The Sky People

Marc Vitrac was born in Louisiana in the early 1960's, about the time the first interplanetary probes delivered the news that Mars and Venus were teeming with life—even human life. At that point, the "Space Race" became the central preoccupation of the great powers of the world.
Now, in 1988, Marc has been assigned to Jamestown, the US-Commonwealth base on Venus, near the great Venusian city of Kartahown. Set in a countryside swarming with sabertooths and dinosaurs, Jamestown is home to a small band of American and allied scientist-adventurers.
But there are flies in this ointment—and not only the Venusian dragonflies, with their yard-wide wings. The biologists studying Venus's life are puzzled by the way it not only resembles that on Earth, but is virtually identical to it. The EastBloc has its own base at Cosmograd, in the highlands to the south, and relations are frosty. And attractive young geologist Cynthia Whitlock seems impervious to Marc's Cajun charm.
Meanwhile, at the western end of the continent, Teesa of the Cloud Mountain People leads her tribe in a conflict with the Neanderthal-like beastmen who have seized her folk's sacred caves. Then an EastBloc shuttle crashes nearby, and the beastmen acquire new knowledge... and AK47's.
Jamestown sends its long-range blimp to rescue the downed EastBloc cosmonauts, little suspecting that the answer to the jungle planet's mysteries may lie there, among tribal conflicts and traces of a power that made Earth's vaunted science seem as primitive as the tribesfolk's blowguns. As if that weren't enough, there's an enemy agent on board the airship...
Extravagant and effervescent, The Sky People is alternate-history SF adventure at its best.