Rivista di Antropologia (Roma) Vol. 79 (2001), pp. 45-52 Allometric analysis of the skull in Pan and Gorilla by geometric morphometrics Emiliano Bruner, Giorgio Manzi UNIVERSITÀ DI ROMA “LA SAPIENZA”, DIPARTIMENTO DI BIOLOGIA ANIMALE E DELL’UOMO, P.le Aldo Moro 5, Roma Summary – Classical metric analyses have shown that much of the morphological variation between Pan and Gorilla is due to size-related shape differences. Geometric morphometrics, which operates a removal of size (as a consequence of GLS superimposition of landmark points), can shed further light on the issue. With these methods and with particular reference to facial and basal regions of the cranium, we examine here a sample including skulls at different ontogenetic stages of Gorilla gorilla and Pan troglodytes. Landmark co-ordinates were acquired in two dimensions from lateral view. Multivariate analysis of transformed data shows that the first principal component (representing 61.2% of total variance) does not separate the two genera, while sets the whole sample in an ontogenetic scaling perspective which involves mostly muzzle growth and airorhynchy The second principal component (16.8% of total variance) is related to an increasing midface flattening along the two ontogenetic series, with Gorilla trajectory displaced toward a larger level of midface projection. The cranial shape variation observed along the first principal component corresponds to the pattern observed when the shape vector is regressed on centroid size, where shape results clearly size-related (r = 0.92). Keywords – Allometry, Thin-plate-spline, Ontogenetic scaling, African apes. Introduction Most part of the inter- and intraspecific morphological diversity among the African apes (Panidae: Pan and Gorilla) has been explained with the different regulation of a common pattern of growth and development, i.e. as the heterochronic result of evolutionary shifts in the onset, rate, or timing of the same ontogenetic “program” (Leigh & Shea, 1996; Shea, 1983a, 1983b). The variation (i.e., extension or truncation) of a given ontogenetic trajectory produces shape differences which are size-related. Thus, “allometry” – defined as “size related changes in shape” (Cheverud et al., 1983) or, simply, as the “study of size and its consequences” (Gould, 1966) – plays a major role in this process. The diversity between taxa or individuals may be produced by a simple regulation of the same ontogenetic pattern; the allometric result, in this case, is called ontogenetic scaling. The morphology of the facial and palatal components of the cranium of the African apes appears particularly affected by ontogenetic scaling, and this was clearly pointed out by analyses based on multivariate traditional morphometrics (e.g., Shea, 1983b). The recent development of geometric morphometrics allows to integrate the quantitative analysis Comunication presented at the 13th Congress of the Italian Anthropologists Roma-Sabaudia 1999.