The word gnathostomes is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 20
Highest Scoring Words With Friends™ Play In The Letters gnathostomes
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Seven Letter Word Alert: (29 words)
amongst, anthems, geostat, ghettos, gnashes, hastens, homages, hotness, magneto, magnets, mangoes, megaton, montage, mottoes, onstage, osteoma, osteons, ottoman, seahogs, seamost, shmoose, smooths, soonest, soothes, stamens, stetson, stomate, stooges, tomatos