Seemingly moody: a hypothetical reconstruction of the eye musculature of Bajadasaurus

GARDERES, JUAN PABLO; WHITLOCK, JOHN ANDREW; TOLEDO, NÉSTOR; GALLINA, PABLO ARIEL

Congreso; 83rd Anual Meeting SVP; 2023

Of the cranial musculature in extant archosaurs, several functional groups have been thoroughlyanalyzed, mainly those related to biting biomechanics and skull stability. In this sense, adductor andcraniocervical musculature, have been primarily analyzed for inference in non-avian dinosaurs.Conversely, other cranial muscular groups, such as the hyoid musculature and the extrinsic eyemusculature, have been shallowly explored. Of the latter, in Archosauria, Alligator has been described, while in Aves, Buteo buteo, Ardea cinerea and Gallus gallus have been described. Here we present the first inferential reconstruction of the extrinsic eye musculature for a diplodocoid sauropod dinosaur, Bajadasaurus pronuspinax, based on osteological correlates and extant phylogenetic bracketing. The eyeball was inferred to have a size of half of the rostrocaudal length of the eye socket parallel to skull roof, similar to the dimensions of the scleral rings of Diplodocus (CM 11161) and Nemegtosaurus (ZPAL MgD-I/9), and an interorbital cartilage septum was inferred. Six muscles have been reconstructed. The insertion of these on the eyeball are inferred to be placed on its medial surface, according to a conservative location of these regarding the major axes of the skull and positions in Alligator, Buteo and Gallus. The four rectus muscles attach their origins surrounding the foramen for CN II. The muscle rectus dorsalis attaches on the surface where the orbitosphenoid and laterosphenoid fuse, leaving a shallow concavity as an osteological correlate. Caudally, the m. rectus lateralis attaches on the dorsal surface of the basisphenoid, without leaving an osteological correlate. The m. rectus ventralis attaches to the edge of the basisphenoid, ventral to the foramen for CN II, leaving a depressed surface as an osteological correlate. The remaining m. rectus medialis is inferred to attach rostral to the foramen for CN II, on the caudal portions of the interorbital cartilage. Onto this cartilage, both obliquus muscles attach rostrally, the m. obliquus dorsalis on the dorsal area, and the m. obliquus ventralis on the rostroventral area. The inferred extrinsic eye muscular system in Bajadasaurus shows a configuration more similar to that on extant Aves than that of extant Crocodylia, based on the position of the origin of the obliquus muscles.