Cranial Morphology and Adaptations in Eocene Adapidae. 1. Sexual Dimorphism in Adapis magnus and Adapis parisiensis

Quantitative analysis of all well-preserved crania of Adapis magnus (n = 8) and Adapis parisiensis (n = 12) together with maxillary and mandibular dentitions preserving canines corroborates Stehlin's hypothesis that Adapis was sexually dimorphic. Males are from 13% to 16% larger than females in cranial length, corresponding to a weight dimorphism estimated at 44% to 56%, and have relatively broader skulls with more prominent sagittal and nuchal crests. Canine dimorphism ranges from 13% to 19%, which is equal to or only slightly greater than that expected as a result of body size dimorphism (i.e., relative canine dimorphism is slight or nonexistent). By comparison with living primates, the observed body size dimorphism in Adapis implies a polygynous breeding system. Cebus apella is a diurnal arboreal living primate with moderate body size dimorphism and slight relative canine dimorphism and one can speculate that Adapis lived in polygynous multimale troops of moderate size like those of C. appella. Adapis extends the geological history of sexual dimorphism and polygyny in primates back to the Eocene. Extant lemuriform primates are generally not dimorphic or polygynous and they clearly do not adequately represent the range of social adaptations present in Eocene primates, The evolutionary lineage from Adapis magnus to Adapis parisiensis exhibits reduction in body size and in relative canine size, and phyletic dwarfing in Adapis is possibly an adaptive response to increasing climatic seasonality and environmental instability in the late Eocene and early Oligocene. The fossil record of primate evolution is important for several reasons. First, it provides unequivocal evidence that primates have changed through time: genera familiar today are not found in Miocene, Oligocene, Eocene, or older intervals of geological time. Secondly, fossils provide a reasonably coherent outline of the phylogenetic history of primates. Finally, fossils provide the only direct evidence of the nature of adaptive mosaics or grades achieved by primates at different stages in their history. In this paper and the following one (Gingerich and Martin, 1981), we attempt to reconstruct aspects of the paleobiology of late Eocene lemuriform primates of the genus Adapis. When similar analyses of other well-known fossil primates are available, it will be easier to characterize major trends in primate evolution in terms of the functional pathways and adaptive grades these represent. Two major, diverse families of primates of modern aspect dominate Eocene and earliest Oligocene primate faunas on the northern continents. Tarsiiform Omomyidae were predominantly small (ca. 10-600 gm weight), insectivorous or frugivorous, nocturnal leaping arboreal forms (Szalay, 1976; Fleagle, 1978; Gingerich, 1981b). Lemuriform Adapidae were generally larger (ca. 100-8,OOO gm weight), frugivorous or folivorous, diurnal leaping arboreal Received March 11, 1981; accepted June 22, 1981. 0002-948318115603-0217$05.0