Ok. The reason to rule out E is that the passage states that "an entirely new - and apparently featherless - coelosaur has complicated the subject." They then theorize on why this is the case, but the fact is that this particular coelosaur lacked feathers, so "ALL" coelosaurs did NOT have feathers.
The answer choice in E doesn't suggest that at one point, we *thought* all coelosaurs had feathers, but rather states "feathers on dinosaurs were a characteristic of ALL coelosaurs." Since this is directly contradicted in the passage, based on the line above, it cannot be the answer.