Their assessment was challenged in 2000 by Thomas Williamson, who found the morphological differences between it and "Kritosaurus" to be due to the skulls in question being from individuals of different ages. Unlike Hunt and Lucas, he considered the type skull of "Kritosaurus" to be diagnostic and thus could compare it to "Naashoibitosaurus". He returned to Horner's argument, noting as Horner had that the nasal crest in other hadrosaurines like "Prosaurolophus" moves back on the skull during growth. He also pointed out that the specimen's provenance was incorrect; instead of being from younger rocks than "Kritosaurus", it is from rocks of about the same age. The next major review, in the second edition of "The Dinosauria", again separated the two. The absence of the beak and lower jaw in the partial skull of "Naashoibitosaurus" makes comparison even more difficult.