418 million years ago

Silurian Period

In the shallow seas of the Silurian a a small fish the size of a human's hand swims by a rock.

This was Celphalaspis. She's a peaceful grazer, who sucks up algae through her jawless mouth. But she's also developed a tough protective head and thick scales.

Then the fish swims off just before a pair of claws snatches her. The culprit is a large scorpion.

Their ancestor's arthropod enemies have also been evolving and their ready for round 2.

Place: South Wales, UK

Oxygen Content: 30% Below Today

Hazards: Giant Scorpions

Twilight and the others been brought out of time warp and entered the Silurian seas. And like when they were at the Cambrian they have breathable bubbles around their heads.

Right now I really wish I know what is going. Twilight said.

I'm sure I'll figure it out eventually. Spike said.

Come on, we better that a look around. Rarity said.

A hundred million years have passed and the fight for survival has filled the Silurian seas with variety. Some creatures here would be recognisable today. As they swam around they saw sponges as they filter food alongside sea urchins.

Well, at less their's something that looks familiar. Fluttershy said.

As well as the sponges and urchins and saw a familiar creature from the Ordovician an Orthocone. And the orthocone is a distant relative of squid and cuttlefish but their as long as a truck.

As they continue to swim they ran into some Celphalaspis and they appear to be swimming from something.

Where are they going in such a hurry? Rainbow said confused.

I'm not sure. Applejack said.

This world is terrorised by a new improved generation of armored arthropods. Then they saw what was scaring the fish. As they meet Brontoscorpio.

Whoa, this is one big scorpion. pinkie said stunned of his size.

Let's just let him be. Twilight said as they leave.

After a short swim they saw more Brontoscorpio.

His kind may look like modern scorpions, but their 3 feet long and have gills. The arthropods have up their game, this monsters have a improve armory to Anomalocaris. They have a stinger the size of a light bulb. Scorpions are the top predators in the Silurian seas, and prey is plentiful is their seabed hunting ground. Then one of the scorpions turned having it's attention on something, as Brontoscorpio zeros in on his next meal.

He has spotted a lone Celphalaspis as she filters in some algae. But Celphalaspis has evolved an early warning system. Special sensors on her skin detect the tiniest vibrations in the water. We've inherited similar senses as well as them. They much us and them sensitive to touch.

Then the Celphalaspis swims off with the scorpion chasing after her. But every once in a while she stops for a second. Because with her defensive head gear, Celphalaspis can't swim fast for long. She must rest frequently. Soon she'll tire completely. The gang watched as each time the scorpion got close the fish swims off making them more nervous each time. As the fish swims off she strangely made a loop around swimming in the other direction.

Celphalaspis suddenly changes her path. She's picking up bad vibrations. Something Brontoscorpio can't detect.

As the Brontoscorpio continues after the fish part of the sand in front of him raised up and then, a claw snatches the scorpion from below and a large arthropod came out the sand and begins to tear the scorpion apart.

Pterygotus is the titan of sea scorpions. The biggest arthropod of all time. More than 12 feet long she's the size of a crocodile. And probably has the power to match one.

And I thought that was a big scorpion! Rainbow said.

Aw, she has babies. Fluttershy said.

As the colossal scorpion continues to rip the smaller scorpion apart, her young that were nearby swam over to feed. And the mother has turned the tables on Brontoscorpio. And he'll make a good meal for her young.

The group of watches the family of eurypterids feed for a little before swimming off.

Then they saw a huge school of Celphalaspis swimming in the same direction.

Where are they going? Rainbow Dash asked.

Let's follow them and find out. Fluttershy said.

And so the group begins to follow the fish.

In such dangerous seas there's nowhere to hide. When breeding season comes the Celphalaspis congregate to head for the one place they might escape the scorpion's grasp. Freshwater, Inland.

Land at this time is like an alien planet. It's a barren expanse of roasting rock hotter than the Sahara. The air would be toxic to us or the gang. It has much less Oxygen and 300 times more Carbon Dioxide than today.

But some forms of life have gained a foothold in this furnace. The first pioneering plants. Cooksonia has a unique survival strategy. It's the first plant to send shoots upwards, trapping extra light to help it grow. This basic design will eventually lead to our and their tallest forests.

Meanwhile the Celphalaspis convey and the group of 8 ploughs upriver, away from the sea. And Twilight and Fluttershy theorized they're making the marathon journey back to the spawning grounds where they hatched.

Incredibly their fish ancestors already use memory. They use familiar landmarks to navigate. Their toughed heads protect a vital weapon. One of the first complex brains. It's much more developed than their scorpion rivals who have no memory at all.

It's thanks to these primitive fish that we can think and solve problems today. As well as Twilight and her friends.

But the fish have underestimated their enemy.

Some distance away a gang of Brontoscorpio crawls out of the water in walks on to the beach.

It is the arthropods and not their and our ancestors who have taken the first momentous steps out of the sea on to dry land.

Brontoscorpio has a huge advantage.

As well as gills he has simple lungs made up of hundreds of thin layers of tissue. He can't breath in and out like we do and the pony gang. He just absorbs the Oxygen into his blood.

Equipped to maximise the little Oxygen available, and with their armor to protect them from the sun, the scorpions patrol the shoreline, scavenging on whatever the sea washes up next.

And the army of Brontoscorpio begin to search to any food.

Back with Twilight and the others Finally they and the fish approach their destination.

They've navigated their way back to the spawning pool where their lives began. Weak from their long journey now they have to cross a ridge of rock to make it from the river, to the pool.

Luckily the ridge between them is smooth and Twilight and the others climbed out of the water to get a good look and their breathable bubbles are still on their heads.

And they watch as the first fish slides across the ridge as they make it through and start to lay their eggs.

But soon the exhausted Celphalaspis have company when soon as 4 Brontoscorpio came crawling by as the first 2 snatch one of the fish and began to feed.

Passing scorpions have stumbled on this bounty.

And soon a 3rd scorpion snatches his own meal and strikes with his light bulb sized stinger. Twilight and the others watch sadden even though it's the way of life.

But the fish have numbers on their side. The clever Celphalaspis have navigated their way, while Brontoscorpio are only here by luck. They're soon stuffed to the gills while the fish keep jumping.

Sometime later the group have notice one brontoscorpio not eating. And strangely his bending his body.

One scorpion is still hungry, but he can't feed. He's become a prisoner in his own skin.

Twilight and the others went for a closer look and saw that he is molting.

His rigid skeleton is now a handicap. It can't grow with his body. He needs to shed his hard skin and then grow another, expanding while the new one is still soft. For such a large creature, this is a long process.

And the group watches as the scorpion crawls out of his skin.

The next morning, there's no sign of life in the spawning pool.

And sadly, the scorpion has missed his chance.

The Brontoscorpio begins the long trek home.

Their ancestors have survived. They've laid their eggs and are returning to the sea. As the the group of 8 swims alongside.

Brain has triumphed over brawn, and soon they won't be such soft targets.

Evolution starts to give them weapons to fight back.

Then the group was again sucked into a time warp and to be taken to their next destination in prehistory.

Over millions of years the fish's gills adapt to form the first jaw with the very first teeth. Now they're equipped to go on the attack. Some develop tougher bones and muscles in their fins and shoulders, which become the first limbs. This is where our arms and legs began. As well as Twilight's and her friends. With this four-limbed design their ancestors finally haul themselves out of water on to land.

But that's another chapter.

Next up they'll move forward 58 million years to swim in the 5th deadliest sea of all time. And to meet a armored giant.