Fans have pointed out that the dinosaur in ‘Deep Breath’ is too big. It towers over Parliament, and its head hovers around the face of Big Ben. In real life, Big Ben is 96 meters in height, while fossil records show that Tyrannosaurus rex topped out at roughly 12m nose to tail. Of course, in a show like Doctor Who, the opportunity to have an immense dinosaur rampaging through London may have been too good to pass up. The writers even attempt to rationalize, giving Jenny and Vastra an argument about how big the things actually were.
A fan somewhere on a page I no longer recall posted a comment that the argument referred to the Big Finish title ‘Bloodtide,’ which features the Silurians and genetic manipulation. Intrigued, I purchased the story. Today I finally listened to it.
The commenter lies. Dinosaurs are not the subject of genetic manipulation in the story.
It’s possible the commenter meant that given their ability to perform genetic manipulations, as demonstrated in ‘Bloodtide,’ the Silurians could have been responsible for an extra-large dinosaur. Or, since Vastra claims they were really that big in her time, perhaps the Silurians were responsible for shrinking them. (Though there should still be fossil records of the behemoth.) Either way, the Big Finish story does not address the question directly. ‘Deep Breath’ visual effects people have no in-universe excuse.
Not that they need one. This is Doctor Who.