cf. Dimetrodon grandis

Now how can this crumb of a tooth be attributed to Dimetrodon??

First, it's serrated. It could be shark? The enamel is not smooth (not very visible in this image, a little at the bottom), so no (additionally, the serration shape is different from those of Orthacanth sharks). That narrows it down to serrated Synapsids. It turns out that very few animals at this time and location had "true" serrations, not just enamel serrations, but bumps in the dentine beneath the enamel. The enamel on this piece happens to still be clear, allowing one to see the globular dentine underneath! From Brink and Reisz (2014), I'd posit that D. grandis is a suitable candidate. I'm also not an expert, so I welcome contrarian arguments. I highly doubt it's Therapsid, as I haven't heard of any from the Waurika locality. 

D. grandis: