Morphological Diversity and Evolution of the Jugal in Dinosaurs

Abstract

In dinosaurs, as in other reptiles, the homologue of the mammalian zygomatic bone is the jugal. The dinosaurian jugal was primitively triradiate, with posterior, dorsal and anterior processes that respectively contacted the quadratojugal, the postorbital, and the maxilla and lacrimal. However, the jugal evolved along different lines in the three major dinosaurian clades. In theropods this cranial element remained relatively conservative in morphology, apart from being reduced to a rod-like structure in most birds and a few non-avians. In sauropodomorphs the jugal eventually became small, plate-like and nearly restricted to the area below the orbit, even being excluded from the ventral margin of the skull in many derived taxa. Among ornithischians the jugal was highly variable, but in many cases became large and/or adorned with ornamental features such as horns, flanges, and rugosities. The jugal does not appear to have been a site of muscle attachment in most non-avian dinosaurs, but represented an important structural element in the akinetic dinosaurian skull. The conspicuous jugal ornaments seen in many ornithischian dinosaurs, like the less striking ones documented in some saurischians, may have played an important role in the social behavior of the species that possessed them. In many cases they have a weapon-like aspect suggesting use in aggressive displays, if not actual combat, adding to the evidence that agonistic behavior was likely widespread among ornithischians in particular. Anat Rec,

Peer Review #2 of "Notes on the cheek region of the Late Jurassic theropod dinosaur Allosaurus (v0.1)"

“…The morphology of the cheek region of theropod dinosaurs has recently been reviewed by Sullivan & Xu (2017) and Wang et al (2017), focusing primarily on the morphology of the jugal. Apart from a few exceptions, the anterior process of the jugal in theropods participates in the posteroventral margin of the antorbital fenestra.…”

Cranial functional specialisation for strength precedes morphological evolution in Oviraptorosauria

“…Incisivosaurus has a cranial morphology that combines features of non-oviraptorosaurian tetanuran theropods and the more unorthodox oviraptorids 44 , 45 . Incisivosaurus retains teeth, which are unusual and heterodont 44 , and has a more typical theropod jugal shape 44 – 46 , not the rod-like form of the oviraptorids 18 , 24 . Incisivosaurus has some degree of cranial shortening and a relatively large circular orbit.…”

Peer Review #1 of "Notes on the cheek region of the Late Jurassic theropod dinosaur Allosaurus (v0.1)"

“…His 149 interpretation was probably influenced by the curved rim of the antorbital fossa on the jugal, 150 which stands out prominently in articulated skulls, and was interpreted as the jugal-lacrimal 151 suture, and the very thin bone anterior to it, which resembles the distal end of the ventral process 152 of the lacrimal. The morphology of the cheek region of theropod dinosaurs has recently been reviewed by 157 Sullivan & Xu (2017) and Wang et al (2017), focusing primarily on the morphology of the 158 jugal. Apart from a few exceptions, the anterior process of the jugal in theropods participates in 159 the posteroventral margin of the antorbital fenestra.…”