ccording to the letter column of Creatures on the Loose#27, "Thongor's adventures... take place almost half a million years ago, eons before Conan or Kull, when Lemuria was the only continent on which modern man dwelt. The rest of the world - apparently at any rate - was still in the Pleistocene period and inhabited by Cro-Magnon and Neanderthal man." This means Thongor's world existed a little under 500,000 years B.C. They gave this as the reason for why there would be no crossovers with Conan or King Kull, but then go on to make a big deal over that "apparently" they slipped in there. "Let us add considerable emphasis to that 'apparently', however. Without giving away too much of what we have in mind... we think it safe to say that in other similarly isolated parts of the planet, a civilization or two may have sprung up at that time." Since (as far as I am aware) no further Thongor comics were made after the end of the Dragon King saga in CotL#29, one can only speculate what those plans for the future were. Any ideas, people? What other Pre-Cataclysmic civilizations were active around that time period in Marvel Comics? The Savage Land?
http://www.marvunapp.com/Appendix/starseed.htm reveals that (Marvel Premiere#28 [fb]) - During a primitive era of man, around the age of the Neanderthals, a group of more intelligent, peaceful people built their own society on an unnamed mountain. This society developed into an age comparable to that of the Roman empire, many hundreds of thousands of years before the rest of the world.
Also, I seem to recall the Dark Rider storyline in Marvel Team-Up revealed the presence of an Age of Wizards. Olshevsky in his Marvel Team-Up Index stated that he was not sure where the Age of Wizards fell with relation to the visit of the Celestials, but he presumed that it happened thousands of years before the time of King Kull.---Per Degaton
The dating employed in the CotL#27 lettercol makes it problematic to place Thongor's adventures within the greater framework of established Marvel history. The mentioning of Hyperborea as existent before Lemuria also further muddies the waters. If the Hyborian Era runs from roughly 17,000 B.C. to the beginning of recorded history, and the Pre-Cataclysmic Era predates that from about 18,000 to 100,000 B.C. this would mean that Thongor's Lemurian Era predates the vast bulk of events on Marvel's prehistoric earth, including Varnae's transformation into a vampire, the sinking of Atlantis, the destruction of the Deviant's empire by the Celestials, the rise and fall of the Hyborian Era, and the Stone and Bronze Ages. Moreover, it mentions a previous Hyperborean Age (which might or might not be connected to Clark Ashton Smith's Hyperborean Era which emerged between Ice Ages). It would likely be safest to just say Thongor's adventures occurred outside the normal Marvel Universe (Earth-616), but with the true love of the obscurantist fanboy I can't help but want to fit it in somewhere. It could be that while the rest of the world was populated by Cro-Mags and Neanderthals, the progenitors of the Lemurians, Phondrath and Evalla, represented highly evolved creations of the Nineteen Gods' that were later absorbed into mainline humanity which the Celestials had allowed to evolve normally.
There is evidence that the Clark Ashton Smith version of Hyperborea could be a part of the Marvel Universe. In Doctor Strange III#8, Doctor Strange was shown as having The Book of Eibon, a magical book from Clark Ashton Smith's Hyperborea stories.