They were revolutionary findings, delivered, fittingly, in a city in revolt. In 1796, at a Paris conference, the French anatomist George Cuvier presented his analysis of the so-called “Ohio animals.” These American fossils were elephant-like. But Cuvier showed they were in fact the remains of vanished animals – mastodons and mammoths. It was the first proof our planet’s fauna has undergone dramatic changes through time. Following Cuvier, paleontologists have since traced the epic story of evolution and extinction – with fossils as their guides.
Yet how do bones become fossils? Why do some bones get preserved while others don’t? The science that studies that journey – from dead animal to fossil – is called taphonomy. And Dr. Rachel Laker, of Hanover University, says Big Bend National Park is an ideal place to practice it.
“We're using fossils,” Laker said, “but I really don't care who it is or what it did or anything like that. I just want to know how it preserved. All I care about is that it's bone.”
Big Bend has a world-class fossil record, and its fossil-bearing strata include the Aguja Formation. These 70-million-year-old rocks have yielded tyrannosaurs, “duck-billed” dinosaurs and more.
They reflect a very different Big Bend. This was then the swampy edge of the Western Interior Seaway, which, at times, split North America. Some of the Aguja rocks were created in a shallow marine environment, others in a river delta.
Hiking and backpacking to Aguja outcrops across the park, Laker finds tiny fossil shards. With taphonomy, she glimpses their story – how long they were mixed on the seafloor or exposed on land before being sealed and buried.
“It's not perfectly clear cut,” Laker said, “like I can tell that it took 20 years for this bone to get buried – but I can distinguish bones that underwent really long amounts of reworking and shuffling versus bones that were one-and-done.”
Because for a bone, death isn’t the end, but a beginning. It may be scavenged or trampled, swept away by a stream or storm, waterlogged or desiccated. Looking at tiny fossil slices through a microscope, Laker sees where microbes tunneled into a bone to feast on its organic matter. Eventually, the bone was buried and underwent “diagenesis” – becoming part of the sedimentary rock.
“It’s nice to think that when we’re fossils we’ll all be in the same thin layer of rock,” the poets Jim Harrison and Ted Kooser wrote. And indeed, in thin layers of marine rock, Laker has found bones that may have been tumbled together across tens, even hundreds of thousands of years. And unexpectedly, she’s also found bones of terrestrial – or dry-land – origin in marine deposits. This is a bit of a mystery – but it may reflect dramatic mountain-building or other geological processes here.
To clarify what happens to bones as they bleach and weather on dry ground, Laker has expanded her study. She’s examining contemporary bones – primarily mule-deer antlers – in the park, to understand how the decay process unfolds in a hot place. She’ll compare that to bone decay in colder locales.
As much as scientists have learned since Cuvier’s lectures, our understanding of the Earth’s history remains partial. Geology and paleontology are key tools in unlocking that story. But taphonomy, Laker said, can reveal nuances of the planet’s changing lifeforms and landscapes.
“Not all the time does sediment tell the whole story,” she said. “It's sometimes erased; sometimes it's missing. Sometimes we don't have much to go on in terms of how these bones all ended up in that spot. I'm hoping to ask the bones themselves.”
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