The unlikely rise of the dinosaurs.

Dinosaurs started off as pipsqueaks in a world of  heavy weight competitors. How they ascended to glory is a mystery we’re only just starting to unravel, says palaeontologist Steve Brusatte.

ABOUT 250 million years ago,  a creature raced along the edge  of a lake in what is now Poland,  leaving prints behind it in the mud. It  was a meek and forgettable animal called  Prorotodactylus, about the size of a pet cat  and with slender limbs. But those prints  weren’t the only legacy it left: its descendants  somehow became the rulers of Earth.  

Those descendants were the dinosaurs.  The very word invokes majesty. These  were among the most successful groups of  animals ever, dominating the planet for more  than 100 million years. They proliferated into  creatures of all shapes and sizes, some even  larger than a jet plane, and filled the land.

Palaeontologists like me were long  obsessed with understanding why these  mighty animals were snuffed out 66 million  years ago. We now know the answer: their  days were ended by an enormous asteroid  impact. 

Today, the greatest mystery of  dinosaur evolution is how they rose to  glory in the first place.  The early descendants of Prorotodactylus  would have stuck to the shadows, skulking  away from much larger and more fearsome  animals. So what was it that allowed them  to take centre stage? Piecing together  the answer is no easy matter. But over the  past few years, a surprising new idea has  gained traction. Perhaps the reason for the  dinosaurs’ ascendency lies not in their teeth  or claws or muscles. It may instead be thanks  to a series of strange anatomical adaptations  invisible from the outside – adaptations  that allowed them to thrive in one of the  most extreme periods of climate change  the planet has seen up until now.

The world was almost unrecognisable back  when Prorotodactylus made those lakeside  prints. Our planet had just experienced one of the worst mass extinctions ever. A blast of global warming, fuelled by volcanic  eruptions of unimaginable scale in Siberia,  had caused more than 95 per cent of Earth’s  species to die. From this catastrophe sprang  the dinosaurs’ ancestors and closest cousins,  including Prorotodactylus. Within 20 million  years, they had evolved and diversified into  the three main subgroups of dinosaurs: the  meat-eating theropods, the long-necked,  plant-guzzling sauropodomorphs and  the beaked, herbivorous ornithischians.  Much later, these lineages would spawn  recognisable dinosaurs: Tyrannosaurus,  Brontosaurus and Triceratops, respectively  (see “An interrupted reign”, page 38).  Before this, in the Triassic period, most  dinosaurs were horse-sized or smaller.  And they weren’t alone.

Proliferating  alongside them were all sorts of other  reptiles, including a particularly successful  group called the pseudosuchians. This is  the lineage to which modern crocodiles  and alligators belong. 

They are a paltry  bunch today, about 25 species all told, living  in warm, semiaquatic environments. But  back in the Triassic, there were scores of  them, including armoured ones that ate  plants, toothless omnivores that sprinted  on their hind legs and apex predators called  rauisuchians that were 9 metres from nose  to tail and had teeth like steak knives. 

See you later, alligator If the pseudosuchians sound impressive,  that’s because they were. So how did  dinosaurs replace them as the dominant  creatures on land? Back in the 1970s,  some palaeontologists thought that early  dinosaurs were unusually well-adapted to  rapid running compared with their close  relatives, says John Hutchinson, an expert on  animal muscles and locomotion at the Royal  Veterinary College in London. They tended  to walk on long, erect legs and were often  bipedal. This view was articulated by leading  dinosaur experts such as Robert Bakker,  then at Harvard University, and Alan Charig   at London’s Natural History Museum.  

of Earth’s species were killed as a result.

Yet the pattern in the fossil  record unmistakably reveals that the  dinosaurs  sailed through this period.  The pseudosuchians, on the other hand,  were devastated. Nearly all of their rich  Triassic diversity was extinguished,  leaving only a few twigs on the family tree.

“During the entire Triassic,  pseudosuchians  were completely outpacing the  dinosaurs”

There are many hypotheses that attempt to  explain this, all of which fall into one of two  camps. One says that the dinosaurs really did  have some advantage over the ancient crocs –  be it speed, agility or intelligence – and,  although this didn’t allow them to gradually  outcompete them in the Triassic, it did finally  give them the edge after Pangaea split. The  other says that there is no single reason  why the dinosaurs won. The rise in global  temperatures was so quick and so brutal that  animals survived mostly or only by chance.

Emma Schachner at Louisiana State  University doesn’t think dinosaurs survived  by mere chance. She has proposed an  interesting idea that has been getting plenty  of attention: that dinosaurs had a hidden  superpower that helped them cope with the  toxic atmospheres of the late Triassic.

