Modern-day dinosaurs: Meet the mass extinction survivors
With lush green jungles and the sweet scent of the first flowering plants filling the air, the Cretaceous period saw the planet at its prehistoric prime. Giants dominated the land, sea and skies up until around 66 million years ago, thriving in the abundance of life occupying Earth. That was of course until a huge piece of space rock fell from the sky, obliterating the dinosaurs and changing the course of history forever. But, some species managed to cling on to life and avoid extinction.
Defining the dinosaurs
Dinosaurs are a group of reptiles that evolved from a class of creatures called archosaurs (‘ruling reptiles’). The archosaurs evolved around 250 million years ago, eventually dividing into two different lineages: one evolutionary branch gave rise to the ancestors of crocodiles (Pseudosuchia) while the other led to the evolution of pterosaurs, dinosaurs, and eventually birds (Ornithosuchia). All these creatures – even the birds and crocodiles alive today – share a common archosaur ancestor.
One of the first dinosaurs to take its steps some 230 million years ago during the Triassic period was a small, speedy, two-legged omnivorous dinosaur called the eoraptor. 15 million years later, any animal on Earth with a length of one metre or more was a dinosaur. These beasts evolved to fill lots of different ecological niches, from towering titanosaurs to miniature microraptors. Dinosaurs were widespread on our planet, dominating the land for over 160 million years before a cataclysmic event wiped out 75 per cent of all life on Earth.
The end of an era
The exact explanation for the demise of the dinosaurs had been debated for many years until a huge crater was discovered in Chicxulub, Mexico, in 1991, a finding that finally shed some light on the truth behind the violent end of these ancient titans.
Now known as the Cretaceous-Paleogene (K-Pg) extinction event, the dinosaurs were wiped out following the impact of a ten-kilometre-wide asteroid colliding with Earth at over 64,000 kilometres per hour. The explosive power of the impact gauged a 180-kilometre-wide hole in the Earth’s surface and killed 80 per cent of the plants and animals living within its vicinity. Yet despite the incredible power of this event, the impact itself was not the sole cause of the global extinction: the atmospheric aftermath also played a key part.
Due to its sheer scale and size, the asteroid vaporised on impact, raining red-hot ash and creating a vast cloud of particulate pollution. Evolutionary biologist and broadcaster Dr Ben Garrod describes the devastation. “The area of impact was full of gypsum, and when gypsum is vaporised it makes this horrific acidic concoction that then falls as acid rain. This huge cloud of toxic gas, along with molten ash, was thrown into the atmosphere, creating a pizza oven effect, resulting in the Earth baking for months.”
This fiery rain increased the Earth’s global temperature to levels fatal to large reptiles with nowhere to go. However, it was the darkening of the sky that lead to the ultimate destruction of the dinosaurs. Acting like a curtain across Earth, this atmospheric debris shrouded the surface from sunlight. By removing light, plants could no longer photosynthesise properly. Since plants are an integral part of every food chain, without them a chain reaction of starvation swiftly followed. It was only those that could rely on alternative food sources that would live to witness the dawn of a new era.
Survival of the fittest
It may have caused the death of the dinosaurs on a global scale, but not all species were wiped out in this devastating mass extinction. To escape the intense heat of the falling ash and the cold global winter that followed, many of those that could dig or dive lived to see another day. In fact, it was the death of the dinosaurs that gave rise to the age of mammals.
Having sheltered from the impact below ground, the first true mammals soon emerged; creatures that could sustain themselves on invertebrates and plants. Arguably the most peculiar mammal members that survived the mass extinction were the egg-laying mammals: the platypus and echidna.
Those that took to the depths of the oceans also fared well, feasting on those relatively unaffected by the climatic change occurring above the waves. Sharks, for example, had hunted the oceans long before dinosaurs had taken their first steps and have lasted long after their departure. But there is one common feature that links all those that survived the catastrophe: their size.
“Size is definitely a big factor, anything bigger than the ten to 20 kilogram mark was gone. At the moment there is no evidence for anything even cow-size that survived. Most dinosaurs were relatively on the larger side. [On land] a lot of other reptiles, birds at the time, even quite a few mammals survived, but nothing other than that,” explains Dr Garrod.
The ability to survive above and below the water enabled prehistoric crocodiles and alligators to gain the upper hand. Thick scales, a long jaw housing razor-sharp teeth and a substantial tail proved a useful arsenal in the quest for survival.
“Crocodiles are perfectly adapted to their environment; they can deal with really hostile situations, such as environments lacking in oxygen. The order Crocodilia evolved separately to dinosaurs. They are true reptiles and evolved 86-85 million years ago. So they were quite new on the scene,” explains Dr Garrod.
As exotherms, crocodiles have been to seen to enter a stasis-like state to ensure their survival; a beneficial quality when dealing with a global climatic change, a quality the dinosaurs unfortunately did not possess. Size, as it did for any of the other species that survived the asteroid impact, played a massive role in the longevity of the crocodiles. Dwarf crocodiles living today are about the same size as their ancestors that escaped the brink of extinction. Charles Darwin once wrote, “It is not the strongest of the species that survives, nor the most intelligent that survives. It is the one that is most adaptable to change.” With the planet labouring under such harsh conditions, only those able to adapt to their new environment survived and evolved into the species we see today. There is one group of animals in particular that not only survived the mass extinction but thrived after it.
As direct descendants of dinosaurs still roaming the planet, birds have lost tooth and nail to maintain their stronghold on Earth. The first signs of a feathered future for dinosaurs began around 245 million years ago in the form of a group of dinosaurs called the theropods. This collection of reptiles balanced themselves on two hind legs with the help of a long tail, while two short forearms enabled them to grasp and pull apart the flesh of their prey. The foot of a theropod is the first visible similarity to a current-day bird, with three extended clawed toes protruding in front of a smaller back toe. Feathers soon followed in non-avian theropods such as the oviraptorosaur, but these dinosaurs still had not harnessed the power of flight.
Over millennia these dinosaurs steadily evolved into the first known winged dinosaur, the archaeopteryx, taking to the skies around 150 million years ago. During the next 80 or so million years, this clade of dinosaurs became smaller and smaller, losing the claws at the tips of their wings and replacing their teeth-laden jaws with beaks, and it was this transformation that was key to surviving the mass extinction.
Prehistoric birds similar in appearance to the ones we know today began to develop, such as the crow-like confuciusornis. Beaks and wings were the real saving grace for birds following the extinction event. As plant resources were declining, a lack of teeth enabled them to access seed and invertebrate food resources in a world where food was scarce. Their ability to fly gave them a distinct advantage over less fortunate land-dwelling animals, enabling them to reach areas of refuge.
“There must have been pockets of little oases around the world. Havens where nothing was touched, such as gorges or valleys. We don’t know where they are yet, but there could be two or three little places potentially, or maybe dozens of places that were untouched, beautiful, lush, tropical places,” says Dr Garrod.
Without the now-deceased carnivorous giants hunting them, or their herbivorous counterparts consuming their weight in vegetation, birds and other small animals were able to sustain themselves, thus birthing the lineage of the living fossils we see today.
This article was originally published in How It Works issue 109
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