Far from carrying the stigmata of some hypothetical former diurnal lifestyle, the tarsier has a superb adaptation to the nocturnal niche, equal in sophistication to that of the owl. Owl and tarsier both have huge eyes whose immobility is advantageous in the difficult task of superimposing the two views for binocular summation and stereopsis. The fovea might be an adaptation to that niche also, for example to aid in focussing which is prodigously difficult with a low f-ratio optical system and dim light. There is no need for the stretched argument that the fovea somehow betrays a diurnal past. Imlications for primate phylogeny and the use of sperm in systematics.
Tarsiers have unusual sperm.
Tarsier sperm do not definitively place them in one of three phylogenetic arrangements shown above, although the prosimian-tarsier link is least supported by the sperm evidence.