By far the longest of period of the Mesozoic Era, the Cretaceous began 145 million years ago. Characterized by the rise and fall of many of the world’s iconic dinosaurs, the period ended with the best known and one of the most devastating extinction events of all time. In Part 12 of “Journey through the History of Earth,” we’ll be exploring the era of the triceratops, ankylosaurs and, of course, the infamous tyrannosaurus rex, one of the most epic apex predators of all time.
By the beginning of the Cretaceous period, the dinosaurs had already been dominating terrestrial ecosystems for some 86 million years. Their reign would continue for another 79 million years, which is more time than has elapsed between their ultimate extinction and the modern day. Their global empire was now in full swing, with new species appearing alongside many other iconic animals, including new species of pterosaurs, mosasaurs and mammals.
The Cretaceous represents the rise and fall of the dinosaurs and the final period of the 186-million-year-long Mesozoic Era. The name is derived from the Latin word for chalk, owing to a rock type that was particularly characteristic of that period. It was first defined in 1822 based on rock strata found in France dating from the time. The period is often defined by the letter K, from the German translation Kreide, such as is the case when referring to the K-Pg boundary, when the period ended.
Highlights of the Cretaceous
- Atlantic Ocean forms
- Mammals feed on dinosaurs
- Flowering plants and grasses appear
- Australia and Antarctica split apart
- Tyrannosaurus rex evolves
- Impact event devastates Earth
Moving Continents Remodel the Path of Evolution
The Cretaceous was a period when the continents were on the move. Gondwana, which formed during the Cambrian Period, started to break apart, forming the Atlantic Ocean. This chain of events ultimately led to a global map that resembled the modern-day continents by the end of the Cretaceous. It also led to biological isolation, causing new, and often very distinct, species to evolve separately from one another. No longer could the world’s creatures roam across the supercontinent of Pangea. It’s for this reason that a Cretaceous wildlife tends to be quite different between that of the Americas and Eurasia.
Continuing from the Jurassic Period, the Cretaceous also saw significant climate change. The world continued to warm, and oxygen levels rose to their highest ever levels since the Carboniferous. At their peak, they reached levels of around 150% higher than they are today.
Most of the world was tropical or subtropical, and forests thrived even as far south as the Antarctic. The first angiosperms, or flowering plants, appeared early in the period too, an event that also gave rise to the first bees. Meanwhile, south polar dinosaurs, such as cryolophosaurus, dominated southern regions in what is now Antarctica and southeast Australia. Eventually, even Australia broke away from the Antarctic during the Late Cretaceous, which is precisely why that continent is home to some of the most unusual species on Earth. Since then, it has enjoyed some 85-million of independent evolution!
Bizarre Creatures Evolve in Isolation
Although mammals have been around since the Late Triassic, some 225 million years ago according to T. S. Kemp’s definition of a mammal, the class is usually associated with the Cenozoic Era that followed the dinosaurs and continues to this day. After all, Mesozoic mammals were, for the most part, small shrew-like creatures living in the shadow of the dinosaurs. That’s not the whole story, however.
Repenomamus was the largest known mammal of the entire Mesozoic Era. Its remains were found only in 2001 in China. Reaching the size of a typical domestic dog, one species of repenomamus was significantly larger than some of the smaller dinosaurs, such as the contemporary graciliraptor. Not only that; fossils of this unusual mammal have been found containing the remains of baby dinosaurs.
Repenomamus was, however, still quite different to today’s mammals. Although it vaguely represented a large opossum, it was actually a gobiconodontid, a long-extinct mammalian order that disappeared some 94 million years ago.
Repenomamus certainly wasn’t the only peculiar creature found in what is now China. Its contemporary, the microraptor, was perhaps even stranger. Discovered in 2000, the microraptor belonged to a clade of four-winged gliding dinosaurs. The largest of the group was the changyuraptor, which grew bigger than an albatross. It even had magnificent tail feathers reaching a foot (30 cm) in length.
Throughout the Early Cretaceous, the geographical isolation of what is now China and Mongolia continued to give rise to distinct fauna. The therizinosaur, which survived right up until the end of the period, was another addition to this unusual selection. This feathered creature boasted a bizarre set of characteristics, including a wide torso and a long neck. Therizinosaurs were largely herbivorous although, with its enormous, sickle-like claws and intimidating size (sometimes up to 33 feet (10 metres) long), it would undoubtedly looked terrifying!
Tyrants of the Land and Sea
The Late Cretaceous, which accounts for the second half of the period, is characterised by the evolution of some of the most spectacular predators that have ever lived. The last few million years of the dinosaurs was the Maastrichtian stage, and although species diversity had started dropping globally leading up to this time, it was the time of some of the most well-known prehistoric animals of all.
In just about every environment on Earth, be it land, sea or air, there were apex predators that put anything alive today to shame. This was the age of the infamous tyrannosaurus rex, colossal marine reptiles and incredibly large flying reptiles. Other apex predators of the time included the semi-aquatic spinosaurus of North America and the giganotosaurus of South America, both of which may have been even larger than the more famous tyrannosaurus. Even the far smaller predators, such as the woefully misrepresented velociraptor, were terrifying hunters of the time.
