That will be West of Eden by Harry Harrison, first published in 1984. It's an alternate history in which the dominant species on Earth is indeed reptilian.

The intelligent reptiloid species is called the Yilanè, and represents the dominant life form on most of the planet. However, during the evolutionary process, the species became non-viable on the North American continent and Caribbean area, leaving them free of Yilanè for millions of years and opening an ecological niche for a top predator. A human-like species, the Tanu, evolved to fill the niche in North America, but are only found on that continent. Unlike humans, which evolved from African primates, the Tanu have evolved from a lineage of New World monkey. By the time the novel begins, the humanoids have reached a late stone age level of technology and culture, with a number of societies having developed farming skills.

That will be West of Eden by Harry Harrison, first published in 1984. It's an alternate history in which the dominant species on Earth is indeed reptilian.

The intelligent reptiloid species is called the Yilanè, and represents the dominant life form on most of the planet. However, during the evolutionary process, the species became non-viable on the North American continent and Caribbean area, leaving them free of Yilanè for millions of years and opening an ecological niche for a top predator. A human-like species, the Tanu, evolved to fill the niche in North America, but are only found on that continent. Unlike humans, which evolved from African primates, the Tanu have evolved from a lineage of New World monkey. By the time the novel begins, the humanoids have reached a late stone age level of technology and culture, with a number of societies having developed farming skills.