First Record of Middle Eocene Elephant Ancestors’ Footprints in the Gonjo Basin, East Tibet Plateau

It is the first time that the fossil footprints of a group of Middle Eocene elephant ancestors have been discovered in the Gonjo Basin, East Tibet Plateau. The Gonjo Formation is attributed to the Middle Eocene Epoch (U-Pb age = 44.7 ± 1.2 Ma) and consists mainly of purplish-red, medium- to coarse-grained sandstones, siltstones interbedded with mudstones, and conglomerates with sedimentary structures like ripple marks, rip-up clasts, and trough-cross bedding, suggesting fluvial-lacustrine systems. The group of fossil footprints has a characteristic oval-concave shape, and the toe impressions are absent. Some fossil footprints are overstepped with a pockmarked texture resembling Proboscipeda enigmatica. More than 165 fossil footprints of the group are relatively well-preserved with different diameters, which is evidence of highly social behavior and trackmakers of different ages, including calves, juveniles, adolescents, and adults. The size frequency of the fossil footprints enabled us to deduce the body mass, shoulder height, and hip-height distribution of the trackmakers that crossed the East Tibet Plateau 44.7 Ma ago. The trackmakers comprised an estimated average hip-height of 111.8 cm, an average shoulder height of 172.8 cm for males/155.9 cm for females, and an average body mass of approximately 1 218.1 kg for males/907.8 kg for females. The abundance of fossil footprints reveals that in the Middle Eocene Epoch, the environment was extraordinarily conducive for the elephant ancestors to live in the East Tibet region.