2. Three times, when we include birds (see Chapter 10).
from brain endocasts (see Figure 12.3b) suggests that the sense of smell was a powerful tool in evaluating a variety of environmental cues.
Clearly, however, sharp vision was key for theropods, so it is not surprising that their eye size was large. In deinonychids generally, but especially in troodontids, the eyes have migrated to a more forward-looking position, indicating overlapping fields of vision. Overlapping fields of vision almost certainly mean that these animals saw stereoscopically - that is, they merged the two separate independent images from each eye into a single image, much as humans and many modern carnivorous birds do today. Recent work suggests that the narrow snouts of even tyrannosauroids allowed a 55° range of binocular vision, not nearly as much as a human or an owl, but far exceeding what one might find in a hadrosaurid (Figure 9.19).
Hearing, too, is important to predatory animals and so it is not surprising that many theropods likely had good sound perception. Indeed, the middle ear cavity of troodontids and ornithomimosaurs was greatly enlarged, suggesting that these theropods were especially able to hear low-frequency sounds. In troodon-tids, detailed anatomical study of the outer and middle ears suggests that the group was capable of identifying the direction from which sounds came; knowledge that would have been of extreme use to a predator.
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