From that first link it says: "Their burrows go down in a steep spiral to a depth of around two metres". There is an extinct marmot-like animal related to beavers that made spiral tunnels about that deep. Now they are finding fosselized tunnels all over the world that they are saying are made by the same creature... I wonder if some were made by ancient bilbies instead... maybe the beaver-like critters are actually squatters.

Here are some links about the fosselized spiral burrows:
http://annbat.livejournal.com/1695.html#cutid1