Dinosaur bones are not always permineralized, however. In Dinosaur Provincial Park in Alberta, dinosaur bones were sometimes encased in ironstone nodules shortly after they were buried 75 million years ago. The nodules prevented water from invading the bones, which for all intents and purposes cannot be distinguished from modern bone…A more spectacular example was found on the North Slope of Alaska, where many thousands of bones lack any significant degree of permineralization. The bones look and feel like old cow bones, and the discoverers of the site did not report it for twenty years because they assumed they were bison, not dinosaur, bones
Curie, Philip J. and Koppelhus, Eva B Questions about Dinosaurs, Dover Publications, 1996, Pg.12.
Note: Dr Philip Curie is the founder and curator of the Royal Tyrrell Museum of Paleontology in Drumheller, Alberta, Canada, and is Professor and Canada Research Chair in Dinosaur Paleobiology at the University of Alberta.