For Mary Schweitzer, an accidental discovery — and an open mind — led to a new field of paleontology.
It’s June in Montana. Paleontologist Mary Schweitzer is with a small group of other scientists, prospecting for fossils. Carrying heavy backpacks, they scramble up and down the rugged badland slopes, picking their way carefully through brush and stones and keeping a wary eye out for rattlesnakes.
If they’re lucky and the weather holds, they can cover 7 to 10 miles of terrain a day. As they walk they scan the ground for slivers of color that stand out against their surroundings, because those slivers may be fragments of dinosaur bones.
Montana is prime dino-hunting country. Dinosaur bone can “weather” out of the ground naturally, leaving small exposed pieces that are easy to spot — if you know what you’re looking for.
“We use geological maps to figure out where the right age rocks are, then we get up early to avoid the heat, load up with all the water we can carry and other tools we need, and start walking,” says Schweitzer, a professor of biological sciences at NC State and research curator at the North Carolina Museum of Natural Sciences. “You can recognize the paleontologists because they’re hunched over, sweaty, with red eyes, wrinkled and sunburned skin, and dirt under their fingernails. It’s not glamorous, but it’s the only way to get what we need.”
The bones she finds won’t end up on display in a museum – they’ll end up inside test tubes and microscopes.
She wants bone that she can destroy to unlock the secrets preserved
within: original blood vessels, cells and even proteins that still
remain. Schweitzer is a pioneer in the field of molecular paleontology,
which aims to recover original molecules preserved inside fossils for
millions of years.
An Unconventional Approach
In 2004, Schweitzer decided to place a piece of a Tyrannosaurus rex’s thigh bone in a solution that removes minerals from bone. To say that this was not standard paleontological practice is an understatement. Conventional wisdom was that fossils were essentially bone-shaped rocks, so removing minerals would destroy the fossil.
It was serendipity, but I also like to think of it as prepared luck.
“It was serendipity, but I also like to think of it as prepared luck,” Schweitzer recalls. “If I hadn’t studied reproductive physiology for a completely different reason I wouldn’t have recognized the medullary bone to begin with.”
Schweitzer’s discovery was the first of its kind, and interest in her work exploded when she published her findings in the journal Science. She immediately started working to support her findings and in the process, she became a pioneer in an emerging field of paleontology — molecular paleontology.
Ancient Materials, Modern Tools
To argue that the blood vessels and cells she sees after demineralizing fossilized bone are original to the dinosaur, Schweitzer has to be able to do two things: rule out contamination and repeat her results. And she draws from a variety of disciplines to help her do so.
Starting in the field, the samples she takes are prepared very differently than those of traditional paleontologists. “Normally paleontologists uncover the bone and then slather it with glue so they can transport it in one piece,” Schweitzer says. “Then they take it back to the lab, uncover it completely and prepare it for study or display. I don’t do any of that.”
After identifying an area with rocks of the right age, Schweitzer heads out into the field with the most fundamental of tools — her eyes. She and her team walk up to 7-10 miles per day, scouring the landscape until they find an area with bone fragments weathering out. Tracing the layers where these bones originate may lead to well-preserved bones.
Because preservation of the bone is key for Schweitzer, she uses special recovery processes to get it from the field to the lab. When possible, she removes the whole bone and surrounding sediment and covers them with plaster. When the whole bone isn’t available, she uncovers and removes the sample and surrounding sediment from the rock. Wearing gloves, she wraps it in sterile tin foil to protect it from contaminants and seals it in a sterile glass jar with desiccating crystals to keep it from absorbing moisture.
Back in her lab, Schweitzer’s research team examines thin sections of bone under a microscope. Then, they use a mildly acidic solution to dissolve the mineral in the bone to reveal any soft tissue that’s present. If they are able to find enough of the soft tissue’s key components, her team uses mass spectrometry and antibody tests to identify proteins that give her important information about the dinosaur she’s studying. Each test yields subtly different information about aspects of fossil preservation.
Schweitzer’s recovery process focuses on keeping the bone free from exposure to contaminants and in its original sediment as much as possible. “The idea is to keep it away from modern water and air and isolate it from other bones or tissues, because as soon as you take it out of the environment it’s been in for 65 million years it becomes subject to degradation and contamination,” she says.
When she gets the bone back to her lab, she and her team look for soft tissue. If the tissue meets certain requirements, they use mass spectrometry, which identifies protein fragments by their molecular weight, and antibody testing, which uses antibodies that react to specific proteins, to confirm the presence of proteins in the sample. Together, these processes provide chemical evidence that the bone tissues are original to the dinosaur and not contaminants.
Being able to repeat these tests with the same results gives additional weight to the evidence. Schweitzer has done this, working with samples taken from 68- and 80-million-year-old fossils.
Schweitzer says, “Conventional wisdom has always been that DNA and proteins are very fragile molecules — they would never last longer than a few hundred thousand years. That time frame has gradually been extended, but there’s still work to do. I hope our group can finally move from simply defending the idea that these molecules can persist to investigating deeper questions.”
Questions, Answers and the Future
Some of those questions relate to climate change, which dinosaurs
experienced over the course of millions of years — much longer than
humans ever have. How did they adapt? Why did some of them survive
longer than others? The answers are in the molecules, and Schweitzer
sees opportunities to apply that knowledge to the climate change
questions of today.
She has the perfect classroom. A supporter of Schweitzer’s work has allowed her access to several hundred thousand acres of privately owned land in Montana, which she has been surveying for fossils.
“I want to expose my students to the Montana that I love, to give them experiences they may not get otherwise,” she says.
The chance to move seamlessly between Montana’s vast landscapes full of enormous fossils and microscopic samples of dinosaur soft tissue in a North Carolina lab is what makes molecular paleontology unique. And these tiny molecules are expanding paleontology in ways Mary Schweitzer never could have imagined.
“I keep trying to buck conventional wisdom,” she says, “because I think the knowledge we can get from these molecules will be critical to where we’re going.”