I'm pretty sure you are refering to Littlefoot's grandfather at the beginning of LBT 5, when he solves the quarrel about the last treestar that had been eaten by Mr. Clubtail while Littlefoot considered it his own (and blamed his friends who thought different about that that it had gone to non of them).
Am I right?
By the way, we were both wrong about Dimetrodon. It lived neither in the triassic, nor in the carbonian period, but a period between those two that began about 285 million years ago. I don't know the English name for that period, and non of my books tells it (can you help out Arvens?), but the German name for it would be "Perm".