Evolutionary morphology, cranial biomechanics and the origins of tarsiers and anthropoids

Abstract

During the Time of Messel, the dominant groups of primates were the adapiform strepsirhines and the tarsiiform haplorhines, both important in discussions of anthropoid origins. Living tarsiers are at the centre of these ideas as one school of thought, representing the Tarsier-Anthropoid Hypothesis, holds they are the sister-group of Anthropoidea. The Tarsier-Tarsiiform Hypothesis, however, maintains that tarsiers are phyletically nested among the (paraphyletic) fossil tarsiiforms (~omomyids). Orbital morphology is crucial to this debate: The possibility that the postorbital septa of tarsiers and anthropoids are synapomorphic and that it evolved in their last common ancestor to insulate the eyeballs from muscular interference. Our biomechanical model of forces acting on the enormous eyeballs and orbits of tarsiers especially during locomotion provides a strong counterargument to this proposition. The uniquely specialised orbita of Tarsius, which include prominent circumorbital flanges that are continuous with the postorbital septum, are designed to sustain enormous inertial loads transmitted by the eyeballs during the acceleration and deceleration phases of powerful leaping, for which Tarsius is also famous. The eyeballs are thus secured, and pressure absorbed by the retina during acceleration is minimised, by enlarging its surface area of contact with a "walled socket", i.e., by the extra-fossa expansion of these flanges. The tarsier septum is, therefore, a form-function convergence on the small-eyed anthropoid condition. Several related Eocene tarsiiforms exhibit a precisely Tarsius-like morphology of the rostrum relating to eyeball hypertrophy, although they lack the exaggerated circumorbital and septal morphology and only rarely exhibit postcranial features indicative of super leaping abilities. © Senckenberg Gesellschaft für Naturforschung and Springer 2012.