The land before time probably does take place in the Cretaceous considering the number of dinosaurs there who didn't live before that era (and also some flowers which didn't begin to develope before the Cretaceous). However there is also a huge number of species from Jurrassic (e.g. Apatosaurus, Diplodocus, Stegosaurus, Ophthalmosaurus, Rhamphorhynchus). The "Tinysaurs" as I said may well be designed after a species from the Triassic period, and in the original movie we even have a Dimetrodon (who lived in the Permian period).
While the land before time is probably supposed to be in the Cretaceous, they were never hesitant to put any species in, no matter from what era that species was (of course this is just a story, so "adjusting" science a bit for its need is not exactly a capital crime )
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