I had instance just like that last fall. The picture posted could have been a picture of my calf as that was all that was left of mine. Ribs intact and everything. When I got there about 10 in the morning, the cow was there trying to fight off the buzzards. all red headed buzzards by the way. Could not find any identifiable tracks, never found the hind legs. Another strange thing, was there was no blood on the ground that I could find. Cow had a long scratch down her side, not deep, looked like something she would have gotten from a tree branch type of thing. Older cow, never had a problem calving before. I have no idea what happened.