The Dawn of the Dinosaurs
Around 252 mya, the Permian Period ended in a mass extinction where around 90% of all species disappeared. This was the result of tremendous and prolonged volcanic activity. The lava drowned great swathes of land, dust blocked the sun and noxious gases polluted the air. Carbon dioxide levels increased, seeing temperatures soar; and dissolved in rain and the oceans, further damaging plants and sealife.
When the lava stopped, the Triassic Period began.
A new type of animal emerged: archosaurs. These creatures evolved from the reptilian sprawlers of the Permian. With their upright posture, these archosaurs were able to run faster for longer.
The first true dinosaurs appeared 240-230 mya, during the middle to late Triassic Period. True dinosaurs are defined by muscle anchors on the upper arms and neck vertebrae, and an open-window-like joint where the thighbone meets the pelvis.
However, true dinosaurs by no means dominated the Triassic: they were still overshadowed by larger and more diverse amphibians, mammal cousins and pseudosuchians.
Dinosaurs Rise Up
When dinosaurs first evolved, Earth was a giant supercontinent (Pangea) surrounded by a single ocean (Panthalassa).
Earth was warmer, due to high carbon dioxide levels, and warm ocean currents could also flow unimpeded from equator to poles (no ice there, then).
Pangea experienced megamonsoons: alternations of dry seasons and storms as violent air currents changed direction with the seasons, across this single lump of land.
During the late Triassic, a couple of key factors increased the population and range of the dinosaurs: many large plant-eaters disappeared, meaning less competition for the plant-eating sauropods; and 215 mya dinosaurs arrived in the sub-tropical arid belts, possibly due to a shift in climate.
Coelophysis and a handful of small meat-eating therapods were able to survive in the desert but the landscape was still dominated by dinosaurmorph precursors and other animals. This was probably because of hyperseasonality where the high carbon dioxide levels led to dry and hot weather for much of the year followed by wet and cool periods. These fluctuations made survival difficult for large plant-eating dinosaurs.
Dinosaurs Become Dominant
At the very end of the Triassic, 201 mya, the supercontinent finally cracked, creating an east-west divide.
Lava, noxious gases and global warming led to a mass extinction with around 30% of all species being lost.
Following this event, as the Jurassic Period began, the diversity of non-dinsaur tracks dropped dramatically. Whereas before the volcanoes dinosaur tracks made up about 20%, they now made up 50%, including many new ones.
For some reason, the dinosaurs thrived. By the middle of the Jurassic, the major dinosaur groups could be found across the globe.
However, it was the sauropods that dominated. During the early part of the Jurassic, the sauropods evolved ‘noodle-necked’ bodies which grew to monstrous sizes. But how were they able to grow so big?
- Long necks allowed them to reach new food sources and browse for hours expending little energy
- Fast growth rate
- Bird-style lung (unidirectional, meaning that oxygen could be extracted during inhalation and exhalation – this was also a feature of therapods)
- Light skeleton
- Efficient heat dissipation
Dinosaurs and Drifting Continents
Sauropods were able to coexist successfully as they avoided competing with each other. This is called niche partitioning. Different neck types, heads and teeth allowed them to eat different foods.
This was the case globally, as Laurasia and Gondwana were still connected by land bridges.
No major event defined the end of the Jurassic, 145 mya. Pangea continued to split and Gondwana started to fracture, too. Sea level and climate changed slowly over millions of years.
20 million years into the Cretaceous Period, many of the familiar sauropods had gone extinct. A new subgroup called the titanosaurs began to proliferate. However, these never achieved the dominance of the sauropods of the Late Jurassic.
As a result of the declining sauropods, the small plant-eating ornithischians blossomed.
The decline of the sauropods also forced the meat-eating theropods to adapt to new diets, leading to a greater diversity within this group.
Carcharodontosaurs, evolved to become larger, stronger and fiercer when their Allosaurus ancestor died out. As the continents continued to drift apart, carcharodontosaurs became isolated in different areas.
However, this dominance did not last for the remainder of the Cretaceous: the tyrannosaurs were coming!
The Tyrant Dinosaurs
During the last 20 million years of the Cretaceous, the tyrannosaurs rose to dominance.
Small tyrannosaurs were spread throughout the world from the middle of the Jurassic and survived the changes that lead to the extinction of many of the sauropods, Allosaurus and stegosaurs.
These tyrannosaurs developed a larger brain and more sophistocated senses. They got smart before they got big.
But how did they end up taking over as the apex predators?
- One possibility is that they outsmarted the carcharodontosaurs
- The other is that they simply took the opportunity when the carcharodontosaurs went extinct due to environmental changes
The King of the Dinosaurs
T.rex was the largest predator to ever live on Earth. The King of the Dinosaurs evolved from Asian species that crossed the Bering Land Bridge, which once connected Asia with North America.
T.rex conquered North America but nowhere else as by this time, this continent had completely separated from Asia. His reign was also destined to be short-lived, having evolved just a couple of years before the end of the Cretaceous—and all land-living dinosaurs.
So why was T.rex dominant?
- Strong teeth, jaw muscles and a rigid skull
- Agile
- Unidirectional lung for bursts of energy
- Large brain with heightened sense of smell, hearing and vision
- And they hunted in packs. As if one of these wasn’t formidable enough!
Dinosaurs at the top of their game
By the Late Cretaceous, the world looked like a very different place. North of the equator were North America and Asia. South of the equator were South America, Africa, Antartica and Australia. Much of Europe was flooded due to greenhouse gases and the lack of polar ice. India existed as a separate continent off the esat coast of Africa.
This geographical and ecological complexity lead to the greatest diversity of dinosaurs that had ever lived.
Dinosaurs Take Flight
Birds are dinosaurs!
The oldest fossil of a true bird found is Archaeopteryx. This dinosaur lived 150 mya.
Birds are a type of therapod, sharing features such as feathers, wish-bones and 3-fingered hands that fold against the body.
They belong to a sub-group of therapods called the paravians. These are identified by bigger brains, keener senses, and a more compact and lightweight skeleton. The paravian group contains birds and non-birds.
Dinosaurs Die Out
(Except the birds!)
66 mya a comet or asteroid, around 6 miles wide and travelling somewhere in the region of 67000 mph, hit the Yucatan Peninsula of Mexico.
The result: scorching air temperatures, fires, sonic booms bringing 600 mph winds, earthquakes, tsunamis and liquified rock from the collision raining down in the form of glass bullets.
And that was just the initial devastation.
A nuclear winter reigned for several years, casued by the soot and dust blocking the light and heat of the sun. And when this finally cleared, global warming followed as a result of the copious amount of carbon dioxide in the atmosphere.
Earth was not a good place to be.
A few hundred years later and the only survivors were some reptiles, small mammals and birds.