Seeing through the eyes of Thylacosmilus atrox (Metatheria, Sparassodonta)

GAILLARD, C.; MACPHEE, R.D.E.; FORASIEPI, A.M.

Mendoza (virtual)

Jornada; 34 Jornadas Argentinas de Paleontología de Vertebrados; 2021

IANIGLA, CCT-CONICET, Mendoza, MHNSR, MCNAM

We estimated the eyeball diameter and orbit orientation of three specimens of Thylacosmilus atrox (FMNH P-14531; MLP 35-X-4-1, and MMP 1433-M) based on 3D models reconstructed with computed tomography. Thylacosmilus is the only sparassodont known to have possessed a postorbital bar, a feature uniquely shared among metatherians with the extinct marsupial predator Thylacoleo carnifex (Diprotodontia) but present in some carnivores (e.g., Barbourofelis spp.) and universal in certain other clades (e.g., modern primates). Eyeball diameter of Thylacosmilus is estimated to have been between 33?35 mm, which represents 4?8% of the total volume of the cranium. Orbital convergence in Thylacosmilus varies between 22?34°, with a visual field overlap of 46?62°. This angle is surprisingly narrow compared to that of other sparassodonts examined (e.g., Cladosictis, Sipalocyon, Arctodictis) (96?118°), extant didelphids (110°) and selected carnivorous mammals (120°). However, orbital verticality in Thylacosmilus is much higher (62?98°) than in other sparassodonts (30?55°) and extant marsupials (40?70°). Verticality values of Thylacosmilus may be correlated with its implied carnivorous diet, but also with a reorientation of the orbit due to the replacement of the postorbital ligament by an osseous postorbital bar. The unique orbital configuration of T. atrox among sparassodonts might also be related to the peculiar architecture of its cranium, which featured a great enlargement of the maxilla housing the ever-growing saber-tooth upper canines. However, some degree of stereoscopy was probably achieved through frontation of the orbits, thus enabling a more vertical gaze than in other sparassodonts.