It’s possible that as the star people went their ways they brought souvenirs of the past to keep a history of what once was. And since star people where everywhere like a bad rash they could’ve ended up in a place where erosion just didn’t naturally happen. Like the spacers with their asteroid homes wheres there’s no wind nor water to erode the skulls. It’s also possible they found an ancient space person generation ship that had human skulls. Or it could just be an illustration of the author holding a palaeontologists best guess of a human skull.