The nonavian theropod quadrate I: standardized terminology and overview of the anatomy, function and ontogeny

By allowing the articulation of the mandible with the cranium, the quadrate of diapsids and most other tetrapods plays an important role morphofunctionally. In Theropoda, its morphology is particularly complex and varies importantly among different clades of nonavian theropods so that the quadrate possesses a strong taxonomic potential. Inconsistencies in the notation and terminology used in discussions of the theropod quadrate anatomy have been noticed, a number of no less than height different terms being sometimes given to a same structure. A standardization list of terms and notation for each quadrate anatomical entity is here proposed, with the goal of facilitating future descriptions of this important cranial bone. An overview of the quadrate function, pneumaticity and ontogeny in nonavian theropods is also given. The quadrate of the large majority of nonavian theropod is akinetic and the diagonally oriented sulcus of the mandibular articulation allowed both rami of the mandible to move laterally when opening the mouth in many of them. Pneumaticity of the quadrate is also present in most of tetanuran clades and the pneumatic chamber, invaded by the quadrate diverticulum of the mandibular arch pneumatic system, was connected to one or several pneumatopores on the medial, lateral, posterior, anterior or ventral sides of the quadrate. Absence of a quadrate foramen in allosauroid embryos and a poor delimitation of mandibular condyles in both embryonic and juveniles tetanurans seems to be ontogenetic features of some theropods. Finally, the numerous morphological differences existing in the quadrates of the two specimens of Shuvuuia deserti, interpreted by some as juvenile and adult individuals, are considered as ontogenetic, taphonomic, and perhaps also taxonomic variations.