Orbital soft tissues, bones, and allometry: Implications for the size and position of crocodylian eyes

Although the visual system of crocodylians has attracted interest regarding optical parameters and retinal anatomy, fundamental questions remain about the allometry of the eyeball and whether such scaling is the same across all crown groups of crocodylians. In addition, anatomy and identities of adnexal soft tissues that interact with the visual system are not well understood in many cases. We used contrast-enhancing iodine stain and high-resolution micro-computed tomography to assess the anatomy of orbital soft tissues, including extraocular muscles and glands, in crocodylians. We also used regression analysis to estimate the allometric relationship between the bony orbit and eyeball across Alligator mississippiensis and Crocodylus niloticus for the first time. Results revealed tight, negatively allometric relationships between the bony orbit and eyeball. Notably, the eyes of C. niloticus were larger for a given orbit size than the eyes of A. mississippiensis, although the slope of the relationship was no different between these two crown crocodylian groups. Among the findings from our anatomical study, new details were uncovered about the homologies of muscles of the abducens complex. In particular, M. rectus lateralis of crocodylians is revealed to have a more complex form than previously appreciated, being adhered to the tendon of the nictitating membrane, which may be apomorphic for Crocodylia. Our calculation of the orbit-eyeball allometric relationship and study of the adnexal soft tissues of the crocodylian visual system, in combination with previous work by other teams in other crown saurian clades, is a critical, formerly missing, piece in the Extant Phylogenetic Bracket for restoring the visual apparatus of extinct crocodyliforms and other archosauriform groups.