A salamander that chews using complex, three-dimensional mandible movements

Abstract

It is generally accepted that most non-mammal tetrapods have a hinge-like jaw operation restricted to vertical opening and closing movements. Many mammal jaw joints, by contrast, operate in more complex, three-dimensional ways, involving not only vertical, but also propalinal (rostro-caudal) and transverse (lateral) movements. Data on intraoral food processing in lissamphibians and sauropsids has prompted a generally accepted view that these groups mostly swallow food unreduced, and that in those cases where lissamphibians and sauropsids chew, they mostly use simple vertical jaw movements for food processing. The exception to that generally accepted view being some propalinal chewing in sauropsids. We combined 3D kinematics and morphological analyses from biplanar high-speed video fluoroscopy and micro-CT to determine how the paedomorphic salamander Siren intermedia treats captured food. We discovered that S. intermedia not only uses intraoral food processing, but that the elaborated morphology of its jaw joint facilitates mandibular motions in all three planes, resulting in complex three-dimensional chewing. Thus, our data challenge the commonly held view that complex three-dimensional chewing movements are exclusive to mammals, by suggesting that complex chewing mechanisms might evolved early in tetrapod evolution.