The tall treetops of Sinnoh, with their deep roots that attach them to the back of the Torterra that forms the land itself, have typically served as nests and refuges for eons of flying and tree-climbing pokemon. But with the rise of the Cranidos, destructive headbutts shook the trees and sent their inhabitants plummeting to the ground, in a technique similar to that used today to obtain certain pokemon in Johto.
Tragically, one Cranidos' headbutt (Many had thought it a Rampardos, but the imprint has recently been discovered and it was surely a Cranidos skull) shook a tree too hard – not hard enough to topple the tree, but hard enough to bury much of Sinnoh, for the branches shook and an enormous wave of dirt, twigs, and leaves washed over the land, burying countless pokemon alive. Thus was Sinnoh's underground formed, and it should not be surprising that Cranidos are among the most common fossils to be found buried there, along with the short, slow-moving Shieldon, who are probably overrepresented in the fossil record.
Unlike fossils elsewhere, Cranidos are often found buried with vast caches of rare treasures, a fact which has sent treasure hunters digging and scientists speculating. The following can be surmised: Because Sinnoh's trees show no evidence of these items, they must have been placed intentionally beside the pokemon whose fossils are now so numerous in the underground. Cranidos themselves are by far the most likely candidate, for Shieldon do not have hands. Most likely, they collected these just as Meowth collect coins and Dragonite treasure. Yet Crandios' vast brain size and the value of many of these items for pokemon trainers today suggests another explanation: that pokemon training did not begin with Man's evolution, but is far older than he could possibly have imagined.