The swollen thumb is supported by a dorsoventrally flattened spine that does not extrude through the skin.
"A Review of the Middle American Tree Frogs of the Genus Ptychohyla"
William E. Duellman
Presumably the fragment is too deep dorsoventrally to be a piece of the dentary, and no sign of the Meckelian canal is present.
"Two New Pelycosaurs from the Lower Permian of Oklahoma"
Richard C. Fox