It’s not quite Jurassic Park, but closer than most paleontologists ever imagined they would get. In April, North Carolina State University paleontologist Mary Schweitzer reported that she and her colleagues had sequenced proteins preserved in soft tissue remains from the 68-million-year-old leg bones of a Tyrannosaurus rex. When compared to all collagen sequences in available protein databases as well as sequences Schweitzer obtained in the lab, the T. rex protein fragments most closely resembled those of today’s chicken, making the first molecular link and bolstering the idea that contemporary birds descended from dinosaurs.
When Schweitzer first began working with the fossils, the specimens looked like others she had encountered, in which all the organic material has ordinarily decayed and been replaced by rock within a million years of the animal’s death. But something unexpected happened when Schweitzer dissolved samples of the fossilized bone in weak acid in preparation for analysis.
“It didn’t dissolve all the material, which is what is supposed to happen to normal, well-behaved dinosaur bones,” she recalls. “We were left with this scablike material that stretched and bent and folded.” Under the microscope, the tissue held more surprises, revealing structures that looked like blood vessels and bone cells.
Even more astounding, the team, which included Harvard University mass spectrometrist John Asara, was able to obtain sequences from proteins formed tens of millions of years ago. In addition to taking on the T. rex project, Asara also succeeded in wresting 76 collagen sequences from a slightly younger find—mastodon bone fragments with soft tissue estimated to be 160,000 to 600,000 years old. Some matched the mastodons’ closest living relative, the elephant. (Elephant protein sequences in present databases are incomplete, so other matches for the mastodon turned up among more distantly related mammals, including dogs, cows, mice, and humans.) Since then, Asara and his team have gotten more than a hundred total collagen sequences, showing an even greater similarity to today’s elephant.
The dinosaur sample, however, proved the most challenging. Asara estimates that collagen protein represented less than one one-hundredth percent of the material he received from Schweitzer, but following a difficult process of extraction and purification, he found six protein fragments, or peptides, that could be sequenced. Five were identical to sequences in chicken collagen, while frog and newt collagen tied for the next closest matches. Additionally, he found one novel T. rex peptide that did not match any other known organism.
Asara points out that his find does not mean chickens are the closest tyrannosaur relatives among modern birds, since he was able to compare the T. rex sequences only to species present in public protein databases. But identifying a bird as the nearest match validates researchers’ expectations based on skeletal evidence.
The real breakthrough, say the scientists, is not proving the link to chickens, which was expected, but learning that this kind of fossil preservation and molecular analysis is even possible in material over a million years old. “What’s really important,” Asara says, “is that now you can get these sequences and make evolutionary comparisons with them, which was never possible before with long-extinct organisms.” Schweitzer adds that paleontologists need to rethink how they handle fossil finds that could yield sequence data. “They’re pretty much doing the same field work as 300 years ago,” she says, “but now we really need to start thinking about paleontology differently.”
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