AFTER A HIGH-PROFILE paper in 2002 confirmed that mysterious pimple-like dots covering the snouts of amphibious alligators and crocodiles were, in fact, sensory receptors alerting gators and crocs to the movements of potential prey in the pond, dinosaur experts like MU’s Casey Holliday were more than a little intrigued.
“Currently, we rely on alligators, crocodiles and birds to provide us with information about how ancient reptiles, such as pre-historic crocodiles and dinosaurs, functioned. However, the first thing we have to do is to understand how the living animals function,” says Holliday, an assistant professor of pathology and anatomical sciences whose work on dinosaur locomotion was featured in the Spring/Summer 2011 issue of Illumination.
The earlier research had determined that the snout bumps were hard wired, neurologically speaking, to detect even the smallest ripples in gators’ and crocs’ watery habitat. Even a single droplet hitting the surface several feet away, for example, was sufficient to generate a signal that, when routed through a bundle of nerves called the trigeminal, alerted the reptiles’ brains to the possibility that dinner was on offer.
But the anatomy of this stimulating nerve — a version of which humans share — remained murky.
In a study co-authored with doctoral student Ian George, Holliday has helped bring things into focus. “We’ve never measured the size of the nerve bundle, or ganglion, in their skulls, until now,” says Holliday. “When compared to humans, this trigeminal nerve in crocodiles is huge.”
The trigeminal nerve is rooted inside the head, then travels through a large opening in the skull before it branches out to reach the crocodile’s facial skin. Holliday and George examined how skull size, brain size and ganglion size relate, and used their findings to estimate face sensitivity. The goal, Holliday says, was to extrapolate from these results information about animal behavior both ancient and modern.
After comparing the size of the opening for the trigeminal nerve in alligators to those of fossil crocodilians, George found the holes in equally sized, land-dwelling crocs were much smaller. This implies that these ancient crocodilians did not have the vibration-sensing adaptations that their modern, aquatic relatives possess.
“Some species of ancient crocodiles lived on land and they probably wouldn’t have a use for a sensitive face that can detect disturbances in the water,” says George.
“So our next step is to trace back and determine when the nerve got bigger and see how that might have paralleled the animals’ ecology.” Holliday says future research in a similar vein could lead to a better understanding of the anatomical basis for behavior in other living animals, including fish, electric eels, platypi and humans.
“The same way that we would look at the size of the visual cortex in the brain to understand how well an animal might see, we can now look at the trigeminal nerve in animals to determine how sensitive their skin on their faces is,” Holliday says.
The study was published in the May issue of the journal The Anatomical Record.