A whole note is a semiquaver - I believe the fractional system came into development long after the breve had fallen into near-obsolescence.
(Footnote - it’s a strange, and as far as I know unexplained phenomena that across the history of European music, note values have gradually reduced, and the longer ones fallen into disuse. Beyond the breve were the lunga and the maxima (ie ‘brief’, ‘long’ and ‘maximum’, with the lunga once being the primary beat. Gradually the breve became the primary pulse, then the semibreve, then the minim (originally minima, ie minimum), crotchet, quaver, and there’s now composers who often write using semiquaver or demisemiquaver primary beats.)