STEREOPHONIC TYRANNOSAURS: EXTERNAL EAR CHANNEL PROVIDED PRECISION IN LOCATING PREY

The fossil record of apex predators on land shows strong trends for faster locomotion and greater sensory precision. The earliest mammalian apex predators, Paleocene mesonychids and oxyaenids, had eyes directed mostly laterally so stereoscopic vision was limited. Flexed limb joints, spreading feet and short lower limb segments indicate relatively low pursuit speeds. By Miocene times, cats, wolves, and hyenas had orbits more forwardly directed, providing stereoscopy and great depth perception. Longer, more compact limbs provided higher speed. Most mammals also posses external ears that can be flipped from facing laterally to facing forward. Did dinosaur communities show similar trends? In early, large predatory dinosaurs, the lower limbs were of modest length. Orbits face mostly laterally and only modest stereoscopy was possible. The latest common apex predators in North America and Asia, the tyrannosaurs, had elongated, compact lower limbs, suggesting faster sustained speeds. Tyrannosaurs display extraordinary expansion of the posterior skull width, producing dramatic forward reorientation of the orbits, conferring stereoscopy.

Did tyrannosaurs develop stereophony? Primitive dinosaur eardrums were set in tympanic sheets stretched across the otic notch, bounded anteriorly by the squamosal-quadratojugal strut. Mammal-like external ears almost certainly were not present, so the ears faced laterally. However, modern clades of small, flying dinosaurs, the owls and harrier hawks, achieve stereophony with a face disk of feathers and a skin-and-feather auditory channel that directs sound from the front into the ear drum. In most early dinosaur predators, there is no indication of any channel to redirect sound coming from the front. But in tyrannosaurs a channel seems to be produced by forward expansion of the quadratojugal strut, which carries a ledge along its lower border. We suspect that an auditory meatus lay along the ledge. The great width of the skull would amplify the stereophonic function. Tyrannosaurs may have had the greatest depth perception of any large dinosaurs. This notion can be tested with recovery of skin-covered skulls from Liaoning-style quarries.