Although Deborah asks for dinosaur bones, several bones that the protagonist finds do not belong to dinosaurs.Arthur can collect bones in New Austin through a glitch or exploit, upon returning to Deborah, the ensuing dialogue suggests that the mission was originally able to be completed by Arthur.Because many dinosaur bones are in the regions which cannot be accessed by Arthur Morgan, only John is able to complete the mission.
Red Dead Redemption 2 - All Dinosaur Bones Locations Guide Trivia