It is not yet known how old parental care in mammals is. One fossil trackway ascribed to Ameghinichnus patagonicus of a mouse-sized primitive mammal from the Middle Jurassic of Patagonia, Argentina, of almost 170 Ma, shows deviation from the usual bilateral symmetry of posture and motion: the left forelimb digits were dragging in the swing phase, and the tail was bent to the right and dragged over the right footprints. However, the footprints are evenly spaced, indicating that the animal was not limping. The opposition of the tail and the forelimb dragging marks could possibly be explained by a heavy load carried on the left side. This hypothesis is supported by experiments on laboratory rats. A plausible load for Ameghinichnus track-maker could be babies riding on a milk-producing mother, as they do in extant opossums and many other mammals. Baby-riding is a missed possible feature of basal mammals, which potentially integrates evolutionary acquisition of profound mammalian features, such as milk-feeding, endothermy, hairs, and limb parasagittalism.