Fossilized Dinos Are Bones Turned to Stone—But Sometimes, Part of the Original Dino Survives

Every fossilized skeleton is a unique snowflake. We now know that some contain biological bits of tooth and claw

The well-known dinosaur skeleton known as "Big Mama" has been hiding a secret. © AMNH / M. Ellison

Everyone learns in school that that dinosaur skeletons are bones turned to stone. Mineral-rich water percolated through the earthly remains of the ancient reptiles, transforming their osteology into something much harder over time. But over the past decade, paleontologists have begun to realize what “everyone knows” isn’t quite right. Every skeleton fossilizes a little differently, and, in special circumstances, some of the original biological tidbits of ancient dinosaurs can withstand millions upon millions of years.

The first such find was announced back in 2005, when biologist Mary Schweitzer and colleagues announced that they had found the remnants of blood vessels and other soft tissues in the femur of a Tyrannosaurus. At the time, the announcement was controversial; no one expected that such tissues could last so long. But over the following decade, Schweitzer and others have built a convincing case that dinosaur bones are not wholly stone, but to varying degrees retain some of the original tissues that grew when the animal was alive.  

Now, paleontologists have identified an even more surprising substance frozen in time: The claw of a famous skeleton, its original material still intact. It’s another sign that the fossil record is capable of preserving far more than previous generations of paleontologists ever expected—including the potential to offer a spate of new clues about dinosaur biology.   

Drexel University paleontologist Alison Moyer, working with colleagues Wenxia Zheng and Schweitzer, announce the find today in Proceedings of the Royal Society B. The fossil under the microscope was a skeleton of a roughly emu-sized, parrot-like dinosaur called Citipati that had been found perched atop a nest of eggs. Paleontologists know it affectionately as Big Mama. And while the image of a dinosaur brooding its unborn young presents a bittersweet moment caught in time, there’s something else remarkable about Big Mama as well: the dinosaur’s preservation. At the tips of the fingers appear to be remnants of the tough sheath that once covered the dinosaur’s claws.

Moyer had been studying beta keratin—the tough protein common to reptilian skin and claws – as part of her PhD research in Schweitzer’s lab. “Mary told me she had a sample of it and never got around to analyzing it,” Moyer says. Knowing how beautifully-preserved Big Mama is, Moyer decided to set about seeing whether there might be any original material left in those curved claws.

Moyer and colleagues analyzed the claw by comparing its microscopic structure to those of living birds, as well as using microbiological techniques to test for the presence of beta keratin. They found that the long-dead dinosaur’s claw had a similar structure and biological makeup as those of living, avian dinosaurs. Remarkably, some of the original proteins are still present after over 75 million years.

So does this discovery mean we’ll soon be getting Jurassic Park? Sadly, no. These are remnants of long-lived proteins, not DNA, which decays at such a rate that even in the best circumstances tatters of genes can only last about 6.8 million years before being completely obliterated. Nevertheless, Moyer points out, the discovery underscores that exceptional circumstances can preserve dinosaur biology to a microscopic levels of detail that are only just now being appreciated.

“The animal was obviously buried very rapidly in a sand dune event, as indicated by its death position sitting on a nest of unhatched eggs,” Moyer says. This shielded Big Mama’s body from scavengers and the harsh outside world. High levels of calcium in the claw, Moyer says, suggests this mineral maybe have helped the preservation of the proteins inside—a happenstance that allowed Big Mama to come to us in such high fidelity. “This adds to the growing body of evidence that soft tissues and biomolecules preserve over time,” Moyer says, “not just in bone but other structures as well.”

Multiple lines of evidence—from anatomy you can see with the naked eye to biomolecular signatures—are reiterating that there’s more to dinosaur bones than we ever knew. Paleontologists haven’t yet come across soft organs or skin, but they could be out there, waiting in the rock. The task is to go find them, Moyer says. “We don’t know if or what is preserved unless we look.”

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