Favreau and screenwriter Mark Fergus wanted to make their script faithful to Burroughs' novels, retaining John Carter's links to the American Civil War and ensuring that the Barsoomian Tharks were 15 feet tall (previous scripts had made them human-sized). Favreau argued that a modern-day soldier would not know how to fence or ride a horse like Carter, who had been a Confederate officer. The first film he envisioned would have adapted the first three novels in the Barsoom series: "A Princess of Mars", "The Gods of Mars", and "The Warlord of Mars". Unlike Rodriguez and Conran, Favreau preferred using practical effects for his film and cited "Planet of the Apes" as his inspiration. He intended to use make-up, as well as CGI, to create the Tharks. In August 2006, Paramount chose not to renew the film rights, preferring instead to focus on its "Star Trek" franchise. Favreau and Fergus moved on to Marvel Studios' "Iron Man".