October 8, 2016:

by Sandhya Ramesh

Until now: After Earth was left impoverished by the Permian-Triassic extinction event 252 mya, life took a very long time to recover. In fact, it was 10 million years before life could reawaken, and some thirty million more to diversify and flourish. This is a long period of time even by evolutionary and geological standards. With the onset of the Triassic period, life then slowly bloomed into some of the groups we are familiar with today: true mammals, true trees and the exciting new type of swimming and walking creatures called the dinosaurs.

The Triassic period lasted from 252 mya to 200 mya. It is also the only period to begin and end with a major mass extinction as the continent Pangea started to break up and drift apart, causing cataclysmic upheavals of Earth’s innards.

Today, the surviving descendants of the gigantic, ferocious dinosaurs are crocodiles and birds. But in the early Triassic, dinosaurs started out small and meek. The ones that roamed Earth then, called the archosaurs, were ultimately to be the ancestors of all giant dinosaurs, crocodiles and birds. While the lystrosaurus pretty much dominated the first half of the Triassic, the herbivore group rhynchosaur saw a meteoric rise in numbers, and then a drastic fall. Indeed, they were probably the second most abundant type of life on Earth at the time. And sadly they all died out at the end of Triassic.

Several of the archosaurs were reptilian, both by looks and nature. Just like the modern-day crocodiles, they lived on land, spent a considerable amount of time swimming in waters, had long snouts and hunted fiercely. They soon evolved to several sizes, ranging right from about the size of a pig to that of a horse. Some looked like heavily armoured anteaters while others like scaly dogs, The phytosaur, the most common type of archosaur that lived during the Triassic, resembled the modern crocodile very closely. Postosuchus, a kind of archosaur, looked like it had T-rex’s head and a crocodile’s body. The plateosaur was striped like a tiger, had a large, heavy lower body like the T-rex and a thin stretched out upper body – like it was squeezed too much. The unaysaurus had either a giraffe-like mottled pattern or a zebra-like stripes (depending on who you ask) and were as tall as our largest dogs.

Also, the ancestors of modern mammals appeared on Earth at this time. They were called the cynodonts. They had typical mammalian characteristics – of differentiated teeth and bulged skulls that accommodated large brains. They were small in size, having had to wait a few million years, before the dinosaurs died out, to proliferate. Other animal life included the now-extinct primitive amphibian class that were the ancestors of modern day salamanders and frogs. The first turtles also made their appearance at this time.

Proto-dinosaurs, the descendants of archosaurs that were one step away from being full dinosaurs, appeared in the form of theropods. These dinosaurs were quite small, about the size of a cat or a dog, and didn’t evolve much in size during the Triassic. They came in many varieties, especially in terms of dietary choices. There were theropods that were herbivorous, carnivorous, omnivorous, piscivorous and even insectivorous. They also had several other characteristics that are familiar to us today, like feathers. Some had them only when they were young or newly hatched. Several groups of these theropods could also swim and hunt underwater.

In the oceans, new kinds of corals appeared, in turn building new kinds of reefs. A single line of molluscs – called shelled organisms – had survived from the Permian era and slowly diversified into the recognisable, whorl-shelled ammonites. The ichthyosaurs and plesiosaurs, the uniquely identifiable marine dinosaurs, grew in size and numbers. Conodonts, large eel-like swimmers, were all over the place.

And then Pangea started to breakup. The events that followed were nothing surprising (anymore). There was an immense release of carbon dioxide into the atmosphere. There were mega flood-basalt events, global warming, toxins in the air, climate change, a drop in sea levels, oceanic acidification and everything else we’ve seen before caused by supercontinents breaking apart. The extinction event that followed is called the Triassic-Jurassic extinction event, and is the fourth of the Big Five.

Where we are. Credit: Ita Mehrotra

And just like the symptoms, the effects of a major mass extinction are also familiar to us: devastation. Several animal species that were transitionary between amphibians and mammals, such as the placerias that looked like a mix between a walrus, hippopotamus and Zoidberg, were lost forever. Marine animals suddenly and quickly began to disappear. The eel-like conodonts were completely eliminated. A large portion of plant life was also lost, and the exposed land then killed several archosaurs that had just started to grow in size. Over 50% of all life on Earth became extinct. And all of these occurred in less than 10,000 years.

Then, the age of dinosaurs dawned.

By the onset of the Jurassic period some 201 mya, Pangea had separated into two landmasses: Laurasia in the northern hemisphere and Gondwana in the southern. This splitting up of a giant landmass led to the opening up of new oceans, which in turn increased water circulation around the two continents. As a result, several of the arid desert regions that formed in the Triassic turned into thick, green forests bathed in water. In this time, both continents remained relatively close to the equator. There were no landmasses near the poles and so no ice caps anywhere on the planet.

In the freshly swirling oceans, aquatic dinosaurs flourished. The ichthyosaurs and plesiosaurs came out of the mass extinction intact and were diversifying rapidly. Different types of reef-building organisms multiplied – as did the reefs they had built. There was a spike in the number of shelled marine animals. Plankton diversified intensely, forming new groups and species.

On land, the dinosaurs dominated. Among them, the herbivores emerged at the top. The diplodocus and brachiosaurus – the one Ellie and Alan first see, then Tim and Lex touch, in Jurassic Park (1993) – were particularly common. There were fewer carnivores; the most populous of these were the megalosaurus and allosaurus, both cousins of the T-rex. Perhaps the most recognisable of the animals was the stegosaurus, which would have been hunted by the former two.

In the next instalment: the next time period, small extinction events, diversification of dinosaurs, a surprising splash of colour – everything leading up to a giant space rock.

Sandhya Ramesh is a science writer focusing on astronomy and earth science.