What Fossils Alone Can’t Explain About Dinosaurs

At the base of a pale hill in the badlands of northeastern Wyoming, Susie Maidment hits her hammer against stone. She breaks off a fist-size chunk, grabs a loose piece between her fingers, and places it on her tongue. “Silty,” she announces, as the sediment brushes the roof of her mouth.

Maidment’s graduate student, Joe Bonsor, takes note on his clipboard, then brings a piece of rock close to his face and squints at it through a hand lens. The layer below this one has slightly larger sand particles, Maidment says—suggesting that the two formed under different conditions. It’s one of many bits of data needed for the job the two paleontologists have come over from the United Kingdom to do: piece together, layer by layer, the history of the Late Jurassic, from details in the rocks that formed at that time.

The hills around us on this June day sprawl with dusty prickly pear cactus, juniper, and sagebrush. Scorpions and rattlesnakes pose the most immediate threats. But during the Late Jurassic, streams and ponds would have flushed through the landscape, and dinosaurs—the creatures that make this spot so compelling to Maidment and Bonsor—would have sent prey scurrying into the shadows.

Along our path, we stop to huddle over a two-inch fossil fragment that Bonsor picked up from the dry rubble—tangible remains of these long-departed animals. Maidment notes that every creature larger than one meter in size that lived on land during the Late Jurassic would have been a dinosaur—and any bone as thick as this one would have come from one. “If it’s big and it’s from the Jurassic,” she says, “it’s a dinosaur bone.”

Dinosaur research has been steadily expanding in recent years, with new fossil discoveries and ever-improving fossil-scanning technology reshaping the way scientists understand these animals, which dominated terrestrial ecosystems for more than 180 million years. But fossils on their own can reveal only so much about bigger-picture questions. Do differences in the head crests of hadrosaurs, say, or the skeletons of stegosaurs represent evolutions through time, or the difference between males and females from the same time? If changes through time, how long did that evolution take, and what caused the shift? Where on the planet were dinosaurs most prevalent and diverse? Who fell prey to whom, and what type of terrain did these creatures carve their lives through? Unearthing additional fossils won’t tell you all these things. The answers more often rest in the rocks that surround the bones. And those rocks are, in many cases, not well studied.

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Maidment, a paleontologist at the Natural History Museum in London, is leading the push to change that, at least for North America’s Late Jurassic. This summer, she and Bonsor teamed up with an international group of paleontologists in a dinosaur dig dubbed “Mission Jurassic” that aims to excavate new museum specimens and explore the surrounding sediments for deeper details. They’re working in the Morrison Formation, a suite of rocks that has produced more Jurassic dinosaur bones than any other collection of rocks on the continent. Maidment’s ultimate goal: to develop the first-ever comprehensive chronology of the entire Morrison that maps out how the landscape changed through time and how different fossils fit into it.

Only once this framework has been established can researchers really begin to tease apart who’s related to whom and how these Late Jurassic dinosaurs evolved. “We think of dinosaurs as really, really well known,” Maidment says, “but they are actually not that well known at all.”

Mapping the chronology of the Morrison isn’t trivial. The formation stretches across roughly 1.2 million square kilometers, from New Mexico and Arizona in the South all the way to Montana in the North. But it’s a challenge worth tackling, given what the formation holds. “These rocks have all of your favorite dinosaurs,” Maidment says, rattling off well-known names including Stegosaurus, Diplodocus, and Brontosaurus. “All the ones you knew when you were 7.”

At 38, she’s focused on stegosaurs, and has distinguished herself as one of the world’s leading experts on this group of dinosaurs. In 2015, she led a team that described the most complete stegosaur skeleton ever discovered—a specimen that came from the Morrison (though she was not involved in excavating it).

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She first visited this fruitful formation as a graduate student at the University of Cambridge in 2006, and has since returned five times to study fossil beds and sleuth out the Morrison’s ancient environmental history. “That’s going to be amazing information she can bring,” says Victoria Egerton, a paleontologist with positions at the Children’s Museum of Indianapolis and the University of Manchester, and one of the lead organizers of the Mission Jurassic dig.

Maidment also brings a somewhat uncommon mix to the research: of prestige for her paleontological lab work and a strong knowledge of field geology—experience she gained as an undergraduate geology student at Imperial College London and by working as a geologist for an oil company before landing at London’s Natural History Museum in 2009.

The geologic work she and her colleagues have conducted within the Morrison suggests that it formed over the course of 9 million years, give or take a few million, between about 156 million and 147 million years ago. But beyond that, researchers still have a poor sense of the ages of individual layers within the rocks where many fossils have come from. So paleontologists have resorted to grouping these fossils into a single unit of time—a practice that can lead to seriously flawed interpretations, Maidment says.

