******************** POSTING RULES & NOTES ********************
#1 YOU MUST clip all extraneous text when replying to a message.
#2 This mail-list, like most, is publicly & permanently archived.
#3 Subscribe and post under an alias if #2 is a concern.
*****************************************************************
LRB, Vol. 41 No. 1 · 3 January 2019
What lives and what dies?
by Francis Gooding
The Rise and Fall of the Dinosaurs: The Untold Story of a Lost World by
Steve Brusatte
Macmillan, 404 pp, £20.00, May 2018, ISBN 978 1 5098 3006 0
‘It was a bad time to be alive,’ Steve Brusatte tells us. A comet or
asteroid about six miles across had just collided with the Earth, in the
area we know as the Yucatán Peninsula in Mexico. The speed of its
arrival compressed the atmosphere ahead of it with such force that air
temperatures became hotter than the surface of the sun; the energy
released on impact was equivalent to a billion atomic bombs. It smashed
through 25 miles of the Earth’s crust, plunging down into the mantle
below, leaving a crater a hundred miles wide. Identified in 1991, it has
been named the Chicxulub Crater, after the nearest town.
The shockwaves generated global earthquakes of immense magnitude. What
had been solid ground rippled and bounced like liquid for thousands of
miles. Vast volcanic eruptions were triggered, and giant tsunamis surged
across the oceans and far inland. Winds of six hundred miles an hour
howled across the planet, and the molten rock thrown up into the
atmosphere by the impact rained down in a hail of hot glassy blobs and
spears, heating the air as it fell until the forests ignited and living
things cooked. All this within the first two hours or so. Soot, dust and
smoke filled the atmosphere, eclipsing the sun, and for years afterwards
the Earth was cold, dark and bathed in acid rain. This ‘nuclear winter’
was sufficiently severe and long-lasting to halt photosynthesis on land
and in the oceans, causing the collapse of those ecosystems that had
survived the initial cataclysm. Some 70 per cent of living species were
eliminated. Foremost among those that perished were the charismatic,
mysterious creatures whose lineage had dominated the planet for well
over a hundred million years – the dinosaurs.
Brusatte’s The Rise and Fall of the Dinosaurs is a biography of these
magnificent lost beings and their world, and it closes, as it must, with
an account of the catastrophe that destroyed them. It could hardly end
otherwise; after all, if there is one thing that truly defines the
dinosaurs it is that they are all – well, almost all – dead, and no
matter how many bones are dug up or how far the science of their lives
advances, the climax of the story is always the same. Their certain
extinction is a fixed point in a fast-moving field, and it’s the
necessary finale to any account of their reign.
The current accepted theory – that an asteroid strike caused the
end-Cretaceous mass extinction – was first put forward in 1980 by the
geologist Walter Alvarez, working with his father Luis, a physicist and
Nobel Prize winner. Brusatte, a reader in vertebrate palaeontology at
Edinburgh, describes being taken as a student by Alvarez to a rocky
gorge in Perugia near Gubbio, where a band of fine clay running through
the limestone marks the place where the geologist had his initial
insight. This thin band, now known as the K-Pg boundary, is ‘a bookmark
separating the limestones of the Cretaceous below from those of the
post-extinction Palaeogene period above,’ Brusatte writes.
It was while looking at these rocks for clues to the formation of Italy
and the Alps that Alvarez first became intrigued by the fact that,
beyond this marker, all the pages of time’s book seemingly went blank.
Below the line was a profusion of diverse fossil shells from the
planktonic foraminifera that crowded Cretaceous oceans; above it, almost
nothing. And not only did tiny sea creatures disappear at the boundary
line, but fossils of larger creatures are absent immediately above it
too, both on land and at sea. There are no dinosaurs above it at all:
‘Nothing,’ Brusatte writes, ‘not a single bone or a single footprint
anywhere.’ That this boundary marked the end-Cretaceous extinction was
already evident to geologists, but what precisely it signified was not
known. Alvarez’s key intuition was that the band of clay itself might
provide evidence for what sort of event had caused much of life on Earth
to vanish 66 million years ago.
