a crowded landscape containing many other similar-looking dinosaurs, for reasons of mutual protection. In such a model, sexual selection would be of minor importance among dinosaurs.
Different feather types from modern birds.
In a direct riposte, Rob Knell and Scott Sampson argued that species recognition might be a secondary function of the horns, crests, and feather arrays of many dinosaurs, but that the only valid argument to explain the costliness of evolving and maintaining such structures is sexual selection. Further, they noted, the shapes and extents of the bizarre structures are so variable among members of a single species that they might not have been very useful as unequivocal labels of species identity, but rather that they were under selection for other functions including mate competition, weaponry for fighting other males, or ornaments for showing off to females.
The debate rumbles on, but all of the evidence for quite complex social behaviour in dinosaurs suggests that they were perhaps not so stupid as they have been portrayed.
Were dinosaurs brainy or not?
Are birds (and dinosaurs) intelligent or not? Sexual display, and all the complex behaviours often associated with it, might imply high intelligence. And yet we say ‘bird-brained’ to mean stupid, and although birds have high-domed skulls and bulging little brains, much of the brain tissue is given over to powering their exquisite senses, especially sight.
It’s also a given for many that dinosaurs were stupid. They are portrayed in museum displays and kid’s books as mindless automata that barged around knocking over Jurassic trees, and surviving only because all the other dinosaurs were equally stupid. Famously, we learn that the plate-backed Stegosaurus had a brain the size of a kitten, and that it even had
a larger brain in its hindquarters to control its tail and back legs.
Humans, and mammals in general, owe their claim of intelligence to the cerebral hemispheres, the forebrain, the two great spheres of wrinkled brain tissue we see when a brain is presented in the surgeon’s hands. These wrap around the so-called ‘primitive’ regions of the brain, the midbrain and hindbrain. In fishes and reptiles, the brain is more linear, with the hind-, mid-, and forebrain regions in a row. In simple terms, most of the reptile brain operates the sense organs – nose, eyes, and ears, as well as reflexes, and repetitive behaviours such as fight or flight and seeking food.
The relationship between brain size and body size for mammals, birds, and reptiles, including dinosaurs.
Dinosaur brains are not preserved, but their impressions are there deep inside the fossil skulls. We tend to think of the head as mainly full of brains, as it is in humans and other mammals. In reptiles, including dinosaurs, the brain is actually pretty tiny. It sits inside the braincase, a bony structure located deep within the skull, and the relative sizes are like a match box inserted inside a shoe box. Most of the dinosaur’s head is full of jaw muscles at the back, and eyes and nasal cavity inside the snout.