You need to check very carefully before you can conclude they were bio-degradable. I have only seen degradable plastic carry bags in the UK which were not in fact bio-degradable. Traditional plastic bags manufactured to be falling into pieces don't mean that their pieces are good for the environment (rather the inverse). Real bio-degradable bags do exist but they are weaker and feel rubbery, and they are probably more expensive to make and people likely don't appreciate them, so you generally don't see them (the bags distributed by some London councils for collecting compost are of the real kind). See this older comment of mine:
https://news.ycombinator.com/item?id=10763281