Fig. 4. Forces involved in the static equilibrium calculations of the lower and upper jaws of H. francisci. F_B, bite reaction force; F_E, reaction force at the ethmoidal articulation; F_H, reaction force at the hyomandibular articulation; F_JR, jaw joint reaction force; F_PO-, force generated by the preorbitalis-; F_QM-PO, force generated by the quadratomandibularis-preorbitalis complex; F_QM-, force generated by the quadratomandibularis-; F_R, resultant adductive force; , angle of incidence of F_E relative to the articular surface of the upper jaw at the ethmoidal articulation. Arrow size does not indicate force magnitude, and angles of force vectors are approximate. CT-scan image used with permission of A. Summers.