Because Lovecraft was an early believer in Wegener's continental drift theory, and this is only 252-290 MYA, the correlation works surprisingly well. I can really get a feel for how Pangean geopolitics work in the Mythos world.
The land portion of the empire of the Elder Things hangs together even better than it would today, with easy reinforcement routes between all points, and the logical central part of their realm is, indeed, Antarctica. The one addition is India, which given its position and Elder Thing aquatic capabilities, has to belong to their realm. Naval dominance of the Paleo-Tethys Ocean, the mostly-sheltered "Mediterranean" sea that all lands border, is contested between the Elder Things and the forces of R'lyeh. The Elder Things dominate the southern and eastern portions of the great Panthalassic Ocean, the super-Pacific of this bygone era. Their northwestern border with Valusia runs along the south edge of a desert which the reptilian Valusians easily dominate, then across Africa in a doubtless-unstable frontier. I assume at least some mountain range where Antarctica and Australia join, to be a natural border with the Yith. The Elder Things basically control the southwestern part of Pangea.
The Great Race of Yith find themselves with a fairly small realm, which no doubt is magnified in importance in their self-promoting histories. They can make no headway against Antarctica, the Elder Thing homeland, and to the north they may control the small Southeast Asian continent, but since this is actually part of the remnants of the sundered former continent of R'lyeh (known to our less poetic scientists as Rodinia), I suspect that they found rough going there. The Great Race basically controls part of the southeastern part of Pangea.
R'lyeh itself has been sadly-reduced from its great days a couple of hundred million years ago. The Cthulhi are powerful underwater, but they are a slow-breeding species and by now leave most of the fighting to the Deep Ones, which they created back in the Devonian on an early amphibian model. The Deep Ones are cheap, fast-breeding, and extremely effective in the water while having some land capabilities, allowing them to contest the South Chinese-Southeast Asian continent and easily control the linked lands of Siberia, Khazakstania and North China. The city of R'lyeh, where Great Cthulhu waits, would be somewhere in the north tropical Tethys at this point. R'lyeh basically controls the northeastern part of Pangea.
Those are the three most advanced civilizations of Permian Earth. There are also two less-advanced but geographically-great powers, each of them controlled by a different kind of reptile folk, who are probably at least as closely related as are, say, the great apes and the neotropical monkeys.
Valusia, in central-western Pangea, has an uneasy border with the Elder Things, being most secure in the western desert which the Valusians find balmy but the Elder Things detest. It does not even try to control anything beyond its own coastal waters, demurring to the Elder Things in the Pan-Thalassic and to both the Elder Things and the R'lyehians in the Paleo-Tethys. Its northern border, with Irem, is secured by the Central Pangean Mountains, a mighty range then at least as high as the Himalayans.
Irem, in northwestern Pangea, enjoys the benefits of a firm frontier with Valusia, guarded by the Central Pangean Mountains, and a homeland centered on a comfortably dry desert, which due to its climate is safe from invasion by the two great Permian naval Powers. The Elder Things and R'lyehians here share control of the Pan-Thalassic, but the R'lyehians clearly dominate the northern Paleo-Tethys. Its unstable border is with R'lyeh, which repeatedly colonizes the eastern coast, only to lose interest in the military effort needed to maintain long-term land colonies.
Diplomacy can be fluid. The central fact is the deep hatred between the Elder Things of Antarctica and the Cthulhi of R'lyeh. They never ally; though they haven't fought an apocalyptic war since the one which caused the Ordovician Extinction Event, they consider their current state of affairs a "truce" rather than a genuine peace. The Yithians, both literally and politically caught between these two Permian Powers, shift alignment as required by the dictates of power politics, and in fact their presence is probably why no decisive war is possible: the Yithians don't want to see either of those two Powers dominate the Earth.
The Valusians are technologically less advanced than either of the three extraterrestrial Powers, but make up for it by being sneaky masters of chemical and psychological warfare. The Iremites are nowhere near as sneaky, but have the good fortune to have reasonably safe borders, given that neither the Cthulhi nor their Deep One servants really want to venture far onto the land. Both the Elder Things and the Cthulhi look down upon the two "native" races, but neither of them wants to launch a war of extermination against the natives, for fear that such an offensive would unite all the other Powers against the aggressor.
And so things remain, seemingly stable, for that last 40-million-year long dry summer that ends the Paleozoic, until someone's scheme of subversion amongst the shoggoths works better than anyone could have dreamed, and everything comes crashing down, and the curtain closes on the active and open empires of Those From Beyond this planet.
So, what do you think?