dino damage
Y'know.
If T. rex WERE an obligate scavenger like Horner thinks, then there should be megafauna remains with obvious bite marks or found in association with titanosaurs and dromaeosaurs which couldn't tackle something that big (like the big hyaenids tied to megafauna later, or the way ravens basically call wolves over to open up prey for them).
There are hadrosaurid and ceratopsian remains with bite marks but I can't find anything suggesting these guys ever ate titanosaurs.