Meeting Abstract
The otic joint is functionally important in many sauropsids, as it often allows movement during feeding, a condition known as streptostyly. This type of cranial kinesis is found in many birds, as shown by in vivo studies and skeletal manipulations. Many inferences of kinesis at this joint in fossil theropod dinosaurs have been made using morphological arguments, even though its structure-function relationships are still poorly understood in extant birds. To document the numerous changes that occurred in the morphology, composition and function of the otic joint during theropod evolution, we used microCT scanning and histology of five species of birds that display a range of kinetic behaviors: the emu (Casuariiformes), the mallard duck (Anseriformes), and three species of parrot (Psittaciformes). We also sampled two species of non-avian theropod dinosaur (Tyrannosaurus and Allosaurus) and outgroup taxa including the American alligator, Common snapping turtle and several species of lizard. All the avian species show similar histological characteristics including a synovial microstructure with secondary articular cartilage on the squamosal. We found these histological characters suggestive of avian-style streptostyly in the otic joints of Tyrannosaurus and Allosaurus. These results suggest avian-style otic joint microstructure, and possibly streptostylic movements evolved prior to the origin of birds. This study reveals the histological underpinnings of avian-style cranial kinesis, retraces the evolution of streptostyly in theropod dinosaurs, and illuminates the diversity of cranial joint structure and function in sauropsids and vertebrates.
