They look like concretions to me as well. Plus, they are not really that
big. The two people in the photo are midgets.
G. Thomas Watters, PhD
Curator of Molluscs
Museum of Biological Diversity
Department of Evolution, Ecology and Organismal Biology
The Ohio State University
1315 Kinnear Road
Columbus, OH 43212 USA
v: 614-292-6170
f: 614-292-7774