Because of the presence of iron, the spheres were thought to be remnants of blood cells. But Kaye's team concluded the spheres are tiny clusters of minerals and iron called framboids.
The team then dissolved some of the bone in acid, as Schweitzer's team did, and found the soft-tissue-like structures coating the blood-vessel walls.
They determined the structures were more closely related to modern slime than with proteins associated with bone. Carbon dating suggested some of the slime samples were fewer than 60 years old.
The team also found bubbles in the slime coating like those produced by methane-breathing bacteria, and trackways across the bones likely left by the bacteria as they scooted through the slime.
"Bacteria are nowhere near as exciting as soft tissues," Kaye said of his findings. "We have to go, though, where the science leads us."
Soft Tissue Finds Defended
Schweitzer, the lead author of the 2005 Science study, defended her hypothesis that the tissue-like structures that her team analyzed are indeed—at least partly—preserved soft tissue.
"The idea that biofilms are completely and solely responsible for the origin or source of the structures we reported is not supported," Schweitzer wrote in an email statement.
She noted that the scientific literature lacks any evidence that such slime coatings form branching, hollow tubes, as her team observed in the T. Rex specimen.
Given the force of gravity, she added, if biofilms were the source of the vessel structures, they ought to be thicker towards the "bottom" of the bones and not evenly distributed across the vessel walls, as her team found.
Nor, she noted, is there any evidence methane-breathing organisms were in the bone to produce the "bubbles" seen by Kaye, or, for that matter, evidence that bacteria ever inhabit bone material.
In addition, Kaye's team largely refrained from addressing several follow up studies by Schweitzer and her colleagues that present chemical and molecular evidence to support the soft tissue claim, she pointed out.
(Related: "Dinosaur Soft Tissue Sequenced; Similar to Chicken Proteins" [April 12, 2007].)
Kaye said his research would not refute a protein analysis. "If they say they got T. rex protein, then we're not disagreeing," he said.
But he questioned why Schweitzer's team only found such a small amount of protein.
Hans-Dieter Sues is a paleontologist at the National Museum of Natural History of the Smithsonian Institution in Washington, D.C.
He was not involved with either research team, though he said both make compelling arguments.
For example, fossilized hair and tissue impressions are often petrified bacteria arranged, ghost-like, in the outline of the preserved material, Sues noted.
"So to find something like that in these dinosaur bones is not at all unexpected," he said.
(Related find: "'Mummified' Dinosaur Discovered In Montana [October 11, 2002].)
Nevertheless, Schweitzer's chemical data seems to suggest that the tissue-like structures are at least partially composed of dinosaur proteins.
Yet even that is disputable, Sues added.
In a you-are-what-you-eat fashion, bacteria that fed on blood vessels and became petrified, for example, might retain some of the molecular information from their last meals, he said.
"I think you do have two very interesting alternative hypotheses," Sues said.
"And frankly, at this point, I don't know which one to put my money on."
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