These unusual Plutei appeared during the peak of the mushroom season here in a part of planted forest where there are always a lot of other Pluteus species (P. leoninus, P. umbrosus, P. chrysophaeus etc.). They grew on fallen rotting poplar trunks (Populus nigra or P. alba). I've been watching that patch closely since 2007 but this year was the first time these fungi appeared. The summer was relatively wet and chilly.