As Wargram said, you need to beat everyone of the missions on the circle of missions on Archlyte Steppe, which are missions 56-62
Bear in mind that once you get both Shao Long Gui and Long Gui roaming around the Steppe, normal Adamanchelid, Adamantoise and Adamantortoise won't appear any longer there. The only place that they respawn is on Eden.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