With the help of his father, Alvarez had samples of the clay analysed
for iridium, a heavy metal that is rarely found on Earth’s surface, but
which is common in space, and which falls from the heavens as cosmic
dust at a fairly constant rate. The Alvarezes were trying to ascertain
how quickly or slowly the band had been deposited. If one assumes the
typical pace of accumulation, a large amount of iridium would suggest it
had taken a long time, indicating that the extinction process may also
have been lengthy; a small amount would suggest that a quick event had
overtaken the Earth. But the samples contained a surprise: huge amounts
of iridium, so much that it would have taken tens or hundreds of
millions of years to deposit at background rates. What could explain it?
The Alvarezes came to a radical conclusion: the high concentration of
iridium in the band must have come from outer space, and it must have
been delivered in bulk by a colossal asteroid strike – an event
destructive enough to have triggered the end-Cretaceous extinction. The
iridium-rich clay band at Gubbio was the shroud of dust and debris that
had eventually settled on a devastated world.
As the same iridium anomaly began to be detected at sites around the
world, the Alvarezes’ notion of an asteroid strike followed by the
blotting out of the sun was substantiated. It also had an influence
beyond palaeontology; when Brusatte describes the aftermath of the
impact as being like a nuclear winter, he is gently reverse-engineering
the concept, for the idea of a nuclear winter – the darkening and
cooling of the world by the dust thrown into the atmosphere after a
nuclear exchange – was explored and popularised in the 1980s partly with
reference to the Alvarez hypothesis. Meanwhile, the iridium testing and
the hypothesis itself came of the Alvarezes’ connections to nuclear
science. Luis Alvarez had been a key player in the Manhattan Project,
and had helped design ‘Fat Man’, the bomb that was dropped on Nagasaki.
‘He even flew behind the Enola Gay,’ Brusatte notes, ‘to monitor the
effects of Little Boy when it was dropped on Hiroshima.’ The accepted
theory about the extinction of the dinosaurs is thus intimately
connected to a key inventor of the weapons that could bring about a
comparable extinction event in the present. Quite aside from the dark
poetry of the connection, it seems that in science, no less than in the
imagery of palaeoart, dinosaurs frequently appear as an ambiguous proxy
for contemporary human concerns.
This slippage seems unavoidable when discussing the extinction of the
dinosaurs, and here Brusatte is no different from his predecessors. In
reality, nuclear war may be closer than ever, but the threat of it has
been superseded by the growing realities of climate change, and so an
extinction that was once used as a parable about the devastating
consequences of a nuclear conflict has been adapted to address more
pressing fears. The end-Cretaceous event proved fatal to most living
things, but not all of them. Why did no dinosaurs, large or small,
survive it? This is ‘a key question’, Brusatte writes: ‘We want to
answer it particularly because it is relevant to our modern world. When
there is sudden global environmental and climate change, what lives and
what dies?’ In the case of the dinosaurs there are, it seems, two answers.
The first is that they were victims of a kind of bad luck. Several
factors, in themselves not disadvantageous under normal conditions, may
have told against them under such extreme circumstances. They were
large, they were specialised, and their food webs were underpinned by
photosynthesising plants. They were energetic and needed huge amounts of
food, and they laid eggs which took months to hatch. They could not
adapt their diets like an omnivore, hide in a hole like a mouse or
lizard, sink to the bottom of a lake like a crocodile, or go into
hibernation and not eat for months like a tortoise (creatures like these
survived). Brusatte’s more complex analysis of dinosaur diversity
suggests that a moderate loss of herbivore species had left key
ecosystems – in the regions which are now North America and Europe –
slightly more vulnerable to collapse under pressure. When the asteroid
struck, ‘dinosaurs were left holding a dead man’s hand.’ And it
shouldn’t be forgotten that a majority of all living things, not just
dinosaurs, were also eliminated. A billion atom bombs aren’t fussy.
The second response to the question ‘Why did all the dinosaurs die?’ is
to contest the premise. We have in fact been thinking about it all
wrong: dinosaurs aren’t extinct at all. One important and adaptable
group of them is still found everywhere on the planet: birds. ‘Birds,’
Brusatte stresses, ‘are dinosaurs,’ and he is adamant that this should
be understood in the literal sense. Looking at a seagull outside his
window, he notes that ‘many of the features that allow me to immediately
recognise it as a bird are not actually trademarks of birds. They’re
attributes of dinosaurs.’ Birds are ‘simply a subgroup of dinosaurs,
just like the tyrannosaurs or the sauropods’, he explains, ‘every bit as
dinosaurian as T. Rex, Brontosaurus or Triceratops’. The two groups
share a common ancestor, making birds just ‘one of the many branches on
the dinosaur family tree’.
