The Predentary Bone and Its Significance in the Evolution of Feeding Mechanisms in Ornithischian Dinosaurs

Abstract

The characteristic predentary bone in ornithischian dinosaurs is a unique, unpaired element located at the midline of the mandibular symphysis. Although traditionally thought to only be a plant "nipping" bone, the true functional significance of this bone among feeding mechanisms of ornithischian dinosaurs is poorly known. Recent studies of a select few ornithischian genera have suggested rotation of the mandibular corpora around their long axes relative to their midline joint articulation with the predentary bone. This study aims to re-evaluate these hypotheses as well as provide in-depth qualitative comparative descriptions of predentary bone morphology in ornithischian genera throughout all subclades, including heterodontosaurids, thyreophorans, ornithopods, and marginocephalians. Descriptions evaluate overall shape of the predentary, its articular surfaces contacting the rostral ends of the dentaries, and the morphology of the rostral extent of the dentaries and their midline symphysis. Functionally relevant morphologies in each predentary morphotype are accentuated for further speculation of feeding mechanisms. Three predentary morphotypes are described throughout ornithischian subclades and each plays a unique role in feeding adaptations. Most notably, the predentary likely evolved as a midline axial point of the mandibular symphysis for simultaneous variable movement or rotation of the mandibular corpora in many, but not all, taxa. This simultaneous movement of the hemimandibles would have aided in feeding on both sides of the jaw at once. The function of the predentary as well as other jaw adaptations is discussed for genera throughout all subclades, focusing on both general shape and joint morphology. Anat Rec, 299:1358-1388, 2016. © 2016 Wiley Periodicals, Inc.

Ornithischia

“…For instance, the diverse rounded-to-spade-shaped, denticulate, and often precisely occluding dentition of ornithischian dinosaurs has greatly informed paleontologists and functional anatomists of their past feeding behavior. In accordance with their morphological diversity, macro-and microscopic dental wear patterns have provided direct evidence of true feeding motions of the lower jaw during feeding, including orthal (i.e., dorsoventral, or up-and-down, jaw motion), palinal motion (i.e., caudally oriented, or backward, jaw motion while teeth are occluded), and long-axis rotation of each side of the jaw at the predentary joint (Mallon and Anderson 2014;Nabavizadeh and Weishampel 2016). These feeding motions vary both between and among clades of Ornithischia, and each gives a paleoecological context of the animals' respective time periods (Nabavizadeh 2016).…”

“…1c), such as Parasaurolophus, Corythosaurus, Lambeosaurus, and Edmontosaurus, as well as other iguanodontians, such as Iguanodon. The small-bodied ornithopods, such as Hypsilophodon and Dryosaurus, are bipedal animals with small, rounded teeth with primarily orthal feeding mechanisms with slight mandibular rotation (Weishampel 1984;Nabavizadeh and Weishampel 2016). As ornithopods diversified into the much larger iguanodontians, including hadrosaurs, there is a transition to a graviportal, quadrupedal gait, and a major palinal component is implemented into their feeding mechanism in addition to long-axis rotation of the mandibles (Nabavizadeh and Weishampel 2016).…”

“…They are divided into the subclades known as ankylosaurids and nodosaurids, both of which diversified during the Cretaceous Period. Dental microwear and morphological studies of their small, denticulate teeth and entire jaw system have shown their feeding mechanisms primarily consisted of a combination of palinal jaw movements with long-axis rotation of each side of the jaw at the predentary-dentary joint (Mallon and Anderson 2014;Ősi et al 2014;Nabavizadeh 2016;Nabavizadeh and Weishampel 2016).…”

“…Previous speculation has stated the possibility of frequent upright posture for reaching higher vegetation; however, this has largely been argued against. Stegosaurs are thought to have exhibited a primarily orthal feeding mechanism (Barrett 2001) with possible slight rotation of the jaws at the predentary-dentary joint (Nabavizadeh and Weishampel 2016).…”

“…The small-bodied ornithopods, such as Hypsilophodon and Dryosaurus, are bipedal animals with small, rounded teeth with primarily orthal feeding mechanisms with slight mandibular rotation (Weishampel 1984;Nabavizadeh and Weishampel 2016). As ornithopods diversified into the much larger iguanodontians, including hadrosaurs, there is a transition to a graviportal, quadrupedal gait, and a major palinal component is implemented into their feeding mechanism in addition to long-axis rotation of the mandibles (Nabavizadeh and Weishampel 2016). Other studies have suggested the possibility of a form of cranial kinesis called "pleurokinesis" might have also been a key component in ornithopod feeding mechanisms (Weishampel 1984;Norman and Weishampel 1985), but more recent studies have challenged this notion (Rybczynski et al 2008;Holliday and Witmer 2008;Cuthbertson et al 2012).…”