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Some glassfrogs have the additional structure present in various anuran hands known as the prepollex, sometimes called the pseudothumb.

There’s been substantial interest in the idea that the prepollex might represent the true digit I – if so, this would mean that at least some anurans still have five digits.

Recent embryological studies seem to show that this isn’t the case, and that the prepollex is a wholly novel structure.

A survey of prepollex and prehallux variation in anuran limbs.

Note the variation in prepollex form, as well as the T-shaped terminal phalanges and the distinctive flange on metacarpal III.