Something I thought I’d add to my earlier argument for Ducky’s species: it may be possible that Don Bluth and co.
did intend for Ducky to be an
Anatosaurus, but changed her to look more like a
Saurolophus. (It would be very interesting to look at early conceptual artwork of the characters, so we could see how their designs changed from their initial conceptions to what we see in the movie.) Perhaps they added the crest because they thought it gave Ducky more character (I personally find that the crest greatly enhances Ducky’s cuteness

), which, of course, they could only do realistically if they made the adults of her species crested as well.
Obviously, we have no idea what sources Don Bluth used when he was researching dinosaurs for
The Land Before Time, but when I compare the LBT dinos (the adults, mainly

) to the work of paleo-artists like
Charles R.
Knight and
Zdenek Burian, I can't help but notice similarities. These artists set the standard for how dinosaurs were illustrated for much of the 20th century, and their work is prominently featured in older dinosaur books, so the possibility is high that Bluth used their paintings as the basis for many of his prehistoric characters. However,
Saurolophus is one dinosaur that does
not appear in the works of either of these artists, and indeed is not commonly seen in paleo-art at all.
Its uncrested relatives, however, are virtual staples of prehistoric illustration, and depictions of them were far more likely to have been encountered by Bluth when he was doing his research. As a result, I don't believe it is impossible that the LBT hadrosaurs were based on species such as
Anatosaurus and
Trachodon, with a saurolophine crest added on for aesthetic appeal.
Of course, I could be completely wrong. The majority of this hypothesis is pure speculation, and maybe Ducky was based on a
Saurolophus all along. But it’s an idea I thought I’d throw out there, just in case anyone was interested in hearing it. (Wishful thinking, maybe?

)