For example, studies of Morrison fossils have begun to reveal differences in skeletons found in the southern portion of the formation compared with similar ones found in the North—including stegosaurs that Maidment has studied. But without ages assigned to these fossils, researchers can’t know if their differences represent changes through time or place-based differences from the same time. That’s an important distinction to make as researchers build family trees and try to understand the broader story of dinosaur evolution.

“If you’re dividing time into 10 million years, you are smushing together a whole load of different ecosystems and different animals that never would have lived together,” Maidment says. By way of context: Just 12 million years of evolution produced humans, gorillas, and chimps from a single common ancestor.

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Anjali Goswami, a paleobiologist and colleague of Maidment at the Natural History Museum who studies vertebrate fossils from other parts of the world, says that establishing a robust timeline is key to untangling the Morrison, and that Maidment’s efforts here are vital. “The error in what we are trying to estimate is really huge. She’s doing a lot of really time-consuming fieldwork to try to remedy those errors.”

That fieldwork includes the painstaking task of collecting what geologists call stratigraphic logs: inch-by-inch observations of sediment layers (or strata) from the base of a rock face to the top (from the oldest sediments to the youngest)—sometimes spanning hundreds of feet of stone. It’s why Maidment stuck the silt in her mouth (a common geologic test of sediment size) and what has consumed her time in the Morrison over the past seven years.

The activity is slow but rhythmic: Extend the tape measure; note where you are in the rock face and how far you’ve come from the previous layer; knock off a piece of the layer with your rock hammer; get the sample as close to your face as possible while still able to focus on it beneath your hand lens; note the size of the sediment and the quality of its layers; and, if you’re inclined, put a bit in your mouth.

Jot down notes, confer with your field partner to confirm your interpretation of your observations, and then move on to the next layer directly above. If a plant or other obstruction appears in the way, skirt to the right or left in a straight line to find the next well-exposed area and proceed upward, forward in geological time.

The end product in the field notebook looks like a vertical barcode decorated with symbols that indicate the size of sediments, the thickness of layers, and the ancient environments these layers might represent. Wavy layers often form in watery places where sand ripples might develop, so they may represent a stream bed or coastline. Flat layers may represent a calmer environment such as a lake bottom. Sand and silt fall faster through water than clay, which settles in places where tides and currents slacken.

On their own, these individual barcodes aren’t very helpful. A single ripple layer can form in a number of different environments, including a small stream. But with many barcodes collected across a region, scientists can start to find patterns across corresponding layers, build connections, and sculpt a three-dimensional illustration of how the landscape might have unfurled and morphed through time—shifts from wet to dry to coastal to riverine, each iteration layered one on top of the next.

Since 2012, Maidment has collected more than 20 of these stratigraphic logs across the Morrison and has worked to correlate them with 245 additional ones that others have collected over the years. While collecting them has been a massive, multi-decade effort accomplished by many scientists, Maidment is the first to pull them all together into a cohesive framework, work that’s been accepted for publication in the Journal of Sedimentary Research.

“She’s really someone who is pushing ahead with that in a way that I don’t think other people have been,” says Roger Benson, a paleobiologist at the University of Oxford who wrote an article in the 2018 Annual Review of Ecology, Evolution, and Systematics on the lingering unknowns in dinosaur biology and evolution. He sees the well-studied rocks of the Morrison as somewhat of a Rosetta stone for other, less studied rocks of the same age, and what Maidment finds could help unravel the story of Late Jurassic dinosaurs not just in North America, but elsewhere. “The work she is doing is really important and fundamental,” he says.

As we drive down a dirt road to the Mission Jurassic dig site, over cattle guards and through several barbed-wire ranching gates, Maidment describes her decades-long commitment to unraveling the story of dinosaurs.

She spent her childhood collecting fossil ammonites along the cliffs of the Jurassic Coast in southern England, but she traces her specific fixation on dinosaurs back to a conversation she had with her grandfather when she was 6, when he asked what she wanted to be when she grew up. “At the time, I was wavering viciously between scientist and princess,” she deadpans. Her grandfather, an electrical engineer, gently pushed for scientist. She wasn’t sure what options existed in science, but knew she liked dinosaurs, so he suggested she study them. Since then, that’s been her pursuit. “It’s always what I wanted to do,” she says.

We arrive at the dig site, and I join Bonsor as he crouches with a group of other students. They kneel on pads and methodically brush away dusty layers to excavate the remains of a sauropod—a long-necked, long-tailed plant eater from a group of the most massive animals ever to live on land.

