Cranial anatomy of Bagualia alba (Dinosauria, Eusauropoda) from the Early Jurassic of Patagonia and the implications for sauropod cranial evolution

Abstract

Sauropods are the largest terrestrial vertebrates ever known. During their evolution, they underwent numerous morphological changes, some of which occurred at the end of the Early Jurassic. However, current knowledge of Early Jurassic dinosaurs is scarce, especially cranial information. Here we describe the skull of the basal eusauropod Bagualia alba from the Cañadón Asfalto Formation (Toarcian; Early Jurassic) of Patagonia, Argentina, which represents the most complete eusauropod skull from the late Early Jurassic worldwide. The osteological description and phylogenetic relationships of Bagualia allowed us to determine the acquisition of the common cranial bauplan of eusauropods at the end of the Early Jurassic: a tall and robust rostrum with great retraction of the external nares; a deep narial fossa; a reduced antorbital fenestra; verticalization of the lacrimal; prefrontal without an anterior process; anteroposterior shortening of the frontal; a supratemporal fenestra visible laterally; a robust braincase; ‘U’-shaped jaws in dorsal view; reduction in the number of teeth in the dentary; teeth with crown to crown occlusion; development of lateral plates in the premaxilla and maxilla; and a dorsoventral expansion of the dentary symphysis. These cranial characteristics were recently suggested to be related to a change in the diet of the sauropods toward mainly hard vegetation. In this context, Bagualia contributes to filling a gap in cranial evolution between the gracile skulls of Sauropodiformes and the highly robust skulls widely represented in Middle Jurassic taxa.