‘The realisation that birds are dinosaurs is probably the single most
important fact ever discovered by dinosaur palaeontologists,’ Brusatte
writes. An evolutionary connection between birds and dinosaurs is not in
itself a new idea; the 19th-century biologist T.H. Huxley – ‘Darwin’s
Bulldog’ – had made a similar observation about Archaeopteryx, the
feathered creature whose fossilised remains were first discovered in
1861. This was the first evidence of feathers in the prehistoric record,
and being very bird-like but clearly not a true bird, Archaeopteryx was
hailed by Darwin himself as an example of a transitional fossil – one of
the hallowed ‘missing links’ between different species that were implied
by the idea of evolution through natural selection. But Huxley, a
comparative anatomist, noted that the skeleton was strikingly similar to
that of Compsognathus, a small carnivorous theropod (the same sub-order
that contains tyrannosaurs and Velociraptor), whose fossils could be
found in the same geological formation as Archaeopteryx. Going further
than Darwin, he suggested that birds were in fact the descendants of
dinosaurs.
The issue remained thorny for a hundred years. New discoveries and
arguments during the 20th century periodically boosted the case, with
the discovery in 1969 of the mobile, bird-like predator Deinonychus
eventually ushering in a new consensus that birds had indeed most likely
evolved from dinosaurs, as Huxley had thought. Even then there was one
particular sticking point: though it might be inferred that defining
bird characteristics – feathers and flight especially – must have had
precursors in their dinosaur forebears, the fossil record was silent.
There was Archaeopteryx, and there was a lot of circumstantial
anatomical evidence, but there was no fossil of a feathered dinosaur.
That changed in 1996, with a series of discoveries from fossil beds in
the Liaoning region of north-eastern China. The environment at Liaoning
in the early Cretaceous was one of dense forests and lakes, with a
diverse ecology of dinosaurs, birds, mammals and other creatures. It was
also volcanic, and entire areas were occasionally destroyed by eruptions
that buried the ancient Liaoning countryside in layers of volcanic ash,
preserving unlucky local creatures in miraculous detail. In this
prehistoric Pompeii, a feathered dinosaur was finally discovered. (A
curious correspondence between geopolitical ascendancy and
palaeontological advance can be observed here. During the imperial,
industrial 19th century, England was the principal scene of discoveries
and advances in the field. In the 20th, the mantle was passed to the
United States; so far in the 21st, the site of the most dramatic finds
is China. Success in the present is reinforced and naturalised by
autochthonous possession of the most important prehistoric relics.)
The feathered fossil creature was named Sinosauropteryx (‘Chinese
reptile wing’) – a small, carnivorous theropod about a metre long,
covered in a fuzz of thin filaments. Some years later, these primitive
feathers were the first dinosaur tissues to undergo successful analysis
for traces of their original coloration, as scientists discovered
melanosomes, tiny capsules that contain pigment, in some well-preserved
fossils. The shape of these melanosomes reveals the colour they once
held, and the perennial question of dinosaur colour could in this case
be answered: the fuzzy, birdlike Sinosauropteryx was ginger, with white
and ginger bands along the length of a long, fluffy tail.
The gravity and value of such discoveries were quickly understood. The
farmers and prospectors who knew the land at Liaoning wasted no time in
making more of them; the area has since yielded dozens upon dozens of
feathered dinosaurs in all shapes and sizes. A whole series of creatures
from successive prehistoric ecologies have been found, and they have
shown that feathers and bird-like anatomical features were common to a
large group of dinosaurs. Some have body designs seen nowhere else, such
as the bizarre Microraptor, all four of whose limbs were feathered wings
(the same analysis that was used on Sinosauropteryx indicates that its
feathers were the iridescent black of a starling). Other discoveries
have changed the way we imagine well-known dinosaurs: the fluffy coat of
feathers that covered Yutyrannus, a large tyrannosaur of the early
Cretaceous, suggest strongly that its later, more famous relative T. Rex
would also have had at least some feathering.
