Early Four-Wheeled Vehicles. The earliest evidence for wheeled vehicles in Mesopotamia is in the form of pictographic proto-cuneiform signs on clay tablets from Uruk, dated to the end of the fourth millennium B.C.E. Pulled by oxen, the vehicles appear to have been sledges equipped with a roofed superstructure. Rather than being dragged across the ground, they have been set on wheels or rollers. During the third millennium B.C.E., two- and threedimensional representations of two-wheeled carts and four-wheeled wagons were typically depicted in military contexts. The four-wheeled vehicle termed a battle car was narrow, with low sides and high front breastwork topped by an open rail. The vehicle had solid composite-disk wheels pieced together from three boards; these wheels rotated on the ends of round axles fixed beneath the wagon. Two pairs of onagers, or wild asses, were...