Interesting BBC article on the K-T boundary event 66 million years ago that finished off the dinosaurs, useful background for a visit to the aftermath, perhaps during the 'rain of fire' where ejecta effectively set the Earth's atmosphere aflame. Some useful details (like the event happening in the northern hemisphere's early winter) that could be included.
Of course in the Whoniverse the time-jumping warp freighter captained by Briggs gets the blame though it's possible that there were multiple impacts. While the Chicxulub Crater is generally taken to be the primary impact site there's speculation that the Shiva crater off the west coast of India is the result of an even larger impact. That freighter must have been carrying a *lot* of anti-matter, about five thousand tonnes, or been moving at relativistic velocity to produce the effect modelled. And then there's the iridium deposition to be explained, what was Briggs's cargo?
Maybe the Silurians weren't so environmentally friendly after all...
Hmmm. Internal warfare perhaps? Or conflict with other species? You could do something like the history of the Cthulhu Mythos with various races (Elder Things, Flying Polyps, Mi-Go, Yithians, Serpent People et cetera) warring over the Earth long before humanity evolved.
Time, time,time, see what’s become of me. While I looked around for my possibilities... My AITAS files.