Growing up among the smiling green sauropods of the Sinclair Gas Stations, I fell in love with dinosaurs as a little kid in the late 1960s, and yet I didn't take a comprehensive course about the history of life on Earth until I began volunteering at the Denver Museum of Nature and Science in the mid-1990s. The museum offered classes to volunteer docents in its Prehistoric Journey exhibition, and to volunteer fossil diggers and preparators in its paleontology certification program. In both places, I heard about the "Cambrian explosion." Initially drawn to the museum by dinosaurs, I found myself more and more intrigued by the odd creatures that evolved some 300 million years before the Age of Reptiles.
Fossils from this time, about a half a billion years ago, include some of the earliest heads, mouths, eyes and legs. It was likely during the Cambrian that animals began to stalk, chase and gobble each other. Many of these ancient animals had rudimentary versions of modern adaptations, and yet so many of them looked nothing like the animals we know now. Cambrian fossils sport spikes, tubes, tentacles, grasping claws attached in odd places, eyes on the ends of appendages, too many eyes, pointy heads. It's as if nature cobbled together animals out of worms, crabs and cutlery. A menagerie of body forms developed during the Cambrian and, measured by the slow clock of geology, they developed fast. Something happened half a billion years ago. Something big.
The fossil site that highlighted the strangeness and diversity of the Cambrian Period (roughly 542 million to 505 million years ago) was the Burgess Shale Lagerstätte, now part of a UNESCO World Heritage site in the Canadian Rocky Mountains. Smithsonian head Charles Doolittle Walcott started exploring the Burgess Shale in 1909, though wasn't the first scientist to dig fossils there; Richard McConnell of the Geological Survey of Canada found intriguing remains in the late 19th century. But the number and diversity of fossils Walcott found and took back to the Smithsonian made the site famous.
What sets the Burgess Shale apart from most fossil sites is the exceptional preservation of fossil remains. Fossils are rare to begin with, and they usually only form from hard parts. The Burgess Shale — basically a sampling of Middle Cambrian fossils from the Rocky Mountains — shows soft-tissue preservation again and again. Geologists surmise that there was a succession of burials in the Walcott Quarry, perhaps seasonal, perhaps driven by hurricanes or earthquakes. Animals were likely buried more or less where they lived.
An even greater appreciation of the Burgess Shale's fossil richness came in the latter half of the 20th century when a team led by Harry Whittington began reexamining the fossils in the Smithsonian collections. It seemed that the more Whittington and his students, Simon Conway Morris and Derek Briggs, studied the fossils, the more body plans they uncovered.
In early 2015, I was keen to talk with a Burgess Shale specialist. (A braver person than myself would also want to hike to the famous locality, but have you seen those mountains?) I knew that the Smithsonian still housed the collection founded by Walcott, and I had briefly worked there as part of a contingent of DMNS volunteers assisting Smithsonian paleobotany curator Scott Wing in 2000. Fifteen years later, he still remembered the small army of volunteers who had assisted him, and he helped me find the right paleontologist to contact. But when I reached the invertebrates curator and asked for a behind-the-scenes visit to the Smithsonian's Burgess Shale fossils, I found my enthusiasm unrequited. Fortunately, Dr. Wing pointed out to me that the Royal Ontario Museum (ROM) in Toronto also had "a large and important collection."
Wing's description was nicely understated. The ROM began intensive collection efforts in the Burgess Shale around 1975, and hasn't stopped since. The ROM now has the world's largest collection of Burgess Shale fossils, so being turned away from the Smithsonian was perhaps a blessing in disguise. I contacted the ROM's Cambrian expert, Jean-Bernard Caron, and asked for a visit. He said yes. He had a summer packed with fieldwork, but would be back in the office in late August. So off I went to Toronto.
The Royal Ontario Museum (ROM) opened in 1914 and incorporated a series of architectural styles. Today it's dominated by the Michael Lee-Chin Crystal, added to the north side in 2007.
