The phrase also seems to be used by paleontologists as an adjective to describe fossils found where in the same strata they were originally embedded in. You can google the term "in situ fossils."

Example usages at

http://en.scientificcommons.org/33282359

and

http://onlinelibrary.wiley.com/doi/10.1111/j.1502-3931.2007.00029.x/abstract