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CRACKING the CAMBRIAN New Fossils and Sites Are Helping Make Sense of the Mysterious Flowering of Animal Life Half a Billion Years Ago

CRACKING the CAMBRIAN New Fossils and Sites Are Helping Make Sense of the Mysterious Flowering of Animal Life Half a Billion Years Ago

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on December 3, 2018 CRACKING THE New and sites are helping make sense of the mysterious flowering of half a billion ago

By Joshua Sokol, in in Canada; Photography by John Lehmann

he drumming of the jackhammer Star Wars. “It’s a spaceship landing area During the Cambrian, which began about deepens. Then, a block of shale here,” says expedition leader Jean-Bernard 540 million years ago, nearly all modern open, exposing to crisp Caron, curator of invertebrate animal groups—as diverse as mollusks mountain air a surface that hasn’t at the Royal Ontario Museum (ROM) in To- and —leapt into the record. seen sunlight in half a billion ronto, Canada. Those early marine exhibited a years. “Woo!” says paleontologist Those “spaceships” are carapaces, molted dazzling array of body plans, as though evo- Cédric Aria of the Nanjing Insti- onto a long-vanished ocean floor by a lution needed to indulge a creative streak tute of and Palaeontology new to science. This field season before buckling down. For more than a in China, bracing the top slab of they’ve been spilling out of the rocks here, century, scientists have struggled to make rock upright. where Caron’s team has spent the past heads or tails—sometimes literally—of TIts underside charcoal-colored few years unearthing groundbreaking - those specimens, figure out how they relate smudges that look vaguely like horse- mal fossils from the Cambrian period, the to life today, and understand what fueled

shoe or the Millennium Falcon from coming-out party for animal life on Earth. the evolutionary explosion. MUSEUM ONTARIO ROYAL FOSSIL:

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Published by AAAS Jean-Bernard Caron shows off the “mothership,” insured a specimen for half or even into the present. So, for example, he an enigmatic Cambrian life form his team a million Canadian dollars when it went concluded almost all the creatures resem- found in the Canadian Rockies this summer. on loan, he says—and that was an animal bling today’s were . known through multiple fossils. This is one But later paleontologists had other ideas. Gingerly, Aria and Caron place the top of a kind. Harvard University’s piece of their slab aside. Space is hard to “It’s going to be iconic,” Caron says. “It’s perhaps best captured the charisma of Cam- come by in the quarry, perched on a ledge the most extraordinary fossil I’ve ever found.” brian life in his 1989 book Wonderful Life: the size of a small bedroom at an altitude The Burgess Shale and the of His- of 2500 meters, far above Tokumm Creek. FOR YEARS, CARON SUSPECTED Walcott’s site tory, in which he lavished attention on the For years, an equally forbidding site about might be rivaled elsewhere in the Rocky “weird wonders” excavated from Walcott’s 40 kilometers northwest of this valley of- Mountains. The breakthrough came in city block–size quarry. Gould argued that fered the clearest window on the Cam- 2012, near an area called Marble Canyon, oddballs such as the aptly named Halluci- brian. There, in 1909, U.S. paleontologist where a 2003 wildfire had burned off the genia, a worm with legs and hard spines, Charles Doolittle Walcott discovered the trees. While crossing an avalanche chute seem unrelated to later animals. He slotted Burgess Shale, a fossil formation that pre- filled with broken tiles of rock, his recon- the unusual forms into their own phyla and serves not only hard shells, but also soft naissance party found itself surrounded by argued that they were evolution’s forgotten features such as the legs, , and guts of impressions of soft-bodied creatures, many experiments, later cast aside by contingen- Cambrian creepy-crawlies. with unfamiliar shapes. “It was clear that cies of fate. But in recent years, Caron has shown nobody had ever been walking over this Contemporary paleontologists have

that the richest fossil-bearing rock extends pile of rocks before with this purpose in settled on yet another way to understand Downloaded from many kilometers beyond Walcott’s site. mind,” says Bob Gaines, a geochemist from them. Consider the arthropods, arguably This summer’s excavation marks his latest Pomona College in Claremont, California, Earth’s most successful animals. In a fam- visit to this long Cambrian tapestry. Each who has joined Caron’s expeditions since ily tree, the spray of recent branches that new stop has offered striking views of un- the beginning. end in living arthropods—, , familiar animals, many already described They returned to excavate in 2014. At crustaceans—constitutes a “crown” group.

