WMG / Prehistoric Park

The park is responsible for the events of Primeval
Obviously something went wrong with the time portal device, causing the anomalies. This would explain why they look similar and why most of the anomalies lead to prehistoric times.
  • Objection: Primeval features a version of Nigel Marven who knows nothing of portals and is implied to have been eaten in the episode he appeared in. One of the retcons in Primeval causing this is more likely.

They did end up messing with the timeline

Hence why Ornithomimus is a filter feeder without feathers and all the other inaccuracies.

The whole thing started because of "Sea Monsters"
Remember the Pteranodon Nigel tamed in that special? After the mosasaur attack, he brought it into the future with him and set up a zoo enclosure for it. He was so inspired from this, that he decided to do this with more prehistoric creatures.
  • To develop: first went the Walking with Dinosaurs series, in which they used the proto time machine to send cameras in the past, but no human. Then went Chased by Dinosaurs, in which they tested the time machine on a human being, and Nigel Marven was the most interested and experimented zoologist adventurer they found. Then went Sea Monsters, and then Prehistoric Park in the chronology you say. Walking with Cavemen is the crew of Walking with… sending somebody else in the past.
    • The Ballad of Big Al could have been an attempt to pinpoint more specific dates in the past, i.e. when Big Al was alive. And as with the above WMG, Primeval followed all this.