Mossman was a man of strong opinions and these included some ideas on mammalian evolution that might be considered idiosyncratic. As reported by David Hill, “He'd launch into impassioned monologues about placentas: ‘If taxonomists would just examine the evidence from the placenta, they'd see that lemurs are actually tree-climbing horses!’" Mossman’s unpublished autobiography at UWZM confirms he held something approaching this view. There are indeed some remarkable resemblances in placenta and fetal membranes between horses and lemurs. But this is yet another example of convergent evolution.
Disclosure: I am Adjunct Curator of The Harland W. Mossman Collection.