FIRST PALEOICHNOLOGICAL EVIDENCE FOR BABY-RIDING IN EARLY MAMMALS --- Preprint doi: 10.5710/AMGH.19.09.2018.3184

It is not yet known how old parental care in mammals is. One fossil trackway ascribed to Ameghinichnus patagonicus Casamiquela, 1961 of a mouse-sized primitive mammal from the Middle Jurassic of Patagonia, Argentina, almost 170 Myr in age, shows deviation from the usual bilateral symmetry: 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. The hypothesis is supported by experiments on laboratory rats. A plausible load would be babies riding on a milk-producing mother, as they do in extant opossums and many other mammals. Baby-riding is an underestimated possible feature of basal mammals, which could logically integrate evolutionary acquisition of such profound mammalian features as milk-feeding, endothermy, hairs, and limb parasagittalism.