Morphology of the Interorbital Region of Saimiri sciureus

Abstract

The skull of the platyrrhine primate Saimiri sciureus is distinguished by a large interorbital fenestra. Juvenile skulls still show a bony interorbital septum with some small gaps. A morphogenetic study was undertaken to better understand the structures of the interorbital region, which represents a linkage between the base of the braincase and the nasal skeleton. Already in early ontogenetic stages a reduction of the posterior portion of the nasal capsule and of the cartilaginous interorbital septum are observed, resulting in the formation of a primary interorbital fenestra. A bony interorbital septum is mainly formed in perinatal age stages by ossification of the presphenoid and by medial fusion of the frontals; the primary interorbital fenestra is retained as a small opening. It only occurs in late juvenile stages when the definitive interorbital fenestra develops by secondary transformation of bone into a membrane of dense connective tissue; this process is most probably caused by mechanical friction of the very closely approximated eyes of both sides.

In vivo function of the craniofacial haft: The interorbital “pillar”

“…Indeed, the low strain magnitudes reported here suggest that the interorbital region is not optimally designed to resist feeding forces, and is in fact overdesigned for this role. In support of this argument, the ability of adult Saimiri to resist feeding forces does not seem to be compromised by the bony fenestra in the interorbital pillar (Maier, 1983;Hartwig, 1995).…”

Ontogeny and homology of the paranasal sinuses in Platyrrhini (Mammalia: Primates)

“…As Hershkovitz (1977) and Maier (1983) noted, Saimiri is quite inappropriate as a model of ancestral platyrrhine facial morphology considering its extreme orbital approximation and uniquely fenestrated interorbital septum. Furthermore, it is now generally accepted that callitrichines represent a secondarily dwarfed lineage (Rosenberger, 1977(Rosenberger, , 1992Ford, 1980;Leuttenegger, 1980;Martin, 1990), calling the "primitive" nature of their craniofacial morphology into question.…”

“…Moreover, on developmental grounds Zuckerkandl (1887), Van Gilse (1927), Keith (1948), and Maier, (1983and Maier, ( , 1993and Maier, ( , 2000, among others, argued convincingly that the lamina transversalis posterior of the sphenoid found in strepsirhines and "insectivores," which forms the floor of the sphenoid recess, is homologous with the ossicula Bertini or sphenoidal concha, which forms the inferior border of the sphenoid recess in humans. Both structures represent ossifications of the floor of the cartilaginous posterior nasal cupula, which forms the recessus cupularis ( Fig.…”

Effect of life history on the squirrel monkey (Platyrrhini, Saimiri) cranium

“…Hershkovitz (1977) argued that thinning of the interorbital septum is typical of small New World monkeys such as Saguinus, and that the condition in Saimiri was not a discrete character but rather an extreme expression along a morphological continuum. Maier (1983) observed that the fenestra develops postnatally, presumably due t o secondary resorption of the septum by the closely apposed musculature of the eye bulbs. This study suggests that the unique approximation of the eyes to the midline results from the large relative brain size of squirrel monkeys and the precocial nature of their prenatal devel-opment.…”

“…Biegert (1963) laid further theoretical groundwork and suggested that the potential for midline approximation of the orbits was driven by the size of the intervening nasal capsule. Maier (1993) has recently advanced this work in primates and demonstrated that the ossified parts of the nasal capsule become the ethmoid bone and thus do not directly contribute to the formation of the interorbital septum, which is primarily an aspect of the orbitosphenoid (Maier, 1983). Considered broadly, primates with decreased reliance on olfaction develop smaller nasal capsules and thus express a relative narrowing of the interorbital space.…”