Balaeric slingers were famous (or infamous) for their ability to inflict serious wounds! We think of stones tossed by people as generally only a nuisance, a la kids throwing rocks or slingshots, but skilled slingers can lob a pretty hefty stone a good distance to break bones and dent skulls - even through armor.

In this particular case, the Balaeric slingers killed and drove off the Roman light infantry, and seriously damaged the heavy infantry as they crossed the river before engaging with the main Carthaginian force (the Romans were subsequently defeated).