To understand the idea, you need to know  a little about how lungs work. In mammals,  including us, muscles stretch the lungs out,

which pulls air into them. This means the  membrane of the lung can’t be too thin or  else it would degrade as it moves and rubs  against the ribs. But lungs work differently  in some other animals, including birds, the  direct descendants of dinosaurs. So let’s turn  to bird lungs, and, as Schachner has put it,  “it gets crazy, so hold on to your butts”.

In the chests of birds, the gas exchange  portion of the respiratory system is like a dense sponge that doesn’t move. Because  of this, its membrane can be extremely thin  without rupturing, increasing the efficiency  with which oxygen passes from the lungs  to the blood. What’s more, several separate  air sacs that aren’t part of the gas exchange  system expand and contract to funnel air  through the air exchange part of the lungs  in one direction only. 
This also means that  oxygen is drawn through the lungs during  both inhalation and exhalation, so birds  get more out of every breath.

In short, birds’ lungs are hyper-efficient,  and Schachner has published a series of  papers arguing that Triassic dinosaurs had  similar lungs to modern birds, and that this  helped them thrive.  Lungs are fleshy things that don’t tend  to fossilise, but they can leave telltale signs  behind. In birds, the air sacs often protrude  into the vertebrae, creating indentations  and sometimes hollows – so-called  pneumaticity – in the bones. Do we see  this in dinosaur bones? We sure do. Some  sections of the back bones of Triassic  dinosaurs are commonly pneumatised,  indicating they probably had avian-style  lungs. “This respiratory anatomy had  the potential to give dinosaurs a major  competitive advantage,” says Schachner.

In truth, the jury is out on whether  the lungs alone made the difference for  dinosaurs. In the past, we thought that air  had much less oxygen during the Triassic  than it does today, in which case more  efficient lungs would have been obviously  beneficial. But the latest thinking is that there  was plenty of oxygen around in the Triassic.  We also aren’t sure if the pseudosuchians  had their own special lung adaptations. 

They  certainly don’t have the same pneumaticity  marks on their bones as early dinosaurs.  But Richard Butler at the University of  Birmingham, UK, has shown that they have  depressions on the sides of some vertebrae.  These might be signs of air sacs that were a tad different from those of modern birds.

Cecilia Apaldetti at the National University  of San Juan in Argentina is contemplating  an idea that takes pneumaticity to a whole  new level. Over the past decade, her team  has unearthed a bounty of new dinosaurs  from the late Triassic rocks of the Marayes-El  Carrizal basin in Argentina. Among these  is a species she and her colleagues named  Ingentia prima. This may be the oldest known  dinosaur to get bigger than an elephant. And  its skeleton is riddled with holes, suggesting  the air sacs proliferated widely. Essentially,  this animal’s lungs ran through its whole  body. It is as weird as it sounds.

“These dinosaurs had an improved  breathing system that provided them with  numerous advantages,” says Apaldetti. With  air sacs spread throughout most of their  bodies, they were able to take in oxygen  super-efficiently and circulate air through  their innards, helping them keep cool. This, in  turn, would have supported a fast metabolism  and rapid growth. Their bones were also light.  All of these factors together would have set  dinosaurs up to get gigantic without running  into problems, like getting too heavy to  support themselves or overheating.

It is easy to imagine how any one of these  things might have helped dinosaurs ride out  a few hundred thousand years of global  warming, foul atmospheres and ecosystem  breakdown. Add them together, and they  may have been almost indestructible.

Where does that leave Bakker and Charig’s  hypothesis that dinosaurs were better  runners than the crocs? Hutchinson’s group  is revisiting this through an ongoing project.  “Past ideas about locomotion were based  almost solely on anatomy,” says Hutchinson.  But that doesn’t necessarily tell you how fast  an animal was. He and his team are instead  using laser scans of fossils to build digital  models of dinosaurs and pseudosuchians,  which they put through gymnastics routines  to test how the animals would have moved.  The work won’t be finished for another year or  two, but the team has already cast fresh light  on how dinosaurs got around. One species.

The bones of Ingentia prima, found in Argentina, are full of tiny holes

“Essentially, this animal’s lungs  ran through  its whole body. It is as weird as it sounds”

seems to have walked on four legs when  young, but graduated to two legs as it grew.  There may never be a simple answer  to the 200-million-year-old riddle of how  the dinosaurs took the Jurassic throne.  Our best guess is that they held a winning  hand of adaptations: efficient lungs, high  metabolism, fast growth and possibly  other assets that we don’t yet understand.  Together, they won the pot. But if the  environmental conditions they faced had  been just slightly different, the rules of the  game would have been changed, and the  age of the dinosaurs may never have come  to pass. As it worked out, however, those  footprints on the edge of the lake were the  start of an epic journey to greatness.  ❚

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