In the March 2005 issue of Science, the paleontologist Mary Higby Schweitzer and her team announced the discovery of flexible material resembling actual soft tissue inside a 68-million-year-old Tyrannosaurus rex leg bone from the Hell Creek Formation in Montana. After recovery, the tissue was rehydrated by the science team.
When the fossilized bone was treated over several weeks to remove mineral content from the fossilized bone-marrow cavity, Schweitzer found evidence of intact structures such as blood vessels, bone matrix, and connective tissue. Scrutiny under the microscope further revealed that the putative dinosaur soft tissue had retained fine structures even at the cellular level. The exact nature and composition of this material, and the implications of Schweitzer's discovery, are not yet clear; study and interpretation of the material is ongoing.
Newer research, published in PloS One, has challenged the claims that the material found is the soft tissue of Tyrannosaurus. Thomas Kaye of the University of Washington and his co-authors contend that what was really inside the tyrannosaur bone was slimy biofilm created by bacteria that coated the voids once occupied by blood vessels and cells.