New Insights Into Dinosaur Jaw Muscle Anatomy

Abstract

Jaw muscles are key components of the head and critical to testing hypotheses of soft-tissue homology, skull function, and evolution. Dinosaurs evolved an extraordinary diversity of cranial forms adapted to a variety of feeding behaviors. However, disparate evolutionary transformations in head shape and function among dinosaurs and their living relatives, birds and crocodylians, impair straightforward reconstructions of muscles, and other important cephalic soft tissues. This study presents the osteological correlates and inferred soft tissue anatomy of the jaw muscles and relevant neurovasculature in the temporal region of the dinosaur head. Hypotheses of jaw muscle homology were tested across a broad range archosaur and sauropsid taxa to more accurately infer muscle attachments in the adductor chambers of nonavian dinosaurs. Many dinosaurs likely possessed m. levator pterygoideus, a trait shared with lepidosaurs but not extant archosaurs. Several major clades of dinosaurs (e.g., Ornithopoda, Ceratopsidae, Sauropoda) eliminated the epipterygoid, thus impacting interpretations of m. pseudotemporalis profundus. M. pseudotemporalis superficialis most likely attached to the caudoventral surface of the laterosphenoid, a trait shared with extant archosaurs. Although mm. adductor mandibulae externus profundus and medialis likely attached to the caudal half of the dorsotemporal fossa and coronoid process, clear osteological correlates separating the individual bellies are rare. Most dinosaur clades possess osteological correlates indicative of a pterygoideus ventralis muscle that attaches to the lateral surface of the mandible, although the muscle may have extended as far as the jugal in some taxa (e.g., hadrosaurs, tyrannosaurs). The cranial and mandibular attachments of mm adductor mandibulae externus superficialis and adductor mandibulae posterior were consistent across all taxa studied. These new data greatly increase the interpretive resolution of head anatomy in dinosaurs and provide the anatomical foundation necessary for future analyses of skull function and evolution in an important vertebrate clade. Anat Rec, 292:1246Rec, 292: -1265Rec, 292: , 2009. V V C 2009 Wiley-Liss, Inc.Key words: Dinosaur; functional morphology; jaw musclesThe dinosaur feeding apparatus comprised a complicated network of jaw muscles that span an intricate assemblage of intracranial joints that link the bones of the skull together. Data from jaw muscles, such as their homologies, attachments, and sizes, are important to test hypotheses of cranial function and feeding behaviors such as powered cranial kinesis, bite force estimation, chewing behavior, and mechanical optimization. Not surprisingly, jaw muscle reconstruction is a common practice among paleobiologists. However, despite a long history of muscle reconstruction and soft-tissue infer-

Jaw biomechanics and the evolution of biting performance in theropod dinosaurs

“…Potentially valuable threedimensional information can be lost by this simplification, but the benefits of using explicitly defined simple models outweigh the drawbacks; for example, their explicitness and statistical rigour. Muscle reconstructions are based on anatomical observations of modern avian and crocodilian jaw adductor musculature (M. Sakamoto 2005Sakamoto -2008, personal observation; but also Holliday & Witmer 2007;Holliday 2009; electronic supplementary material, figure S1). …”

Cranial anatomy ofErlikosaurus andrewsi(Dinosauria, Therizinosauria): new insights based on digital reconstruction

“…Clark et al (1994) considered an attachment for m. adductor mandibulae externus dorsal to it on the medial surface. However, based on the muscle topology, such an arrangement would be in conflict with the other adductor muscles of E. andrewsi (Lautenschlager, 2013) and unknown in theropod dinosaurs in general (Holliday, 2009).…”

“…Ventral to it, the surangular is medially inclined and contacts the angular along a rostrocaudally elongated suture. A coronoid eminence (sensu Holliday, 2009) is only moderately developed in the form of a slight swelling. It separates the muscle insertion sites for m. adductor mandibulae externus superficialis (m. AMES) laterally, and m. adductor mandibulae externus profundus (m. AMEP) and m. adductor mandibulae externus medialis (m. AMEM) medially (Lautenschlager, 2013).…”

New materials of Masiakasaurus knopfleri Sampson, Carrano, and Forster, 2001, and implications for the morphology of the Noasauridae (Theropoda:Ceratosauria)

“…This facet is longer and more laterally exposed than in Majungasaurus (FMNH PR 2100). A shallow fossa is also present near the posterodorsal corner, presumably for the anterior portion of the M. pterygoideus ventralis (Holliday, 2009).…”