Using a metal trowel to discard clumps of dirt and a razor blade to carve finer details, Bonsor comes across an object with the distinct reddish hue of bone. “This has always been my goal,” he says, as he gazes at his first-ever dinosaur find. “Pretty much this second has been my life goal.”

The allure of discovering new fossils certainly motivates Maidment, as well. But she says that she often finds the sediments even more enticing than the dinosaurs—especially if they contain datable material.

But locating rocks with that material isn’t easy, says David Eberth, an emeritus geologist at the Royal Tyrrell Museum in Alberta who has conducted extensive fieldwork studying younger dinosaur-rich rocks in Canada. “You have to go where the rocks will talk to you,” he says.

Eberth is referring to rocks that contain the mineral zircon, the preferred material scientists use to date Earth’s oldest remains. Tiny zircon crystals are especially helpful for two reasons: They’re strong and can stay intact across millions of years, and they contain the radioactive element uranium. Uranium decays to the element lead at a known rate, so researchers can measure the ratio of uranium to lead in a zircon to calculate its age.

Zircons form in volcanoes, so researchers look for them in ancient volcanic-ash layers where they would have been buried relatively soon after they formed. But ash doesn’t always fall neatly alongside fossil beds, and trying to process zircons from sediments broken down from ash can be challenging. The cost and difficulty of doing zircon work mean many spots in the Morrison lack zircon dates. This is where stratigraphic logs become helpful in assigning ages to fossils: Though zircons may be absent from some fossil sites, they are present in others, and geologists can extrapolate the age of one sediment layer by correlating it with a corresponding layer of known age elsewhere in the rock formation. Work like this, Eberth says, “is absolutely key to making any sense of patterns we see coming out of the Morrison.”

But you need more than zircon dates and stratigraphic logs, he adds. Sometimes seemingly corresponding layers look alike but do not actually match up; the resemblance could be coincidental. “You can’t tell,” he says. “You need a huge multidisciplinary tool kit to tell you”—including other lines of evidence from the sediment layers.

Researchers also correlate the chemistry of the strata—chemostratigraphy—by looking at the ratios of different elements in the rocks. And they carefully note the orientation of magnetic mineral grains within the strata—magnetostratigraphy. Only when these multiple lines of evidence match up can scientists solidify the timing of layers. “Then,” Eberth says, “you start putting the animals in it.”

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During past field seasons, Maidment has collected cores of Morrison rock for magnetostratigraphy and samples of ash for zircon analysis. This time, she is keeping an eye out for more volcanic-ash layers. Otherwise—tape measure in one hand and hand lens in the other—she’s fully focused on collecting observations for a new stratigraphic log that she’ll transfer to a computer and add to her mounting collection from across the formation.

Maidment’s efforts to compile all existing Morrison logs into a single comprehensive framework will help make the most of the relatively few reliable zircon dates that she and her colleagues have collected over the years. “That would be a big contribution,” says Kenneth Galli, a geologist at Boston College whose team has collected and analyzed zircons from the Morrison.

And by bridging this gap between geology and paleontology, she’s filling a niche that others aren’t necessarily equipped for, says Amanda Owen, a sedimentologist at the University of Glasgow in Scotland who has studied the Morrison extensively and whose stratigraphic logs helped inform Maidment’s chronology.

As Maidment, Bonsor, and I continue our way up the silty hill to complete their log for the day, Maidment knocks off a gray stone and hands a piece to me. I bring it to my face and notice a strikingly familiar but out-of-place odor—the dank, musky smell of a lake.

Maidment confirms that rocks can, incredibly, retain the smell of their origins millions of years after they form. I could actually be holding a piece of lake bottom.

Soon after, ominous storm clouds descend and we hustle back to the central dig site to take cover. But our minds are still stuck in the Jurassic. “It’s very relaxing,” Maidment says of the sensory experience of collecting logs: the smell of rock, the taste of sediment. “I love doing it.”

Before the incoming rain kicks us off the dig site, a hubbub forms around one of the fossil quarries. Paul Kenrick, a paleobiologist from the London museum, has found a fragment the length of a thumbnail.

Maidment examines the find between her fingers and tentatively identifies it as a piece of a femur. Based on its curvature, she thinks it might have come from a small therapod—a meat-eating dinosaur that would have been magnitudes smaller than the sauropods the team has been digging up. “The small stuff is less well known; it’s rarer,” she says, as people huddle close to get a look. “It shows that there are other things in here.”

The rain starts to fall as we pile into trucks and head down the dirt road before it becomes slippery and impassible. As we leave, we rattle over beds of undiscovered bone. Those bones will bring the team back the next day—but it’s the surrounding layers that will bring the bones to life.

This post appears courtesy of Knowable Magazine.