Filed under "Hang on a sec": a new scientific paper has called into question one of the most exciting paleontological finds of the 21st century. Soft tissue discovered deep inside aTyrannosaurus rex legbone may be a recent "biofilm" (what you might call scunge if you found it on a dishrag), not remnants of the Toothy One after all. That's the suggestion of a team led by Thomas Kaye, writing in the scientific journal PLOS One. Avid Smithsoniacs and dino fans may remember bits and pieces of this story. In 2005, paleontologists Mary Schweitzer and Jack Horner were stuffing a T. rex femur inside a too-small helicopter on their way home. They cracked the bone in half to make it fit, and Schweitzer noticed a goopy residue on the 65-million-year-old insides of the bone (see theSmithsonian story). Then this April, Schweitzer and her colleagues isolated a protein called collagen from the sample, analyzed it, and found striking similarities to the collagen of modern birds. Kaye's contradictory opinion comes from using an electron microscope to peer at similar residues he found in different fossils. Studying fossils of 17 dinosaur and mammal species, Kaye and his team saw evidence of biofilms, or slime left behind by bacteria that grew on the bone long after the dinosaur's death. Where Schweitzer's group described the remains of red blood cells, Kaye's team thought they were seeing iron-rich structures routinely built by bacteria. (The iron content and the structures' characteristic shape might have made them look like red blood cells in some analyses, Kaye suggested.) Kaye found these structures time and again in his samples - even in a fossilized shell, which never would have contained blood at all. Worst of all, carbon dating suggested the biofilm was as recent as 1960. Of course, there's still the matter of the collagen's similarity to chickens and ostriches - a detail Schweitzer was quick to point out to reporters. And Kaye didn't sample theT. rex in question, leaving open the chance that Schweitzer's find was the genuine article. Personally, I'm leaning toward believing in the extraordinary. At least until the collagen results are explained (I mean, can anyone tell me if bacteria even make collagen?) Either way, it's fascinating to listen to the well-constructed arguments on both sides. That's what science is all about.