*
The fossils from Liaoning are extraordinary in providing what Brusatte
describes as ‘an almost perfect case study’ of a major evolutionary
transition. The striking range of creatures preserved helps to ‘untangle
one of the biggest riddles of biology: how evolution produces radically
new groups of organisms, with restyled bodies capable of remarkable new
behaviours’. Such long-term changes cannot be simulated or observed;
only fossils can provide evidence for the way organisms changed over
long periods of time in the deep past. Archaeopteryx may have been a
missing link, but Liaoning was the reliquary for a whole lost chain. The
discoveries were so extensive that, ‘when stitched together and placed
on a family tree, they provide something of a running film of an
evolutionary transition in action.’ Most significant, ‘the Liaoning
fossils confirm where birds perch on the dinosaur family tree. Birds are
a type of theropod’ – exactly as Huxley had suggested.
The discoveries also tell us something about the development of feathers
themselves, because although some of the creatures could clearly take to
the air or at least glide, most of these feathered dinosaurs could not
fly. In Sinosauropteryx and many other small feathery dinosaurs, the
feathers were very simple, more a fluffy, fur-like covering. The
creatures themselves were ground-dwelling, and mostly without anything
wing-like. Perhaps their ‘proto-feathers’ developed to keep them warm;
perhaps they were used for display, or camouflage. Some dinosaurs would
develop more complex feathers, of the kind we see in modern birds, but
not all of them flew either. Moreover, it seems that wings themselves
did not originate as a means of flight. The first feathered dinosaur
discovered in the US – a large, ostrich-like ornithomimosaur, sporting
‘a mange of cotton-candy fuzz’ – has what are, anatomically speaking,
feathered wings; but like those of a modern ostrich, they could not
possibly have been used for flying. The creature was too large – as big
as a horse – and its wings far too small. No one knows what purpose such
wings served, or why they developed; to judge from modern birds, display
is one possibility.
The anatomical link with birds also provides insight into the most
baffling aspect of dinosaur biology: their enormous size. The biggest of
them, a late Cretaceous group of Diplodocus-like sauropods named
titanosaurs, were the largest land-dwelling animals since life began,
and by a very large margin. Felicitously named creatures such as
Dreadnoughtus, Austroposeidon and Patagotitan could grow to around
thirty metres in length and weigh more than fifty tons – for scale,
Brusatte notes that this is ‘more than a Boeing 737’. Such beings seem
to have slipped the bonds that govern terrestrial growth and size. How
could they function?
There are various factors at work, but part of the answer to the problem
of dinosaur gigantism seems to be that they had unusually efficient
lungs, which worked on the same design found in birds. The lungs of a
bird are ‘unidirectional’ – they do not work like a bellows, but
circulate air in one direction, thanks to a complex arrangement
involving a series of air sacs that extend into special cavities in the
bones. This means that birds can extract oxygen from air on inhalation
and exhalation – ‘twice the bang for the buck, a continuous supply of
energy-sustaining oxygen’. The fossil bones of some large dinosaurs show
similar cavities, which are called pneumatic fenestrae: ‘They are
exactly the same structures as in modern birds, and they can only be
made by air sacs,’ Brusatte explains. They also made the skeleton
lighter. In fact, the bones of some large sauropods ‘were little more
than honeycombs, featherweight but still strong’, hollowed out by the
elaboration of ‘ultra-efficient lungs that could take in enough oxygen
to stoke their metabolism at huge size’. Beyond birds and some
dinosaurs, no other animals have the same design. Its unique efficiency
seems to have helped sauropods to grow into giants, giving a T. Rex the
bursts of energy needed to strike its prey from ambush and bite down
with a force of three thousand pounds per tooth; today, the design makes
it possible for modern birds to fly enormous distances in freezing and
rarefied air.
Perhaps just such a skill was key in helping a small number of the
dinosaurs we call birds survive when the fire and darkness of the
end-Cretaceous asteroid strike overtook life on Earth. Their descendants
are intelligent, adaptable and mobile. They have evolved, in dazzling
variety, to endure and thrive in all climates and habitats on Earth.
From the Antarctic to the Sahara to the open ocean, they populate every
corner of the globe. No doubt some hardy creatures will ride out the
current planetary catastrophe, and birds are as good a contender as any;
after all, 66 million years ago they had what it took to evade the
effects of a billion atom bombs. So maybe the dinosaurs will survive the
sixth mass extinction too.
_________________________________________________________
Full posting guidelines at: http://www.marxmail.org/sub.htm
Set your options at:
https://lists.csbs.utah.edu/options/marxism/archive%40mail-archive.com
