In the Late Cretaceous, hundreds of extraordinary, prehistoric creatures roamed the Earth. Armored
dinosaurs battled against ferocious, bipedal predators. Flying reptiles soared through the
skies, while giant, marine animals dominated the Earth’s rivers and oceans. But why don’t these
huge Cretaceous species exist in the world today? Spanning 79 million years, the Cretaceous Period
was the third geological period in the Mesozoic Era. Long before the Cretaceous Period,
our prehistoric planet was already teeming with dinosaurs, pterosaurs, and other giant reptiles.
The Triassic and Jurassic periods stimulated tremendous evolutionary growth,
rearing diverse populations of giant, reptilian species. Many Cretaceous dinosaurs evolved
during the Triassic and Jurassic, as well as mammals and the first flying vertebrates.
Near the end of the Triassic Period, the Earth experienced an episode of major extinctions,
known as the Triassic-Jurassic or Tr/J Extinction. Nearly a third of marine organisms disappeared,
but giant reptiles remained the rulers of our prehistoric planet. Their dynasty matured through
the Jurassic Period and into the Cretaceous, when many of the largest species walked the Earth.
Throughout the Mesozoic Era, the planet was changing rapidly. Roughly 200 million years ago,
all seven of the world’s current major landmasses were joined together in one supercontinent called
Pangea. Geographic obstacles were limited, and creatures roamed freely between North
America and Australia; but, during the Early Jurassic, Pangea began breaking apart. The
Americas drifted west. Eurasia moved east, and vast oceans developed in the spaces between.
The world was no longer one interconnected system. Instead,
distinct landmasses with individual climates and topographies developed unique ecosystems,
bearing thousands of new species, including angiosperms, pollinating insects, and some of
the largest reptiles to ever walk the Earth. On every major landmass, titanosaurs,
a group of sometimes gigantic sauropods, reached their long necks to feast on the tallest trees.
Titanosaurs were some of the largest and heaviest animals in history, with some species stretching
over 25 meters from head to tail. Argentinosaurus, one of the largest titanosaurs, weighed up to
75 metric tonnes and likely intimidated other dinosaurs with its overwhelming size and strength.
In the Western United States, another giant herbivore called Triceratops, wielded enough
power to fight off the largest carnivores. Triceratops had three horns on its face and bony
frills on the back of its head. Like modern-day rams and oxen, these humongous herbivores
clashed with members of their own species, smashing their horns and frills together in
Earth-shaking battles for dominance and territory. Triceratops was not the only herbivore with armor
during the Cretaceous Period. In the United States and parts of Canada, ornithischians
like Ankylosaurus had rows of wedge-shaped bones protruding like spikes from their bodies. On its
tail, Ankylosaurus wielded a massive club that may have shattered the bones of other dinosaurs.
To feed on these giant, armored herbivores, predators fought tooth and nail for every meal.
Few apex predators were more dominant than Tyrannosaurus rex, “the tyrant king” of
the dinosaurs. This bipedal predator may have exceeded 12 meters in length, from head to tail,
and 4 meters in height at the hip. Tyrannosaurus rex hunted in the grasslands of North America,
though its direct ancestors likely migrated from Asia. Scientists estimate Tyrannosaurus
rex had the strongest bite of any terrestrial animal. With its crushing jaws, T. rex wrestled
and killed dinosaurs of all shapes and sizes. T. rex was not the largest carnivorous dinosaur
during the Cretaceous Period. Lurking in the rivers and swamps of North Africa,
Spinosaurus was a semi-aquatic carnivore that fed on ancient sharks, sawfish, and coelacanths,
which exist in the world today. With a narrow snout, long arms, and a fin-like tail, Spinosaurus
grew up to 15 meters. That’s longer than the longest T. rex ever discovered. Spinosaurus
gets its name from a bony protrusion that formed a sail on its back. With its towering sail, which
nearly doubled its height, Spinosaurus was an intimidating sight for prey and predators alike.
Tyrannosaurus rex terrorized prehistoric grasslands and plains. Spinosaurus attacked
anything too close to the water’s edge. Even the skies were patrolled by flying reptiles, called
pterosaurs. These aerodynamic creatures were among the earliest vertebrates to evolve powered flight.
Often mistaken for oversized birds, pterosaurs were close cousins of dinosaurs that dominated
the skies for most of the Mesozoic Era. Some pterosaurs were the size of house cats,
while others were larger than modern-day fighter jets, swooping down on sea creatures,
mammals, and even small dinosaurs. As the positions of the Earth’s major
landmasses shifted, seas and oceans developed in the gaps between each continent, but these
bodies of water were as dangerous as the land or sky. Some oceanic predators died out during
the Tr/J Extinction Event, but one of the largest marine reptiles still lurked beneath the waves.
Mosasaurus was a crocodile-jawed apex predator, stretching up to 17 meters long and well-adapted
to its marine environment. Preying on sharks, cephalopods, and other marine reptiles,
Mosasaurus was a deep-sea terror, reminiscent of sea monsters in myths and legends.
While the Cretaceous Period was dominated by a frightening cast of reptilian giants, the
longest-lasting species were anything but giant or terrifying. They weren’t club-swinging dinosaurs
or towering predators but a myriad of plants and small animals, like pollinating insects.
The first insects evolved some 400 million years ago. Wasps emerged prior to the Mesozoic Era,
but the first bees did not evolve until the Early Cretaceous, followed by the earliest angiosperms,
or flowering plants. Nurtured by a radical growth of floral diversity, bees and
wasps—along with ants, grasshoppers, aphids, and termites—flourished during the Cretaceous Period.
Feeding on flowers and insects, rodent and marsupial species also prospered throughout
the Cretaceous Period. Hiding in burrows, nests, and dark corners of the forest,
the ancestors of modern-day mammals were ignored by the larger dinosaurs stomping over their heads.
In the Late Cretaceous, massive reptiles shook the Earth, but it was these diverse, unsuspecting
species that would transform the planet. Approximately 66 million years ago,
our global ecosystem changed forever. At the end of the Cretaceous, a mass extinction, known as the
Cretaceous-Paleogene or K/Pg Extinction Event, eradicated roughly 75% of all life on Earth.
Most likely, an asteroid struck the Yucatan Peninsula off the coast of modern-day Mexico.
Dust and debris launched into the atmosphere, blocking the sun’s rays from warming the
Earth’s surface, thereby limiting the growth of life-giving plants. Toxic clouds of vaporized rock
suffocated most dinosaur species, and the global food web rapidly collapsed. The K/Pg extinction
eradicated all non-avian dinosaurs, but many species did survive, like smaller reptiles,
mammals, plants, and fungi, which grow on decaying organic matter. These species would come to
flourish in the Cenozoic Era, evolving into the millions of plants and animals we know today.
While most giant reptiles disappeared 66 million years ago, these extraordinary
species were legendary, unforgettable animals, largely unrivaled in power,
size, and ferocity. Today, their relatives and descendants still roam the Earth,
but the Late Cretaceous remains the last great age of monsters.