Aetiology

An interesting new paper is just out today in PLoS ONE. You recall the announcement a few years back that soft tissue that resembled organic tissue had been isolated from a Tyrannosaurus femur. This started off a huge controversy in the field (and beyond)--researchers disagreeing with each other whether the structures seen were indeed blood cells and vessels; creationists crowing about how this finding represented "proof" that the earth was indeed young and dinosaurs had existed just a few thousand years ago; and of course, talk of cloning and DNA analysis. On the side of "soft tissue = dino blood" were findings that reported identification of the iron-containing protein heme (potentially from the red blood cells) and morphology of cells and vessels similar to that seen in modern-day ostriches and emu. However, the new paper by Kaye et al. provides an alternative explanation: that the structures aren't actual vessels and cells, but are instead iron-rich bacterial biofilms. More on that below.

First, a bit about biofilms. These are the sticky structures that form when bacteria adhere to a surface and form "communities" of organisms (as opposed to their free-living, "planktonic" stage). The plaque on your teeth and tongue; the scummy ring around your bathtub; the slippery coatings on ocean rocks are all composed of biofilm, which itself is made up of not only bacterial cells but also the matrix they produce that allows them to adhere to the surface (and also provides them protection from the elements around them). Though we've studied bacteria in their planktonic state for much of the history of microbiology, this is quickly changing as we realize the ubiquity of biofilms not only in the environment around us, but also within our own bodies (where biofilm-associated bacteria are much more resistant to the effects of antibiotics, for instance).

So, biofims are everywhere--but *inside* bone? That was the question examined in the new paper. To do this, researchers fractured a number of specimens from different ages and locations and examined the hard and soft tissue they found inside. They found remarkably similar structures in each case after examining them with scanning electron microscopy (SEM), which allowed them to view the surface and shape of the soft tissue material; and with energy dispersive spectroscopy (EDS) and Fourier Transform Infrared Spectroscopy (FT-IR) to look for molecular "fingerprints."

What they found provides an alternative hypothesis to the previous "dino blood" findings. The iron present (and thought to have come from blood cells) could be explained by the presence of iron-containing framboids: spheres commonly found in sediments. Blood vessel-like structures were found but also could be attributed to biofilm, and when compared by FT-IR to lab-grown biofilms, the chemical signature of the fossil structure more closely resembled modern biofilms than modern collagen. The authors argue that the biofilm hypothesis better explains the data, including the ubiquity of these structures in fractured fossils:

This investigation contends that iron-oxygen spheres are far too common in many formations to be the result of extraordinary preservation. Framboid morphology and elemental signature may superficially make them appear to be related to biological structures but they are, in fact, an inorganically produced mineral feature often found in association with organic matter.

Arrows on this electron microscope image indicate biofilms, or slime, peeling away from the walls of vascular canals in dinosaur bone. Credit: Thomas Kaye.

But what about the dinosaur collagen claimed to be found in a prior study? The authors explain that as well:

Recent protein work by Asara et al. examined ground tyrannosaur bone under a highly sensitive mass spectrometer. This resulted in seven recovered protein sequences attributed to the original tyrannosaur but only in femptogram quantities (10−15 gram moles). The additional detection of bacterial proteins, identified at the species level as the decomposing bacteria Rhodococcus sp. showed conclusively that bacterial contamination was present, even though the original bone was deeply buried. Rhodococcus sp. exhibits morphological differentiation and can be found as both cocci and filaments consistent with forms found in lacunae from this survey (Fig. 10). Recent discoveries of collagen-like proteins in bacteria and viruses add to the problem of unambiguous identification of vertebrate biomolecules.

This paper seems to deal a pretty convincing blow to the "dino blood" theory, but I'm anxious to see what the dino experts have to say about it, including the original proponents of the hypothesis.

Kaye, T.G., Gaugler, G., Sawlowicz, Z., Stepanova, A. (2008). Dinosaurian Soft Tissues Interpreted as Bacterial Biofilms. PLoS ONE, 3(7), e2808. DOI: 10.1371/journal.pone.0002808