Wednesday, June 17, 2015

Understanding Dinosaurs - Part 8: Prehistoric Time

I find it hard to tell time without a watch. Time seems even more complicated before humans even existed. We have come pretty far in understanding what dinosaurs were, where they came from, and we will get to where most of them went. But I think to best grasp that information and to really place what we have learned so far, we should take a look at time. And there has been a lot of it.

The earth formed out of the accretiondisk of the sun, around 4.5 billion years ago. Taking its sweet time to cool (and get bombarded with space rocks), it wouldn't be for half a billion years later that the earth would be right enough for life to exist. Now, we are talking dinosaurs here, so we have to fast forward from the 4 billion year old Archean era to about 250 million years ago. We call this the Mesozoic era and it is the time of the dinosaurs. Before it was the Paleozoic era, during which most complex life evolved. But the Mesozoic saw the emergence of that new hip socket adaptation that made up the dinosaurs.

We can divide the Mesozoic into three distinct periods. These are pretty well known names to most dino-fans, they are the Triassic, the Jurassic, and the Cretaceous.

Beginning about 250 million years ago and lasting about 50 million years, the Triassic world was much different than our own. This period began with the great Permian extinction. This event, sometimes called the Great Dying, wiped out 96% of all marine life and 70% of all terrestrial species. It would take the earth another 10 million years to recover. But as it did, new forms of life evolved from the limited diversity of the survivors.
Among those new lifeforms were the archosaurs, a prehistoric lizard group that would eventually branch off into the dinosaurs. The second major group to come out of this event were the therapsids, who would develop the rodent-like ancestors of mammals.
These creatures lived on hot earth, with a heavy CO2 atmosphere. The land of earth at the time was broken up into two massive supercontinents known as Laurasia (in the north) and Gondwana (in the south), which together formed Pangaea. The air was hot and dry, but would begin to humidify over the course of the period as these two major landmasses began to break up and move apart.
The Triassic period ended like it began, in yet another extinction event known as the triassic-jurassic extinction. It was this event that allowed more room for dinosaur evolution and gave them a foothold into becoming the dominant creatures on earth.
The most well known dinosaur from this period would probably be Coelophysis, a small therapod dinosaur who has appeared in countless works of literature since its discovery over a hundred years ago.

The time period (at least in name) that we are all most familiar with (thanks Mr. Crichton). After the triassic-jurassic extinction event around 200 million years ago, dinosaurs and other ancient reptiles really began to flourish and spread across the earth. This time period has also come to be known as The Age of Reptiles (an amazingcomic book series too BTW).
Named for the Jura Mountains in Europe where this layer of rock was first identified, the Jurassic period contained some of the most distinctive and iconic dinosaur forms we have come to know. As well as dinosaurs, it saw the return of ancient lizards to the sea, giving to the rise of the aquatic reptiles.
The earth, still hotter than today, was now covered in dense plant life. Jungles like those of the previous era, spread across the world. Pangaea broke into the more distinctive lands of Laurasia and Gondwana, while what would become the Gulf of Mexico began to tear into what would become North America.
The Jurassic gave us many of the dinosaurs we know so well now. Brachiosaurs, stegosaurs, allosaurs, and diplodocoids were common. We also had the non-dinosaurs such as plesiosaurs in the ocean and many of the pterosaurs that ruled the air. Even the earliest birds began to show up in Archaeopteryx and others like it.

145 million years ago saw the final period of the Mesozoic era, and the end of most dinosaurs. This period gets its name from the Latin for 'chalk', making up much of the rock's composition in the geological layer.
By the Cretaceous, Pangaea was completely broken up into the present day continents, though none stood where they do now. North America was split in half by a massive inland sea making it a hot bed of both marine fossils as well as the remains of the terrestrial animals that kept close to the coast.
The Cretaceous gave us even more iconic dinosaur species to love than the Jurassic period (I guess Cretaceous Park just didn't sound as good). It gave us Triceratops, Tyrannosaurus, Parasaurolophus, and many others.
Birds became numerous and ranged in diversity as much as other dinosaurs. Even the plants were becoming more like the shapes we know today, as flowers began to develop. But unlike the birds, and the plants, and the crocodilids, the dinosaurs would not survive the end of the Cretaceous period.
It ended with a bang...possibly, around 65 million years ago. And much like the Mesozoic began, it ended in an enormous extinction event we call the Cretaceous-Paleogene Extinction event(often abbreviated at the K-T, or K-PG boundary). We will look more at this event in the next segment.

The Mesozoic gave us the rise and fall of the dinosaurs (but as we discussed before, we still got to keep the birds). By understanding these time periods, we can look at when particular species evolved and further help us understand where they fit on the dinosaur family tree.

Next Time: Part 9 - Dinosaur Extinction

No comments:

Post a Comment