I always thought about the person who could possibly tell Bron about his wife.
Rooter is indeed the most logical candidate. Person who knew the information should have personally seen or heard about wife's death, should have known names of Longneck family minus Littlefoot (or, at the worst, their detailed appearance). I believe Rooter and Littlefoot talked a little more after short scene from movie, and Litlefoot could tell his relatives' names.
Other option is some other dinosaurs from migrating herd who was knew Longneck family in some way and saw battle with Sharptooth, or saw Mama Longneck's dead body with scars left by predator.
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About Mysterious Beyond - there's no mystery. (no pun intended
)
This term was introduced in LBT2, and it was logical.
Put yourself into their position: you made one long one-path travel to some far land (it's like you travel to some city of some distant foreign country for the first time), then you made yourself a home and live there isolated for the majority of time, because it's the safest place, you never want to examine outskirts of your home, you never go outside and explore it, so you don't know much about it. So, for you, area around your town and beyond is indeed mysterious unexplored area.
Well, you HAVE to name this area somehow. Yeah, Great Valley residents could name it differently, but still, it's not the worst name in my opinion.
But in case if OTHER dinosaurs outside the valley call ANY land as Mysterious Beyond in following movies, then yeah, it is strange.