in high-profile papers: the little rela- least one in five of the animals they found But some animals in the Burgess Shale http://science.sciencemag.org/ tive , a ances- at Marble Canyon belongs to species new to probably come from earlier “stems” that tor that Caron now speculates clustered science, the team concluded. Now, they’ve branched off before the crown arthropods. in schools; the pincered Tokummia; and moved on to other sites along the valley. These branches of the tree don’t have sur- the ice cream cone–shaped fossils called How Cambrian species are related to to- viving descendants, like a childless great- hyoliths, which Caron’s Ph.D. student ’s animals has been debated since the uncle grinning out from a family photo. In Joseph Moysiuk last linked to shelled fossils first came to light. Walcott classified that view, many of Gould’s weird wonders animals called , some of which his oddities within known groups, noting are stem group organisms, related to the persist today. that some Burgess Shale fossils, such as the ancestors of current creatures although Other sites around the world are also brachiopods, persisted after the Cambrian not ancestors themselves. Newer fossils opening new vistas of the Cambrian. Scien-

tists can now explore the animal explosion on December 3, 2018 with a highlight reel of specimens, along All in the family A partial schematic of a proposed family tree of arthropods shows the complex with results from new imaging technolo- relations among living and extinct groups. Some extinct Cambrian creatures (red) may belong to “stem” groups gies and genetic and developmental stud- that branched off the tree before the common ancestor of living groups like and insects. ies of living organisms. “There have been a host of new discoveries,” says paleonto- logist Doug Erwin of the Smithsonian In- stitution’s National Museum of Natural History in Washington, D.C. Researchers may be closer than ever to fitting these strange creatures into their proper places Arachnids Crustaceans Velvet worms Insects in the tree of life—and understanding the 0 Horseshoe , “explosion” that birthed them. crabs Each new find brings the simple joy of unearthing and imagining a seemingly alien creature. On a break, Caron cau- tiously shows off this year’s crown jewel, found about a week earlier. It’s an intact,

DATA) JO WOLFE, MIT JO WOLFE, DATA) hand-size carapace with a center spine, like a Prussian spiked helmet frozen in an- Radiodontans and relatives cient rock. Another undescribed species, SCIENCE; ( SCIENCE; it seems to be related to the spaceships. Caron’s team calls it the mothership. He’s nervous just holding it. Burgess Shale fossils are so valuable that Parks Canada keeps the exact locations of Caron’s 541 million sites secret, monitors them with cameras, years

CREDITS: (GRAPHIC) N. DESAI/ N. (GRAPHIC) CREDITS: and prosecutes fossil poachers. ROM once ago

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Published by AAAS NEWS | FEATURES Downloaded from http://science.sciencemag.org/ from the Canadian Rockies help support ing nervous tissue in exceptionally preserved Critics argue that paleontologists such that view. Caron argued in 2015, for ex- Chinese fossils. Those nervous archi- as Ma and Ortega-Hernández overinterpret ample, that his specimens of Hallucigenia tectures offer a parallel way to sort animals some fossils, spotting nervous tissues that have features suggesting the animal be- into evolutionary groups, beyond the usual aren’t there. Many of those structures, the longs on a stem group of the velvet worms, anatomical structures, and other teams have critics say, might just be “halos,” biofilms creatures that still crawl around in tropical presented their own compelling specimens. formed when microbes broke down inter- forests spitting slime. In fossils of the shrimplike Chengjiangoca- nal parts like muscles or guts after death. Similar analysis awaits the spaceships. At ris kunmingensis from southwest China, for But other researchers are convinced. “If you first glance, Caron’s team thinks they are a example, “we have this structure that looks look at the best-preserved nervous systems, new species or group of radiodontans, stem almost like a pearl necklace,” running almost there’s no doubt” that the features are real, arthropods that also include , head to tail, says Javier Ortega-Hernández, says Graham Budd, a paleontologist at Up- on December 3, 2018 the Cambrian’s charismatic —a an incoming professor at Harvard. His team, psala University in Sweden and an architect clawed, fearsome-jawed swimmer half a me- led by Jie Yang at University in Kun- of the current stem-and-crown concept. ter long. Filling out the branches of that stem ming, China, argued in 2016 that the neck- Bold claims that use anatomy to revise group gives a “step-by-step view of how an lace is a nerve cord studded with smaller family trees engender similar controversy arthropod built its body” through evolution- clusters of neurons, themselves sprout- throughout the field. One argument that ary time, says paleontologist Allison Daley at ing tiny nerve fibers. Living arthropods no Hallucigenia fits with the velvet worms, the University of Lausanne in Switzerland. longer have those fibers. But today’s velvet for example, depends on the exact shape of Throughout much of Cambrian paleonto- worms and priapulid worms do, implying its claws. But other teams counter that the logy, that’s the game—a high-stakes, some- kinship between long-vanished stem arthro- claws aren’t diagnostic of ancestry. times contentious race to find diagnostic pods and those groups. The uncertainties leave paleontologists body parts on known or new fossils, make ever hungry for newer, better specimens. arguments about what taxonomic groups “When there is a debate, you bring a new fos- they belong to, and maybe revise evolution- British sil and say, ‘Look, this is the feature we see,’” ary history in the process. Columbia Alberta Caron says, warming up in a tent perched In the past few years, paleontologists Walcott high above Tokumm Creek. “Without fossils, have approached the problem with an array Quarry Lake Louise it’s speculation.” SCIENCE of new techniques. Those include scanning electron microscopes, which can discern a Yoho Banf THE FOSSILS MAKE UP for the discomfort: specimen’s chemical makeup as well as im- National Park National Park 6 weeks in tents above the tree line ward- it, and computerized tomography (CT) Tokumm Creek ing off grizzlies with an electrified fence, scans, which can penetrate fossils without contending with hot days and snow scraping away material. Those tools have Bow River days and wildfire smoke, obeying the also illuminated a startling of internal Marble Canyon smelly requirement to carry everything— Columbia features: fossilized Cambrian . Begin- River everything—out of the national park at the ning in 2011, paleontologist Xiaoya Ma, now Kootenay expedition’s end. 0 25 at the University of Exeter in the United National Park It’s a chilly August morning, 1 day before Km

