Allometry and heterochrony in extant and extinct Malagasy primates

Measurements of the skull and dentition were obtained for three groups of closely-related Malagasy lemurs: Propithecus verreauxi and P. diadema, Hapalemur griseus and H. simus, and Varecia variegata and subfossil Pachylemur insignis. In Propithecus, ontogenetic series for the larger P. diadema and smaller P. verreauxi were compared to evaluate whether species-level differences in skull form result from the differential extension of common patterns of relative growth. In the other two cases, mostly adult H. simus data and adult Pachylemur data were compared to ontogenetic series for their smaller sister taxa, respectively H. griseus and Varecia, to similarly infer whether morphological differences between these taxa result from the ontogenetic scaling of cranial proportions. First, analyses of the data indicate that cranial proportions for both species of Propithecus are ontogenetically scaled. As such, P. diadema apparently attains larger overall size primarily by growing at a faster rate, but not for a longer duration, than P. verreauxi. Second, analyses of the Hapalemur data suggest that facial proportions, but not mandibular dimensions, in both species are ontogenetically scaled. It is inferred that differences in patterns of relative growth for the mandibular corpus and symphysis of bamboo lemurs are correlated with differences in the loading regime of the lower jaw due to variation in the mechanical properties of their diets. Finally, analyses of the data indicate that most cranial proportions for Varecia and Pachylemur are not ontogenetically scaled, thus supporting claims for the generic separation of these taxa. Additional consideration of these comparisons and examples from the literature illustrate potential differences in the effects of selection for body size increases versus decreases, and selection for greater inter- versus intra-specific body size variation, on postcanine dental allometry in ontogenetically-scaled taxa. Possible differences in the development process underlying phyletic size change are also discussed.