Eye Size and Set in Small-Bodied Fossil Primates: A Three-Dimensional Method.

Area of Science:

• Paleoanthropology
• Primate Anatomy
• Evolutionary Biology

Background:

• Accurate estimation of primate eye volume and placement is crucial for understanding visual capabilities and metabolic rates.
• Traditional methods relying on aperture diameter can underestimate eye size, particularly in fossil species.
• The position of the eye within the orbit (eye set) significantly influences visual field and potential for forward-facing vision.

Purpose of the Study:

• To introduce and validate a novel geometric method for reconstructing primate eye volume and placement.
• To assess the method's efficacy on both extant and fossil primate species, including those with damaged orbital structures.
• To quantify eye protrusion using a new metric, the Ectopic Index, and compare it across different primate groups.

Main Methods:

• Utilizing the three-dimensional contour of the intraorbital surface for geometric reconstruction.
• Validating the method with dry skulls and wet dissections of seven living primate species.
• Applying the method to seven Paleogene fossil primate species, including those with incomplete orbits.

Main Results:

• The method accurately reconstructs eye volume and placement, performing well even with damaged or incomplete orbital features.
• The Ectopic Index quantifies eye protrusion, revealing significant variation among primate groups (e.g., 11%-13% in Lemur, Notharctus, Rooneyia; 47%-56% in Galago, Tarsius).
• Reconstructed virtual eye sizes indicate large to hypertrophic eyes in several Paleogene primates, suggesting potential underestimation in previous aperture-based measurements.

Conclusions:

• The developed geometric method provides a reliable means to reconstruct primate eye volume and placement, enhancing our understanding of visual ecology.
• Eye protrusion (Ectopic Index) varies considerably across primates, with some species exhibiting deeply recessed eyes similar to anthropoids.
• The findings challenge assumptions about the relationship between orbital aperture orientation and line-of-sight, suggesting soft tissue and eye protrusion play key roles in determining visual orientation.