Kingdom, published a string of papers trac- a helicopter comes to take all traces DESAI/ N. JOHN LEHMANN; (MAP) (PHOTO) CREDITS:

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The Burgess Shale, a storied rock formation that offered some of the earliest glimpses of half-a-billion-year-old creatures, crops out high above Tokumm Creek. http://science.sciencemag.org/ away. Today is the last chance to stumble loving bacteria while alkaline chemistry en- The Moroccan samples, for example, date on a fossil that could crack a mystery—say, cased the dead animals in coats of carbonate, to a little after the Cambrian, and they show to find the body that belongs in the mother- sealing soft tissues inside. a blend between the Cambrian’s signature ship carapace. In summer 1984, for example, paleonto- oddities and the more familiar fauna that The nine-member team hikes from camp logist Hou Xian-guang of Yunnan Univer- dominated later periods. “We are still at the to their quarry, up steep, rock-littered slopes. sity uncovered an arthropod glistening in point of unpacking fossils,” says Daley, a col- Ridged fossils poke out from ex- Cambrian mudstone, its legs seemingly laborator on that research. “This is a chance posed layers, but on this expedition, they alive. He had discovered the Chengjiang to study why some taxa go extinct and why don’t even warrant a second glance. At the biota, a trove of immaculate fossils that others are able to replace them.” quarry, most people split rock while Caron’s sprawls over a region in southwest China. grad students help ROM curator Maryam Slightly older than the Burgess Shale— ALTHOUGH SHOW-STOPPING ANIMALS keep on December 3, 2018 Akrami pack away the most recent finds in about 518 million years old compared with falling out of the strata, the full signifi- swimming-pool noodles. “It’s the last day,” the Burgess’s roughly 507 million years— cance of the remains Caron says. “No injuries!” those deposits showcase related animals a mystery. Arthropods, the most diverse Each successive excavation in this valley in a different style of preservation. Unlike and common creatures known from the has targeted the same band of rock, which Caron’s sites, where geologic processes have time, littered Cambrian . Judg- records a single slice of geologic time. But squashed the fossils almost flat, the Chengji- ing by the fossils, Daley argued in a paper each dig has yielded a different array of ang animals still retain some depth. Since in May, the Cambrian witnessed both the new species. That’s because conditions var- 2015, Chinese researchers, including Hou, birth and step-by-step diversification of ied across the ancient sea floor, favoring have capitalized on that by using CT scans many modern groups. Another approach different animals. Such variation is “not a to make 3D images of the specimens without yields a different answer, however. Geneti- shock to anybody that has ever strapped on destroying them. Today, three rival Chinese cists use a tool called molecular clocks to a snorkel and swum around,” Gaines says. teams, each with international collaborators, trace back down the tree of life. By start- But this vast, wide-open valley captures that compete to pull out new discoveries from the ing with genetic differences between living kind of diversity at a single moment, al- site. “There is an absolute landslide of mate- animals, which have accrued as a result of lowing glimpses of how the earliest animal rial,” Ortega-Hernández says. random mutations over the eons, molecular ecosystems were structured. Add to that sites such as Bay in Austra- clocks can rewind time to the point where As Caron’s quarries bring this moment lia, where paleontologists announced in 2011 branches diverged. into ever-sharper focus, other sites have that they had unearthed radiodontan fossils According to recent studies using that opened portals on other stages of the Cam- revealing their complex, multifaceted eyes; method, modern animals began to march off brian. Nearly everywhere, the fossils preserve and Morocco’s , which into their separate phyla some 100 million levels of squishy detail that are absent in paleontologist Peter Van Roy at Ghent Uni- years before the Cambrian. The finding im- specimens from later in Earth’s fossil record. versity in Belgium reported in 2010. Each site plies that those groups then hung out, incon- In 2012, Gaines and colleagues proposed a offers distinct insights. “Every fossil assem- spicuous or unnoticed in the fossil record, reason: Perhaps unique chemical conditions blage is horrifically biased,” says paleonto- before suddenly stepping on . suffused Cambrian seas. After dead animals logist Nick Butterfield of the University of Paleontologists have a cryptic set of settled into mud on the sea floor, low levels Cambridge in the United Kingdom, “but clues about life before the explosion. Long of sulfates could have slowed decay by sulfur- they’re horrifically biased in different ways.” before the odd beasts of the Cambrian

