The rise of the Dinosaurs

Posted on Saturday 22 November 2003 to Story So Far


The disappearance of the dinosaurs is an one of those fascinating mysteries but there's an equally interesting riddle which has been given far less attention:

How is it that the dinosaurs came to rule the earth in the first place?

For 160 million years they dominated the planet without rival before being suddenly wiped by the impact of a massive meteorite at the end of the Cretaceous period some 65 million years ago. This event, known in the fossil record as the Cretaceous-Tertiary boundary (also called the K-T boundary) was a mass extinction that wiped at least 75% of all species on the planet.

But this was only one amongst several mass extinction events that occurred over the past 500 million years or so. Before the rise of the dinosaurs there were two mass extinctions which (in evolutionary terms) occurred in quick succession: at the Permian-Triassic boundary (250 million years ago in which 90% of marine species and 70% of land species died out) and at the Triassic-Jurassic boundary (200 million years ago which wiped out at least 50% of marine and land species). The exact causes of these extinction events is not known although theories include meteorite impacts and extemely active volcanism.

The latter theory has a great deal of evidence going for it, especially for the Triassic-Jurassic extinction. Around 200 million years ago, the supercontinent of Pangea began to tear itself apart in the most violent way imaginable. The hundreds of basalt outcrops which today line the Atlantic coasts of North America, Brazil, Spain and West Africa are all the remnants of a single volcanic eruption which drove these landmasses apart. The lava flows at their peak covered seven million square kilometers and spewed noxious fumes and greenhouse gasses into the atmophere.


180 million years ago, the rift valleys that formed between North America and Africa are slowly opening to form the Atlanic Ocean. 1), the Newark Supergroup, 2) the Glen Canyon Group; 3) the Stormberg Group; and 5) the Lias of England. Also shown is 4, the Lufeng Formation of China.


150 million years ago

Furthermore it turns out that massive volcanism of this kind had been going on from around 275 to 175 million years ago resulting in very low oxygen levels during the entire period. Oxygen as a percentage of the atmosphere plummeted from 21 percent to 11 percent, that's equivalent to the oxygen level atmosphere currently found at the 4,200 metres (14,000 feet).

Recently it's has been argued that the combination of these low oxygen levels and hot greenhouse conditions while making life extremely difficult from most of the world's species also favoured creatures which had extremely large and efficient respiratory systems.

In other words, a world just tailor-made for dinosaurs.

Dinosaurs first appeared during a long period of low oxygen and therefore developed highly efficient breathing mechanisms that allowed them to thrive while many other species became extinct...

Birds and dinosaurs both have holes in their bones. And many of the largest dinosaurs, such as brontosaurus or apatosaurus, seem to have had lungs attached to a series of thin-walled air sacs that may have acted something like bellows to move air through the body.

"The reason the birds developed these systems is that they arose from dinosaurs halfway through the Jurassic Period. They are how the dinosaurs survived," Ward said.

"The literature always said that the reason birds had sacs was so they could breathe when they fly. But I don't know of any brontosaurus that could fly," he added.