Crassigyrinus was a big primitive amphibian which lived in the Carboniferous period. It eat small crustaceans. It had not live with Synapsids or Reptiles. They also lived before the Pennsylvanian and appear in the Middle Mississippian.
Crassigyrinus in The Land Before Time
It appears in the first movie, incorrectly portrayed as living during the Mesozoic era also before the reptiles too (Eusthenopteron is much older but it would be a Tetrapodomorph other than an amphibian) and with a sail on its back. It appears being chased after 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 look like a small Longisquama that fell in to the water.