The crushing bite of tyrannosaurids

Abstract

Tyrannosaurids are a group of large meat eating theropod dinosaurs. Newly prepared material of Gorgosaurus libratus, Tarbosaurus bataar, and Tyrannosaurus rex provides additional knowledge of the structure and movement of the intramandibular jaw joint in the family. Paradoxically the theories of jaw movement and the fossil evidence in tyrannosaurids have not been comparable. It has been suggested that the joint had a wide range of movement to allow the gullet to expand, and to allow adjustment of the pitch and position of the mandibular teeth relative to the upper teeth (Maleev, 1974; Barsbold, 1983; Bakker et al., 1988; Molnar, 1991). A moveable joint suggested a feeding behaviour like lizards with a weak bite. On the other hand, the shortening and elevation of the skull in tyrannosaurids for insertion of powerful jaw muscles (Molnar and Farlow 1990), increased tooth robustness (Farlow et al., 1991), bite marks suggesting bone fragmentation (Erickson and Olson 1996), estimation of bite force from tooth-marked bone (Erickson et al., 1996), and bone fragments in coprolites (Chin et al. 1998) all suggests high bite force. Here we describe the intramandibular jaw joint and report on the discovery of the first completely preserved and fused supradentary/coronoid bones in three different tyrannosaurids. The fused supradentary/coronoid bone crosses the intramandibular jaw joint restricting its movement. This leads to a rigid lower jaw in tyrannosaurids that is a secondary specialization for a powerful crushing bite. Institutional Abbreviations-BHI, Black Hills Institute, South Dakota, USA; TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; ZPAL, Institute of Palaeobiology, Academy of Sciences, Warszawa, Poland.