Crassigyrinus ("thick tadpole") was a large tetrapod which lived in the Early Carboniferous period. It also lived before the Pennsylvanian period and appear in the Middle Mississippian.
Crassigyrinus in The Land Before Time
It appears in the first movie, incorrectly portrayed with a sail on its back. It appears in the very first frame, where it swims for a while before being pursued by a Metriorhynchus, but escapes. Since it has a sail, it may actually be a Platyhisthrix (Synapsid-like amphibian) that fell into the water. It also resembled a small aquatic Longisquama.