Laterality — or hemispheric specializations within the brain, the thing that makes us right- or left-handed — was once thought to be a human-only trait. Research has found, however, that this trait is shown by many nonhuman species, too! A multitude of organisms demonstrate differential brain hemisphere control for behaviors like vocalization, escape reactions, and feeding.

Knowing that nonhuman animals exhibit laterality in their behavior raises several questions: how far back in history does laterality go? How many extinct organisms may have had lateral hemisphere control of their behaviors? And, is it even possible to figure out whether prehistoric animals did display laterality?

Much of the present research into lateralization in living organisms uses non-invasive observations of behavior. Since we can’t do that with extinct organisms, we have to turn to fossil records to see what clues we might find about their behavior.

Nobu Tamura, Wikimedia Commons

A recent study cleverly used the fossilized dental records of an extinct reptile, Captorhinus aguti, in an effort to determine if this reptilian ancestor showed a lateralized feeding preference. Investigations of 89 intact jaws of Captorhinus resulted in a clear pattern: there was much greater wear on the teeth of the right side of the jaw, suggesting a clear right side preference for feeding. That is, Captorhinus reptiles were ripping, tearing, and chewing food with and on the right side of the jaw significantly more often than with the left side of their jaw. This preference for right side control of feeding behavior has been maintained in present-day species like toads and domestic chickens .

The Captorhinus fossil records demonstrate that this reptilian ancestor showed lateralized responses with the crucial-to-survival behavior of feeding. But this finding raises further questions. What other functionally relevant lateralized behaviors may have existed in the lives of extinct organisms, and what might such lateralized behaviors tell us about the evolution of hemispheric specializations of vertebrate species alive today?