Functional anatomy of a giant dinosaur beak: Gigantoraptor and the evolution of the oviraptorosaurian jaw

Abstract

The Oviraptorosauria are one of several theropod dinosaur groups that diverge from the typical carnivorous theropod diet. Studying the detailed skeletal anatomy of oviraptorosaurian jaws is therefore important to address uncertainties in their seemingly complex dietary evolution, which encompassed omnivory and herbivory. The two main lineages of oviraptorosaurs – Caenagnathidae and Oviraptoridae – display a number of differences in mandibular morphology, but little is known about how they relate to potential differences in function. Gigantoraptor erlianensis is a giant oviraptorosaur from the Late Cretaceous Iren Dabasu Formation of Nei Mongol, China. Its mandible is the only well-preserved mandible of a basal caenagnathid, so it is particularly promising towards understanding the evolution of potentially diet-related mandibular features in caenagnathids and oviraptorids. The mandible of the Gigantoraptor holotype (LH V0011) was described in detail for the first time. It shows the greatest relative beak depth among caenagnathids. In modern finches, a deeper beak appears to be an adaptation for processing harder seeds. However, Gigantoraptor does not show obvious mandibular features (e.g. a tall coronoid process prominence) that favour the production of a large bite force. A lingual triturating shelf is present in more derived caenagnathids but not in Gigantoraptor. This suggests a possible increased specialisation towards shearing along the caenagnathid lineage. The possession of a dorsally convex articular glenoid in Gigantoraptor and more derived caenagnathids indicates that propalinal jaw movement was probably an important mechanism in food processing, as supported by similar morphologies in living Sphenodon and dicynodonts. Based on a number of osteological features, we find that the mandible of oviraptorids is more suited for producing a powerful bite (e.g. crushing-related behavior) than that of caenagnathids. In general, oviraptorids possess a deeper, more downturned beak, a taller coronoid process prominence and a larger medial mandibular fossa. The disparity in mandible morphology between caenagnathids and oviraptorids potentially suggests specialisation towards two different feeding styles – shearing and crushing-related mechanisms respectively. This study provides new data and functional analogues that reinforces previous suggestions that the two main oviraptorosaur lineages had divergent feeding styles likely to be linked with divergent dietary preferences.