Professor Potter's explanation for Zanthodon runs like this. Approximately 150 million years ago, an asteroid or incredibly large meteor,struck the earth. How big it is, we have no idea. Butit should have produced a terrestrial crater hundreds of miles in diameter. This, of course, would be an extinction level event.
On Earth, we've discovered remnants of craters a hundred miles in diameterwhich have been associated with major extinction events. Ifanything, the Zanthodon crater would have been that big, or perhaps ordersof magnitude bigger. At the very least, that should have been itfor the dinosaurs, and it might well have been it for anything bigger thanbacteria.
Carter's biggest inspiration was Edgar Rice Burroughs. The Callisto series was a take off on Barsoom through and through. The Zanthodon series was a faithful reproduction of Pellucidar. Burroughs also appears as a primary or secondary influence in his Martianbooks, his Green Star series and his Thongor novels.