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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

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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 NEWS FEATURES Downloaded from http://science.sciencemag.org/ on December 3, 2018 CRACKING THE CAMBRIAN New fossils and sites are helping make sense of the mysterious flowering of animal life half a billion years ago By Joshua Sokol, in Kootenay National Park 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 butterflies open, exposing to crisp Caron, curator of invertebrate paleontology animal groups—as diverse as mollusks mountain air a surface that hasn’t at the Royal Ontario Museum (ROM) in To- and chordates—leapt into the fossil record. seen sunlight in half a billion ronto, Canada. Those early marine animals 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 Geology and Palaeontology species 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 bears charcoal-colored few years unearthing groundbreaking ani- 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 crabs or the Millennium Falcon from coming-out party for animal life on Earth. the evolutionary explosion. MUSEUM ONTARIO ROYAL FOSSIL: 880 23 NOVEMBER 2018 • VOL 362 ISSUE 6417 sciencemag.org SCIENCE Published by AAAS Jean-Bernard Caron shows off the “mothership,” insured a Burgess Shale 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 arthropods were crustaceans. 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 Stephen Jay Gould 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 Nature 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, eyes, 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—spiders, insects, familiar animals, many already described They returned to excavate in 2014. At crustaceans—constitutes a “crown” group. in high-profile papers: the little fish rela- least one in five of the animals they found But some animals in the Burgess Shale http://science.sciencemag.org/ tive Metaspriggina, a vertebrate 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 day’s animals has been debated since the uncle grinning out from a family photo. In Joseph Moysiuk last year linked to shelled fossils first came to light. Walcott classified that view, many of Gould’s weird wonders animals called brachiopods, 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 arthropod tree before the common ancestor of living groups like arachnids 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 Centipedes, “explosion” that birthed them. crabs millipedes 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 Trilobites 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 Hallucigenia 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 SCIENCE sciencemag.org 23 NOVEMBER 2018 • VOL 362 ISSUE 6417 881 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 system 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 Anomalocaris, head to tail, says Javier Ortega-Hernández, says Graham Budd, a paleontologist at Up- on December 3, 2018 the Cambrian’s charismatic apex predator—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 Yunnan 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.
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