alzheimer's disease: a progressive form of presenile dementia that is similar to senile dementia except that it usually starts in the 40s or 50s; first symptoms are impaired memory which is followed by impaired thought and speech and finally complete helplessness
presenile dementia: dementia with onset before the age of 65
pelycosaur: large primitive reptile having a tall spinal sail; of the Permian or late Paleozoic in Europe and North America