The evolution of predatory adaptations in vertebrates reached an awesome yet elegant apex in the anatomy and behavior of carnivo- rous dinosaurs. One should not dwell too long on the fate of the dinosaurs that fell prey to these creatures, however, because the prey animals also extended the size, specializations, and population numbers of their kind to similarly impressive levels. The existence of large predators such as Allosaurus and the later tyrannosaurs, spinosaurs, and carcharodontosaurs is evidence that prey animals such as iguanodonts, hadrosaurs, sauropods, and horned dino- saurs existed in great numbers and so successfully occupied their
respective niches in the world of browsing vegetarians that preda- tory dinosaurs had to adapt highly specialized weapons and tech- niques to hunt them down.
The predatory lifestyle of theropods is reflected in many aspects of their anatomy. Clues in the bones of theropods allow paleontolo- gists to reflect on the likely behaviors and lifestyles of these animals as living organisms.
senses
Predatory creatures rely on their senses to find their next meal. The olfactory organs enable a predator to detect the smell of a prey animal or carcass long before it is in sight. The otic, or auditory, function alerts a predator to the spatial proximity of a prey animal before it can be seen. A keen optic function—excellent vision—is important to pinpoint and run down the prey once it has been located. All of these senses were certainly important to predatory dinosaurs, and their skulls sometimes contain clues as to the acuity of these senses.
The brains of modern vertebrates—and particularly the brains of reptiles and birds—are similar in many ways. The braincase in the skull, including the brain cavity, holds the brain and the many con- necting nerve bundles that connect the brain to other parts of the body. Nerves connect the brain to other organs through holes in the braincase. Dinosaur skulls show many of the same kinds of connec- tions found in modern vertebrates. The sense of smell was located at the front of the brain, in the olfactory lobe, and vision was concen- trated in an optic lobe near its center. Study of the fossil braincases of various theropods confirms that their senses were moderately to highly acute, as one would expect for specialized predators.
Vision. The eyes of theropods were larger than those in other
kinds of dinosaurs of similar size. The eyes were especially large in the smaller, later, big-brained theropods such as Troodon.
While most theropods had eyes that could look only to the sides, like those of a horse or a lizard, many of the later theropods, including Tyrannosaurus, had eyes that were more forward-looking,
overlap with each another. Binocular vision allows an animal to more accurately track and pinpoint objects—such as moving prey— in its field of vision, thus making it easier to focus on a prey animal that moves in an evasive manner.
In a recent study of theropod skulls, paleontologist Kent Stevens analyzed the binocular field of vision of seven theropod dinosaurs. He found that the eyesight of less advanced theropods, including the basal tetanurans Allosaurus and Carcharodontosaurus, was not as binocular as that of more advanced and later theropods such as Tyrannosaurus, Troodon, and Velociraptor. The tyrannosaurs had particularly excellent binocular vision, equaling and possibly exceeding that of the modern hawk, a bird known for its impressive vision.
Hearing. The acuity of theropod hearing can be determined by
examining the brain cavity as well as the parts of the skull that may have contained hearing bones.
Some theropods appear to have had special adaptations for hearing. Troodon and some other small- to medium-sized coeluro- saurian predators had large, complicated inner ears that probably helped them detect the source of a sound more accurately. These adaptations also may have allowed the animals to detect low- frequency sounds more readily, such as the distant footfall of a large plant eater or even the low bellow of a duck-billed dinosaur com- municating with its herd.
Smell. In its arsenal of sensory weapons, a theropod’s sense of
smell may have ranked up with its eyesight and good hearing. The olfactory potential of tyrannosaurs has been studied closely by Christopher Brochu of the Field Museum of Natural History. Brochu has done extensive computed tomography (CT) work on the skull of “Sue,” one of the most complete Tyrannosaurus skel- etons ever found. It was nicknamed after Sue Hendrickson, the field paleontologist who found it. Brochu fabricated a digital endo- cast of the brain and olfactory bulbs of Sue. The olfactory bulbs of
this specimen were proportionately larger than those seen in other examples of theropod skulls, suggesting that these giant coelurosau- rian theropods had achieved remarkable senses of smell. The hole in the braincase through which these nerves passed was about the diameter of a peach. By comparison, a smaller hole in the back of the braincase, where the brain connected to the spinal cord, is closer to the size of a grape. Both Allosaurus and Ceratosaurus, from the Jurassic, have been shown to have relatively large olfactory bulbs and therefore good senses of smell, though perhaps not as good as tyrannosaurs