LBT/Yu-gi-oh! - A mysterious outsider comes into the valley and introduces the five/seven to a new game. Then Littlefoot gets possessed by an ancient spirit and its all downhill from there.
I was thinking that as well, except on a different note.
The gang walk into a cave and discover the ancient markings which show the millenium items. Then, the gang and the rest of the dinosaurs of the valley are transported to another dimension, where they become trapped in duel monster cards.
Then, Yugi and his friends buy the cards which contains their souls, and Littlefoot speaks to Yugi, Cera to Joey, etc... They summon the gang as a part of duel monsters, etc...