Can you Identify Strange mounds in Eagle Sandstone Montana ?

[AU79]

I found these Strange mounds in the Eagle Sandstone north of Billings Montana, I spoke to a Geologist at MSU and he said he thought they were fossil Ammonites but they dont have a spiral shape and some of the mounds have interconnecting tunnels as though the organism moved under the sand. Any Ideas photos http://www.flickr.com/photos/74583211@N03/sets/72157628925927805/

 

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near Friant California there are similar mounds. From what i understand they were made by prehistoric ants. For a long time i thought they were tailing piles.