Osteological evidence for the evolution of activity pattern and visual acuity in primates

Examination of orbit size and optic foramen size in living primates reveals two adaptive phenomena. First, as noted by many authors, orbit size is str ongly correlated with activity pattern. Comparisons of large samples of ext ant primates consistently reveal that nocturnal species exhibit proportiona tely larger orbits than diurnal species. Furthermore, nocturnal haplorhines (Tarsius and Aotus) have considerably larger orbits than similar-sized noc turnal strepsirrhines. Orbital hypertrophy in Tarsius and Aotus accommodate s the enormously enlarged eyes of these taxa. This extreme ocular hypertrop hy seen in extant nocturnal haplorhines is an adaptation for both enhanced visual acuity and sensitivity in conditions of low light intensity. Second, the relative size of the optic foramen is highly correlated with th e degree of retinal summation and inferred visual acuity. Diurnal haplorhin es exhibit proportionately larger optic foramina, less central retinal summ ation, and much higher visual acuity than do all other primates. Diurnal st repsirrhines exhibit a more subtle but significant parallel enlargement of the optic foramen and a decrease in retinal summation relative to the condi tion seen in nocturnal primates. These twin osteological variables of orbit size and optic foramen size may be used to draw inferences regarding the activity pattern, retinal anatomy, and visual acuity of fossil primates. Our measurements demonstrate that th e omomyiforms Microchoerus, Necrolemur, Shoshonius, and Tetonius, adapiform Pronycticebus, and the possible lorisiform Plesiopithecus were likely noct urnal on the basis of orbit diameter. The adapiforms Leptadapis, Adapis, an d Notharctus, the phylogenetically enigmatic Rooneyia, the early anthropoid s Proteopithecus, Catopithecus, and Aegyptopithecus, and early platyrrhine Dolichocebus were likely diurnal. The activity pattern of the platyrrhine T remacebus is obscure. Plesiopithecus, Pronycticebus, Microchoerus, and Necr olemur probably had eyes that were very similar to those of extant nocturna l primates, with a high degree of retinal summation and rod-dominated retin ae. Leptadapis and Rooneyia likely had eyes similar to those of extant diur nal strepsirrhines, with moderate degrees of retinal summation, a larger co ne:rod ratio than in nocturnal primates, and, more speculatively, well-deve loped areae centrales similar to those of diurnal strepsirrhines. Adapis ex hibited uncharacteristically high degrees of retinal summation for a small- eyed (likely diurnal) primate. None of the adapiform or omomyiform taxa for which we were able to obtain optic foramen dimensions exhibited the extrem ely high visual acuity characteristic of extant diurnal haplorhines. (C) 20 00 Wiley-Liss, Inc.