Sometimes I'm just THAT Rock & Roll.
Be EXCELLENT to each other.
So, the question then is when is it exactly in the mists of prehistory were the Silurians supposed to have reigned? As you will no doubt guess, the obvious answer is not remotely the correct one.
The Silurian period itself occurred some 443 million years ago, and finds itself somewhat unsuited to the evolution of a race of intelligent Reptiles owing to the fact that we hadn't even got ANY Reptiles at that point, intelligent or otherwise. Hell, life on Earth was only just about working out that land was a thing. So that's a bit out.
Interestingly enough the period was named for the area of Wales in which it was first studied. Obviously Doctor Who has a strong connection to Wales in modern times, but a bit of cursory googling will soon show that calling the original story Doctor Who & the Silurians may lead to some unexpected confusion.
In retrospect changing the title to The Cave Monsters for the novelization comes off a bit racist.
However it was quickly noted that, whilst it undoubtedly makes for a great name, Silurians was historically way off the mark. Thus the show made an effort to rectify this in The Sea Devils, with the Doctor pointing out that they should properly be called Eocenes.
The Eocene period starts 56 million yeas ago. Which you may note is just a little bit different from the 443 million of Silurian period. I guess this goes to show that paleontology really is rocket science to a nuclear physicist. With maths that bad it's no wonder their bloody reactor never worked properly.
Anyway, the other thing you may note is that the Eocene is some time AFTER the extinction of the non-avian Dinosaurs. Whilst on the one hand this does give us some time for intelligent reptiles to have evolved, it doesn't really do much to explain this guy:
Seeing as how the Silurians are normally associated with Dinosaurs to one degree or another it does seem a little odd to claim they come from a period millions of years after they died out. So once again we're left with more questions than answers
And it's only going to get worse.
You see, in their first modern appearance it is explicitly stated on screen the the Silurian race went into hibernation to ride out a predicted catastrophic impact with what would in fact become Earth's moon. So this should give us a pretty accurate estimate right?
The formation of the Earths Moon, quite possibly from some sort of impact occured in the Cryptic Era. Some 4.5 BILLION years ago. Needless to say, back then conditions weren't exactly what you'd call conducive to life. What with the Earth only having recently formed and everything. Sure it's a mysterious and poorly understood part of geologic history, but it's also about as far away from Dinosaurs as it's possible to get. It's about as far away from ANYTHING it's possible to get whilst you still have something to stand on whilst you variously asphyxiate, burn, fry, get crushed, explode and just generally die in a baffling number of hideous traumatic ways simultaneously.
Not being funny, but we all know how City Of Death would have really ended.
So we have 3 possible dates, none of which are remotely close to the age of Dinosaurs. How do we relate this to the Kaijusaurus we saw on screen in Deep Breath?
Well, we don't. The structure of the creatures head is, as far as I can tell basically Tyrannosauroid. Placing it somewhere in the late Cretaceous. The arms however seem a little large, indicating that this is maybe a more primitive type. So maybe even mid Cretaceous. Although to be a meat eater of that scale you'd be going through even Titanosaurs like they were crisps. Supporting this we can also reference the episode Dinosaurs On A Spaceship, featuring as it does a Triceratops and several unspecified generic Dromeosaurids.
So, having considered all the available evidence, both stated and inferred, at the end of the day what can we actually say about the Silurian civilizations place in the history of earth?
I have no idea. AND NEITHER DO THE WRITERS.