The Earth's past from dinosaur footprints--A detective story
D. Leela, Chennai

The story of the Earth started about 4.5 billion years ago, at the same time as the Milky Way galaxy, of which our Sun is also a member. But our Earth did not look as it does today, with a set of continents separated by so many oceans. What did the continents look like, and how did we find this out? That's really a detective story.

First let me tell you about what we know, and then I'll tell you how we found this out.

BOX The beginning of Life on Earth
The Earth was too hot, to begin with. Almost as soon as it cooled and a solid crust formed, early life forms such as microbes began to appear. Fossil evidence for these have been dated to 3.5 billion years ago!

In the beginning there was no oxygen on Earth. Photosynthetic organisms appeared around 3 billion years ago and began enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million years ago. A that time, complex, multicellular life arose and developed over time. This led to what is called the Cambrian Explosion which is dated very precisely to 538.8 million years ago.
This sudden diversification of life forms produced most of the major
phyla and species known today. It is estimated that of 99 percent of all
species that ever lived on Earth, over five billion have gone extinct.
Over 86 percent of these species have not been described. Of these
species, Dinosaurs evolved about 243 million years ago.
END OF BOX

The Earth, Half a billion years ago 
Life on Earth began almost 4 billion (4,000 million) years ago, although early humans appeared only about 2 million years ago. Over all these millions of years, the Earth itself was not static. As the species on the Earth's crust evolved, so also did the crust itself constantly change since its formation. Evidence for the continental shifts therefore can be got from studying fossil evidence.

BOX What is a fossil
Many organisms that lived long ago may have died in such a way that they (or their bones) were preserved unchanged. These remains of the animals give us clues to what these animals were like, what they ate, how they walked (on 2 or 4 feet), whether they could fly (hollow bones), etc. They are called fossils.
END OF BOX

Continental Drift
How much have the continents shifted? The answer is, a lot! And they are still shifting, so the theory is called continental drift. The theory says that Earth's continents move or drift relative to each other over huge (geological) time scales.

BOX Alfred Wegener 
The concept that the continents once formed a contiguous land mass was first put forward by Alfred Wegener, along with the theory of continental drift, in 1915.
END OF BOX

The main reason is that the continents that we see today are a part of the surface, or Earth's crust, and they are actually floating on the mantle of the Earth, like ships on water. In fact, continental crust and oceanic crust both float on the mantle. As the continents drift about, they collide with other land masses and fuse with them. Sometimes they break up. When they fuse together they form large super-continents. One of them was called Gondwanaland.

Gondwanaland
Gondwana was a large landmass, formed about 800-650 million years ago, before the Cambrian explosion of life on Earth. Eventually, Gondwana broke up to form around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia, Zealandia, Arabia, and the Indian Subcontinent (see map). Amazing to think that all these land masses were fused together half a billion years ago!

The process began about 800 to 650 Ma (Million years ago) with the collision of India and Madagascar with East Africa. It ended about 530 Ma with the collision of South America with Africa, and the addition of Australia and Antarctica, respectively. Eventually, Gondwana became the largest piece of continental crust of the Palaeozoic Era, covering an area about one-fifth of the Earth's surface. (The map shows how Gondwana would have looked 420 million years ago, when viewed from the South Pole.) But it would get even bigger.

Pangaea
At about the same time, there was a smaller supercontinent made up of present-day North America, Greenland, North-western Europe and Russia. It was called Euramerica. About 335 million years ago, Gondwanaland fused with Euramerica and Siberia to form the super-continent Pangea. This was during the carboniferous period when coal beds were also being formed. The map shows how Pangaea would have looked like about 200 million years ago. Pangaea is the most recent supercontinent to have existed and the first to be reconstructed by geologists.

Later on, Gondwaland began to separate from northern Pangea during the Triassic period, and broke up into fragments during the Early Jurassic period (around 180 million years ago). First the portion that is both Africa and South America broke off into a separate continent. Madagascar and India separated into different landmasses. The Antarctica separated from South America and Australia. New Zealand also broke away. Most interestingly, Western Gondwana broke up into Africa and South America. The continents now known as Africa and South America started to split around 140 million years ago, researchers said. The south Atlantic Ocean eventually filled the void. See the maps showing the time line of the break-up of Pangaea.

How do we know all this?
Now we come to the question, How do we know all this? There are two different evidences. First of all, regions that were part of Gondwana have similar flora and fauna that are seen even to the present day. This is true even if the regions are now separated by vast oceans. Even more compelling, the distribution of fossils across different continents is one line of evidence pointing to the existence of Pangaea. Let us see how this works.

Tracking dinosaur footprints
A team of paleontologists found matching dinosaur footprints on what are now two different continents, separated by thousands of miles of ocean. The footprints were found in Brazil and in Cameroon, that is, in South America and Africa. If you look at the modern map of these two continents, we can see that the shape of their coastlines are such that they will actually fit together exactly like jigsaw puzzle pieces. (The place where the fossils were found in Brazil is marked with a red spot.) That is, the "shoulder" or "elbow" of South America, where the fossils were found in Brazil, fits into the "arm-hole" of Africa where the fossils were found in Cameroon. And this was exactly the case 200 million years ago, in the Cretaceous Period, when the two land masses had not yet split apart.

Scientists (Paleontologists) realised that more than 260 footprints that they found in mud and silt in Brazil were similar to those found in Cameroon, more than 6,000 km away! They were similar in shape, as you can see from the photos. They also turned out to be similar in age, and even the geological features of the two places were similar.

How did this happen?
Dinosaurs made the tracks 120 million years ago on a single supercontinent, Gondwana, after it had broken off from the larger landmass of Pangea. So they left tracks on both sides of the rift that would ultimately separate the single landmass into two different continents. This also showed that the two continents were actually a single landmass 120 million years ago.

Basins formed as the continents pulled apart; rivers flowed and lakes formed in those basins. The basins where the footprints were discovered can be found on both sides of the split.

What we know about the dinosaurs 
Most of the footprints were made by three-toed theropods, a group of carnivorous dinosaurs. There were also prints left behind by sauropods or ornithischians. Plants fed the herbivores and supported a food chain. Muddy sediments left by the rivers and lakes contain dinosaur footprints, including those of meat-eaters. This proved that these river valleys allowed such life forms to travel across the continents 120 million years ago.

This is just one example of how we know that the Earth once had super-continents and how we can tell at what time they broke up. Today, using the evidence of flowers, trees, living animals and fossils, we have the picture of ancient Earth, as it formed and evolved over the ages. A true detective story.

Sources: Wikipedia, CBS News, IFL Science News