Tyrannosaurus rex, weighing some nine tonnes, was one of the largest terrestrial carnivores that ever lived. The species has been represented or, rather, misrepresented, countless times in popular culture. Nonetheless, the real animal, while probably partly covered in feathers, was no less formidable than the fantasy. Growing 40 feet (12.3 metres) in length, it had teeth up to nine inches (23 cm) long, and its bone-crushing jaws boasted the highest bite force of any animal ever known. Such a monstrous beast was the tyrannosaurus rex that it may even have been able to rip the head clean off a full-grown triceratops.
While monstrous carnivores dominated almost every terrestrial ecosystem of the Late Cretaceous, life in the Earth’s oceans was also far from safe and harmonious. Mosasaurs, the largest ever carnivorous marine reptiles, were far larger than any shark, and they prowled the coasts and river deltas of Western Europe and North America. Many species, such as tylosaurus, grew much longer than a bus, and sported gaping maws of enormous conical teeth to boot. They were likely incredibly fast swimmers too, owing to their long tails and stiff, streamlined bodies.
During the Late Cretaceous, the skies were also home to predatory giants. These were the fish-eating pterosaurs, which had already been around for some 150 million years by the time. Although pterosaurs were already on the decline in terms of biological diversity by the end of the period, it was then that the largest flying animal ever known took to the clouds. Quetzalcoatlus northropi was a truly epic terror of the skies. With a wingspan of up to 36 feet (11 metres), this magnificent azhdarchid pterosaur was no smaller than a giraffe.
Peaceful Grazers Struggle for Survival in the Age of Tyrants
If popular culture is anything to go by, it might seem that almost everything wanted to eat everything else during the Cretaceous Period. However, this was also the time of majestic herbivores such as the much-loved triceratops and the heavily armoured ankylosaurus.
Triceratops is by far the best-known ceratopsian, a family of ornithischian dinosaurs characterized by their particularly large heads. Its ancestors, however, such as the protoceratops, were small, sheep-sized creatures. Nonetheless, all members of the family boasted elaborate sets of horns, frills and bird-like beaks. Triceratops itself is often portrayed doing battle with its arch enemy, the tyrannosaurus rex but, even with its three horns and bony frill, it probably didn’t stand much of a chance.
A contemporary of triceratops was the ankylosaurus which, at up to 20 feet long, was almost as large. Living in the same region of what is now western North America, ankylosaurus was also one of the last non-avian dinosaurs. Sporting a magnificent coat of armour of plates and nobs and a club-like tail, it was nothing less than a living tank. Owing to its elaborate armour, it was originally thought to be a relative of the stegosauruses, which had already been long extinct.
Like most of the now-extinct ornithischian dinosaurs, ankylosaurus and triceratops were herbivorous grazers who fed on low-lying plants. By contrast, the sauropods, with their extremely long necks, concentrated on foliage on higher branches, so the two groups did not need to compete. In fact, it was the Cretaceous period that saw the evolution of the next major group of these massive sauropods.
The dominant group of Cretaceous sauropods were the aptly-named titanosaurs, which fulfilled much the same environmental niche as that of the Jurassic brachiosaurs and diplodocids. Among the most spectacular of these gentle giants was the argentinosaurus, which lived in South America around 95 million years ago.
As evidenced by discoveries of its nesting grounds in Patagonia, argentinosaurus boasted a truly incredible trait: It hatched from an egg no larger than that of an ostrich before growing up to 25,000 times its birth size. At its peak, argentinosaurus grew up to 90 pounds (40 kg) in a day, so much that you would literally be able to ‘see’ it expanding!
The Apocalypse Marks the End of an Era
66 million years ago, the era of the dinosaurs ended with the best-known mass-extinction events of all time. The event, which is now widely believed to have been caused by an asteroid or comet impact, sealed the fate of almost every animal on Earth larger than a sheep, the last of the non-avian dinosaurs along with them.
Based on extensive studies of the Chicxulub crater, part of which lies off the coast of Mexico’s Yucatan Peninsula, scientists have been able to create remarkably accurate simulations of what likely happened on that day. The dinosaur’s 165-million-year reign was wiped out in a matter of a few days all because of this one unlucky event.
Travelling 20 times faster than a rifle bullet, the city-sized impactor hurtled into Earth’s atmosphere, crashing into the sea off the coast of Mexico. Exerting a force some six billion times greater than the atomic bomb dropped on Hiroshima, a megatsunami erupted from the ocean. 300-foot (100 metre) waves travelled in all directions faster than the speed of sound. The entire southern coast of North America was wiped off the map within hours, countless species vanishing forever in the process.
Because the impactor landed in relatively shallow waters, the megatsunami was not nearly as powerful as it could have been. However, it was accompanied by such a vast quantity of dust particles being kicked up into the atmosphere that it heralded an even more devastating impact winter. As death rained down from the heavens, most of the sunlight reaching the surface plummeted. Vast swathes of forest perished, the animals that relied on them for sustenance quickly following.
Within the months following the impact, bitterly cold global temperatures and greatly diminished food supplies saw the last surviving non-avian dinosaurs starve to death, along with the pterosaurs, mosasaurs and countless other species. This was the end of an era, a time marked by death and destruction on a truly biblical scale.
Following the devastating Cretaceous-Palaeogene mass extinction, the destruction heralded new opportunities. As light once again dawned upon our ancient Earth, life would, as it has always done, recover, and the path of evolution would be open once again. In next week’s instalment, we’ll be exploring the rise of mammals and the birth of a new world from the ashes of the Cretaceous.