Wyoming, Susie Maidment hits her hammer against stone. She breaks off a fist-sized 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 a note on his clipboard and brings a piece of rock to his face. The layer below this one has larger particles, Maidment says. Jurassic, from the details in the rocks that formed at that time. It is one of many pieces of data needed for the job.
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 been sent prey scurrying into shadows.
Along our path, we stop to huddle over a two-inch fossil fragment that is picked up from the dry rubble – tangible remains of these long-departed animals. Maidment notes that every creature would be larger than a meter in Jurassic would have been a dinosaur. "If it's big and it's from the Jurassic," she says, "it's a dinosaur bone."
Dinosaur research has been expanding for more than 130 million years. But fossils on their own can only reveal so much about bigger-picture questions. Do differences in the head crests of hadrosaurs, say, or the skeletons of stegosaurs, represent evolution 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 will not 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.
Maidment, a paleontologist at the Natural History Museum 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 to explore the surrounding sediments for deeper details. They are 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.
Only once this framework has been established can researchers really begin to tease apart who's related to who and how these are 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."
Your favorite stegosaurs and more
Mapping the chronology of the Morrison is not trivial. The formation stretches across roughly 1.2 million square kilometers from New Mexico and Arizona to the south to the north of Montana. But it's a challenge worth tackling, given what the formation holds. Maidment says, rattling off well-known names including stegosaurus, diplodocus and brontosaurus. "These rocks have all of your favorite dinosaurs." "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, they described the most complete stegosaur skeleton ever discovered – a specimen that came from the Morrison (though they were not involved in excavating it).
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 bring you amazing information, you 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 Jurassic dig.
Maidment thus brings a somewhat uncommon mix to the research, of prestige for her paleontological lab work plus a strong knowledge of field geology – gained experience 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 and researches within the Morrison suggested that 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 individual layers within the rocks where many fossils have come from. So paleontologists have resorted to grouping 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 been found to reveal differences in skeletons in the southern portion of the formation, But without these differences, but can not differ from each other. 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're living together," Maidment says. By way of context: Just 12 million years of evolution produced humans, gorillas and chimps from a single common ancestor.
Paleobiologist Anjali Goswami, a colleague of Maidment at the Natural History Museum who studies 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. They're 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 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 have you come from the previous layer; knock off a piece of the layer with your rock hammer; get the sample as close as possible while still being able to focus on it; 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 to the right or left in a straight line to the next well-exposed area and proceed upward, forward in geological time.
The end product in the field looks like a vertical barcode decorated with symbols that indicate the size of sediments, 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 like 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 are not very helpful. A single ripple layer can be formed in a number of different environments, including a small stream. But with many barcodes collected across a region, many of the same can not be found in the same region, and a three-dimensional illustration of the landscape has failed 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. While collecting them has been a massive, multi-decade effort by many scientists, Maidment is the first to pull them all together into a cohesive framework Journal of Sedimentary Research.
Roger Benson, a paleobiologist at the University of Oxford who wrote an article in the 2018 Annual Review of Ecology, Evolution, and Systematics last year on lingering unknowns in dinosaur biology and evolution. He sees the well-studied rocks of the Morrison as well as a Rosetta Stone for other less-studied rocks of the same age, and what Maidment finds could not help the story of Late Jurassic dinosaurs just in North America, but elsewhere. "The work is really important and fundamental," he says.
Fascinated from the start
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 fossils ammonites along the cliffs of the Jurassic Coast in southern England, but she has her granddaughter when she was raised up. "At the time I wavering viciously between scientist and princess," she deadpans. Her grandfather, an electrical engineer, gently pushed for scientist. She was not sure what options existed in science, but 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. "He 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 she often finds the sedatives even more dinosaurs – especially if they contain datable material.
David Eberth, an emeritus geologist at the Royal Tyrrell Museum in Alberta, has studied extensively fieldwork with younger dinosaur-rich rocks in Canada. "You have to go where the rocks will talk to you," he says.
Eberth is referring to the mineral zircon, the preferred material scientists use 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 of 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 volcanos, so researchers look for them in ancient volcanic ash layers where they have been buried soon after they are formed. But ash does not always fall apart, and trying to process zircons from sediments. The cost and difficulty of doing zircon work mean many spots in the Morrison paint zircon dates. This is where the stratigraphic logs are useful in assigning to fossils: although zircons may be absent from some fossil sites, they are present in others, and geologists can extrapolate the age of sediment layer in the rock formation. Work like this, Eberth says, "is not there any sense of patterns coming out of the Morrison."
But you need more than zircon dates and stratigraphic logs, he adds. Sometimes seemingly identical; the resemblance could be coincidental. "You can not tell," he says. "You need a huge multidisciplinary tool 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 deal with the timing of layers. "Then," Eberth says, "you start putting the animals in it."
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 one hand in the other – she's fully focused on collecting observations for a new stratigraphic log.
Maiden's efforts to compile all existing Morrison logs into a single comprehensive framework. "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 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.
The smell of ancient eons
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. Thanks, 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 hold a piece of lake bottom.
Soon after, ominous storm clouds descend and we hustle back to the central site to take cover. But our minds are still stuck in the Jurassic. "It's very relaxing," Maidment says, "the taste of rock, the taste of sediment. "I love doing it."
Before the incoming rain kicks off the dig site, a hubbub forms around one of the fossil quarries. Paul Kenrick, a paleobiologist from the London Museum, has published a fragment of the length of a thumbnail.
Maidment examines the finger and tentatively identifies it as a piece of a femur. The small stuff is less well known, it's rarer, "The small stuff is less well known, it's rarer," he says. huddle close to get a look. "It shows other things in here."
The rain starts to fall as it becomes slippery and impassible. As we leave, we rattle over beds of undiscovered bone. Those bones want to bring the team back to the next day – but it's the surrounding layers that will bring the bones to life.
Editor's note: The text was changed on August 15, 2019 to correct geologist Kenneth Galli's affiliation. He is at Boston College.