The mineralized remains of bones and teeth provide vertebrate paleontologists with most of the data they use for interpreting the biology of extinct creatures. By contrast, soft tissues (like skin or muscle) are extremely rare in the fossil record. Only a few cases of soft tissue preservation have been reported for large terrestrial vertebrates, such as frozen mammoths, 'mummified' dinosaurs, and in some cases dinosaur skin (or at least the impressions of skin). As a result, it's no wonder why the recent discovery of blood cells, vessels, and other soft tissues in the bones of Cretaceous dinosaurs like Tyrannosaurus was met with some skepticism.
Burke Museum research associate, Tom Kaye, was one such skeptic. To test the 'soft tissue' hypothesis, Kaye fractured the bones of a number of Burke Museum specimens, which included vertebrate bones ranging from the Pleistocene as far back as the Cretaceous, and looked inside to see if he could find structures similar to those reported by earlier workers. In 2008, Kaye and his colleagues presented the results of their electron microscope research, arguing that microscopic structures previously attributed to 'fossilized' soft tissues were more easily explained as filamentous and spherical bacteria and their associated biofilms. A 'biofilm' is a sticky substance (which may be composed of many different types of proteins) that is secreted by colonial bacteria to adhere to the surfaces on which they live, and can be found in abundance in almost any environment. Kaye and his colleagues suggested that the biofilms formed natural casts of the original biological structures, such as body vessel networks and hollowed recesses (lacunae) for bone cells—they therefore mimicked the shapes of blood vessels and bone cells. Bacterial trails observed, carbon dating on the films, and comparisons of infrared spectra in the biofilms with modern collagen further supported Kaye's claims.