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Perched in their quarry 2500 meters up, paleontologists hammer open slabs of shale to expose the rare fossils inside. http://science.sciencemag.org/ evolved, an even more alien set of ocean Cambrian creatures off their as the sion might also mark the moment when, organisms left impressions on sedimen- first animals, Cambrian science itself con- after millions of years of quiet progress, tary rocks now seen in and Aus- tinues to explode. Caron and others keep animals had finally accrued the develop- tralia. The Ediacarans, as those fossils are hunting for fossil features that could re- mental recipes to build body parts and im- called, taunt paleontologists with the same veal the relationships among , provise on basic themes. That genetic toolkit, kind of interpretive challenge as the Cam- Cambrian, and present-day groups. Other Butterfield argues, is “absolutely, astronom- brian’s weird wonders. But they’re even researchers struggle to explain what caused ically, inconceivably complex. It just took a weirder. Their imprints suggest some grew the explosion of animal forms. Atmospheric while to figure that out.” Or, of course, multi- in fractal patterns; others had three-part may have spiked, enabling animals ple causes could have piled up together. symmetry. Unhelpfully, they don’t have ob- to grow bigger, stronger, and more active. vious mouths, guts, or appendages. “That’s Or erosion could have dumped toxic cal- AFTER A LUNCH BREAK, the paleonto- on December 3, 2018 where the freak flags are going now,” says cium into the oceans, prompting organisms logists chisel into a few more slabs. Gaines Jo Wolfe, a paleontologist at the Massachu- to shunt it into building hard skeletons. takes rock samples from each layer of setts Institute of Technology in Cambridge. Or biology itself could have led the their quarries, hoping to reconstruct each Most Ediacarans vanished before Cam- way. Inventions such as , free environment’s chemistry. Then Caron deliv- brian deposits, perhaps perishing in the swimming, and burrowing into the sea ers the announcement: “It’s over, guys. No world’s first mass extinction. But many re- floor—all first seen in or shortly before more digging.” searchers suspect some belong on the tree the Cambrian—could have transformed a The next day, its last, the ROM team of animal life, perhaps as early stems. One placid global ecology into a high-stakes con- breaks camp. Over several hours, a helicop- Ediacaran, , looks like an ani- test, spurring waves of call-and-response ter ferries nets sagging with fossils toward mal: a snail or that grazed along the innovation between groups. The explo- a staging area by the highway, making the sea floor. In August, , a frond- roughly 10-minute trip again and again. like Cambrian creature already thought Some specimens, like the spaceships, will to be an animal, was pegged as an Edia- be rushed to publication in coming months, caran survivor on the basis of its fractal now that visiting journalists have seen branches. That would make its Ediacaran them. Other finds will sit in drawers, await- relatives animals, too. And in September, ing new techniques or the graduate student researchers announced that an iconic who asks the right question. Ediacaran fossil called , which As the team huddles, waiting for its heli- looks like a halved Christmas ham, con- copter ride, tiny, rabbitlike called tained lipid molecules that resemble those cry out from the hills. Each helicopter of living animals. trip erases the signs of human presence one “We’re seeing the beginning of the advent by one, until only carved-out quarries re- of animals in the Ediacaran,” says paleo- main. More fossils still rest inside, pressed biologist Mary Droser of the University of between folio sheets of rock, waiting for the California, Riverside. “It’s more fun and ex- next season. j citing than just the Cambrian explosion.” A lace —a long-extinct “stem” arthropod called

And yet even as the Ediacarans shove splendens—turns up in a slab. Joshua Sokol is a journalist in Boston. MUSEUM ONTARIO ROYAL FROM FOSSIL LEHMANN, JOHN PHOTOS:

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Published by AAAS Cracking the Cambrian Joshua Sokol

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