RAVOSA, M.J.; Northwestern U & Field Museum: Ontogeny and Phyletic Size Change in Extant and Extinct Malagasy Primates

Recent Malagasy lemurs are notable for encompassing the range of body-size variation noted for all primates past and present – about 5 orders of magnitude. Benefiting from the close phylogenetic proximity of many larger-bodied subfossil taxa to smaller living forms, this study uses allometric data from the skull and postcranium to probe the ontogenetic bases of body-size differentiation and morphological variation across this diverse clade. Thus, growth allometries for extant species are employed as a criterion of subtraction with which to evaluate morphological variation, and putative adaptations, among sister taxa. In lemuroids, a pervasive pattern of ontogenetic scaling is observed for facial dimensions in all 5 genera, with 3 genera also sharing relative growth trajectories for mandibular proportions (Lemur, Eulemur, Varecia). Differences in masticatory growth and form characterizing Hapalemur and fossil Pachylemur reflect common feeding/dietary behaviors, but not taxic affinities. In indrioids, pervasive ontogenetic scaling characterizes the limbs and facial skull in Indri, Avahi and Propithecus, and this size differentiation appears due in part to the effects of geographic variation in resource seasonality on the rate of growth-in-time. Facial growth allometries for these 3 extant genera and 2 larger, extinct sister taxa (Mesopropithecus, Babakotia) also are ontogenetically scaled. Significant interspecific differences are indicated, however in the scaling of indrioid masticatory proportions. Variation in the mechanical advantage of the jaw adductors and stress-resisting jaw elements is again linked to interspecific differences in dietary properties. Such allometric dissociations belie the presence of selection to uncouple shared ancestral growth patterns.