Influence of Orbit Size on Aspects of the Tarsier Postorbital Septum

摘要

Explanations invoking the complex mechanical effects of orbital enlargement in Tarsius have been extended to several areas of the skull, including the conformation of the postorbital septum. The strong cline in the degree of relative orbital enlargement across tarsier species groups presents an unexploited opportunity to test such scenarios. Our goal is to evaluate hypotheses concerning the impact of orbital hypertrophy on the size of specific components of the postorbital region including the frontal, zygomatic, alisphenoid, and maxillary bones. The frontal process is almost always viewed as a functional projection whose bracing role requires a positive morphometric association with orbital hypertrophy. Conversely, the periorbital expansion of the zygomatic is often perceived as functionally unrelated to orbital enlargement and therefore is not expected to track increases in relative orbit size. Interpretations of the alisphenoid and maxillary periorbital processes range from vestigial remnants of once larger structures reduced because of ocular enlargement to structures large in tarsiers because of their functionally relevant role in supporting the enlarged ocular apparatus. We measured these attributes in an extensive sample of 4 tarsier species groups including Tarsius bancanus, T. syrichta, T. spectrum, and T. pumilus. In contrast to proposed functional interpretations, our results indicate that variation in most linear parameters might be better explained by differences in body size than intrageneric differences in orbit size. As expected, width of the zygomatic postorbital contribution does not parallel intrageneric variation in orbit size. However, morphometric relationships between relative orbit size and other parts of the septum are complex but not clearly associated with orbit size differences within Tarsius.