Chambered Cairn

Link

A picture of the cairn on this site - with the rather strange statement that:
"During excavation in 1963, some 5,000 seashell remains were found in the chamber, representing 15 different species. All had been eaten."

I'm not sure how you know if a long deposited seashell had been used for food or not. But more interestingly, why were the shells in the cairn chamber? Did the seashells had some other significance than food? or were they for the dead to eat? Or was the chamber just being used as a shell tip (surely not?)