A comparative analysis of internal cranial anatomy in the hylobatidae

Abstract

Craniometric studies on the hylobatids using external metrics (Creel and Preuschoft, 1976, 1984) sorted hylobatid populations into primary species groupings which are in accordance with the four currently recognized generic-level groupings. The goal of the current study was to assess the relative orientations of the orbits, palate, and basioccipital clivus among the hylobatid genera in an effort to further clarify whether the lesser apes differ significantly in these internal cranial features and how that variation patterns across the groups. Nine angular variables quantifying orbital, palatal, and basioccipital clivus orientations were measured on lateral view radiographs of adults representing three of the four hylobatid genera: Hylobates; Nomascus; and, Symphalangus. The interspecific adult hylobatid means for the angular variables were analyzed using t-test contrasts. The total sample was further subjected to discriminant function analysis (DFA) to test for the ability of craniofacial angular variables to distinguish the hylobatid genera from one another. The three hylobatid genera displayed significant morphological differentiation in orbital, palatal, and posterior skull base orientations. Normal, jackknifed, and cross-validation DFA procedures correctly identified the hylobatids 50-100% of the time. The observed morphological patterns generally mapped onto the findings of earlier external craniometric hylobatid studies and suggest concordance between specific internal and external cranial features. This article is the first comprehensive study of variation in internal cranial anatomy of the Hylobatidae and includes the first published craniofacial angular data for Nomascus.

Ontogeny of Orbit Orientation in Primates

“…Interestingly, reduced frontation associated with a more airorhynch or dorsally deflected facial skull has been argued to be primitive for early anthropoids (Begun, 2015;Leslie, 2010;Moyà-Solà et al, 2009;Shea, 1985Shea, , 1988. While the presence of relatively low encephalization in basal catarrhines (Gonzales et al, 2015;Simons et al, 2007) may explain the low levels of frontation and greater airorhynchy in such anthropoids, it is unclear if such a pattern of morphological integration also characterizes early hominoids.…”

“…Aside from phylogenetic arguments regarding patterns of covariation in orbitofacial position (Begun, 2015;Biegert, 1957Biegert, , 1963Enlow, 1996;Leslie, 2010;Moyà-Solà et al, 2009;Radinsky, 1968;Ravosa, 1988;Ravosa and Shea, 1994;Shea, 1985Shea, , 1988, variation in orbit orientation has been invoked in discussions of circumorbital function. In primates, the postorbital bar, postorbital septum, and supraorbital torus all have been linked to elevated convergence and/or increased frontation (Cartmill, 1970(Cartmill, , 1972(Cartmill, , 1974(Cartmill, , 1980(Cartmill, , 1992Heesy, 2008;Hylander and Ravosa, 1992;Noble et al, 2000;Ravosa, 1988Ravosa, , 1991aRavosa, , 1991bRavosa et al, 2000Ross, 1995Ross, , 1996Shea, 1986).…”