Episode One: Coelurosauravus (aka Rex Rex)

A flying reptile from the Permian period about 250 million years ago and is unusually active and intelligent for a cold blood. He appears in the first programme and immediately bonds with Abby, so that when they try to return him to his time he sneaks back to be with her. For the rest of the series she keeps him - for the most part secretly - in her flat with the central heating on full blast to suit his exotic tastes (there were no ice caps in the Permian).

Rex is based on a smaller reptile (about half Rex's size) called Coelurosaravus jaeckeil found in Germany. His wings were formed by extensions of his ribs but he could not fly only glide. He is, however, the first known vertebrate flyer because he predates the pterosaurs.

Episode Two: Giant Spiders

In an abandoned tunnel off the Underground a hole opens to the Carboniferous swamps 300 million years ago. First through are hoarders of giant spiders. They are boosted to these sizes by the oxygen rich atmosphere of the Carboniferous which also fills the tunnels. These are very primitive relatives of modern spiders - they have no web or venom but they do have two enormous pincers like modern day camel spiders. They are responsible for wounding a number of people but their attacks are not lethal. Despite their size they shun light and so once Cutter brings down torches they are easily rounded up.

These creatures are based on rather fragmentary fossils of 'large spiders' found in the coal swamps from the Carboniferous Era. For a long time, there was a fossil called Megarachnid or 'giant spider' which scientists thought was a 1-meter-wide spider, but this has just recently been reclassified as a type of sea scorpion.

Episode Three: Mosasaur

When an anomaly opens up under water it turns out to be connected to the worst possible time in Earth's marine history, the late Cretaceous Period 70 million years ago. Then the seas were ruled by the mosasaurs - long snake-like sea serpents some almost 20 metres long. In this programme the anomaly moves and in each body of water a different mosasaur comes through. One is about seven metres long, the other almost ten. These predators are like 'dinojaws' ruthless in their pursuit of prey in fresh water or salt. Cutter is attacked by a small one when he swims through to the Cretaceous sea but saved when an enormous twenty meter monster eats his attacker.

These creatures are based on a real group of animals that evolved from the first lizards and took to the sea during the time of dinosaurs. They were the killer whales of their age and fed off anything, including other mosasaurs.

Episode Four: Dodo

In a complete change to what they were expecting in this programme, instead of some ghastly carnivore, a flock of plump dodos appear. Confused and agitated, they take a time to round up, but are not in the least aggressive. By nature these plump, flightless birds are placid and curious. Unfortunately, one escapes and is adopted by a pair of students, who suspect he has been created in some Government laboratory. Sadly the dodo dies, but before he goes he turns uncharacteristically violent and bites one of the students.

Our portrayal of these creatures is very close to the truth as described by sailors in the 18th Century who encountered these relatives of pigeons on the island of Mauritius. It was their benign nature that helped hurry their extinction as they were so easy to kill.

Episode Five: Pteranodon

After a savage killing on a golf course, Cutter and the team discover a huge pterosaur in the skies above. Because of its head crest and lack of tail Cutter identifies it as a Pteranodon. Although it then appears to swoop down and attack Connor it is in fact after the small reptile Rex. Despite his size with a wing span of over 9m this aerial giant has no teeth and feeds on small animals like fish and lizards. The team eventually bring the creature down with a tranquilliser gun and confirm he is harmless by looking at his poo. At the end of the programme they manage to release this gentle giant back to his own time.

Based on Pteranodon ingens that lived about 100 million years ago, this pterosaur was made famous after featuring in numerous Hollywood movies. At 9 metres, he is not the biggest of the pterosaurs, creatures like Quetzocoatlus could reach 13m.

Episode Six: Future Predator

The final episode features a creature with no name because it comes from the future, following the logic that if holes can open to the past so holes in the future can open up. The animal that comes through is a predator, drawn through while hunting Helen Cutter. Genetic tests reveal it is a type of wingless bat with lightning-fast reactions and the ability to stalk its prey using high-frequency sonar. Cutter manages to kill the predator when it corners him in a greenhouse by confusing its sonar with smashed glass. However, it turns out there was a mated pair and he is saved from the jaws of the female by the prehistoric Gorgonopsid from Programme One that kills her and her young in the climax to the series.

This creature is obviously not based on any real animal but it is assumed that as long as the Earth's climate remains as seasonal as it is then mammals will continue to be the dominant group of animals. If that is so than rats and bats are by far the most common groups of mammals (accounting for three quarters of all mammal species) and any large predator could well evolve from them.