We argue on the basis of astrometric and spectroscopic data that the ultramassive white dwarf GD50 is associated with the star formation event that created the Pleiades and is possibly a former member of this cluster. Its cooling age (~60 Myr) is consistent with it having evolved essentially as a single star from a progenitor with a mass M > 6Msolar, so we find no need to invoke a white dwarf-white dwarf binary merger scenario to account for its existence. This result may represent the first direct observational evidence that single-star evolution can produce white dwarfs with M > 1.1Msolar, as predicted by some stellar evolutionary theories. On the basis of its tangential velocity, we also provisionally identify the ultramassive (M ~ 1.2Msolar) white dwarf PG0136 + 251 as being related to the Pleiades. These findings may help to alleviate the difficulties in reconciling the observed number of hot nearby ultramassive white dwarfs with the smaller number predicted by binary evolution models under the assumption that they are the products of white dwarf mergers. Based on observations collected at the European Southern Observatory, Chile. ESO No. 072.D-0362. E-mail: pdd [at] star.le.ac.uk (pdd[at]star[dot]le[dot]ac[dot]uk)