The crystal now identifies the museum from the outside, and it shapes that part of the museum from the inside as well, the angular motif repeated throughout the northern portion of the ROM. Inside the crystal, on the first level, is ROM Boutique. Between August 25 and 27, I suspect I was the boutique's single best customer.
The eastern part of the ROM, constructed in the 1930s, sports Art Deco prehistoric animals. (It just doesn't get any better than an Art Deco dinosaur.)
A sauropod skeleton greets visitors inside the main entrance, and without a doubt the dinosaurs and big fossil mammals comprise the most popular part of the museum. Slightly quieter but still popular are the portions devoted to ancient civilizations, and gems and minerals. If you really want some peace and quiet, head to the museum's Europe: Evolution of Style gallery, where you practically have the place to yourself.
And if you approach the museum by subway, don't fret about missing your stop. Best subway station I've seen yet.
Freshly bought ticket in hand, I headed straight for the information desk and asked if the museum had a Burgess Shale display. A friendly volunteer told me that (1) there would one day be a big exhibition and, (2) for now there's a case. Actually, there turned out to be two cases. He pointed me toward the appropriate staircase, and as soon as I reached the right level, I was distracted by a glittering mosaic. But off to the side, I saw a sign about Earth's first animals.
My photography skills are pretty limited, especially through glass, but I immediately started snapping photos. I suspect this little corner of the ROM doesn't typically see much traffic (though it should). After a few minutes taking pictures, I noticed movement out of the corner of my eye. Looking around, I saw a very puzzled security guard standing several feet away.
It's worth mentioning that, if you want to take some of the Burgess Shale home with you, you can buy an assortment of Cambrian critter toys in the ROM Boutique. Some of them appear in the display case, useful guides to how the animals might have looked while alive.
I didn't realize it at the time, but I eyeballed these cases in their final days. Less than a month later, the ROM opened a preview of its upcoming Dawn of Life Gallery. Dr. Caron wrote me that the preview included "3D prints, animations and images, Hallucigenia, Metaspriggina (for the first time), Anomalocaris, Nectocaris, Wiwaxia and Opabinia."
Delayed by logistics and Covid-19, the Dawn of Life Gallery finally opened in December 2021. The exhibition featured more than a thousand specimens, spanning over three billion years, from early traces of microbes through the dawn of the dinosaurs. If you're interested in seeing some of it (and you should be), visit the ROM's Willner Madge Gallery, Dawn of Life online.
At 3:00 p.m., I reported to the ROM staff entrance, ready for my appointment with Jean-Bernard Caron. A short time later, I was in his office, peppering him with questions.
Dr. Caron grew up in France. Dinosaur fossils weren't that common where he lived, and they weren't the fossils that got him hooked on the history of life. He found himself most fascinated by suture lines on ammonites (understandable to anyone who's taken a good look at a well-preserved ammonite or baculite). He participated in his first dig, at an archaeological site in France, before he turned 18. Despite his youth, the work was exhausting: perching on a plank and leaning way over to reach the ground.
His introduction to the Burgess Shale was a French translation of Stephen Jay Gould's Wonderful Life, and he still keeps the book in his office. Gould focused largely on the work of Derek Briggs and Simon Conway Morris. After reading Gould's book in the early 1990s, Caron contacted Conway Morris, who introduced him to Desmond Collins at the ROM. Caron started volunteering for the ROM, working at the Burgess Shale, in 1998. He earned his Ph.D. at the University of Toronto then joined the ROM as an employee, becoming a curator there in 2006.
C.D. Walcott assembled the first large-scale collection of Burgess Shale fossils, and they are housed at the U.S. National Museum of Natural History at the Smithsonian. But ROM scientists also collected from the Walcott Quarry, continuing to dig for decades. ROM now has the biggest Burgess Shale collection "by far," Caron said. At the time of my visit, the Smithsonian housed roughly 65,000 fossils, and the ROM housed some 150,000. "But these numbers can be misleading," Caron cautioned, "because one slab of rock might have one fossil or many fossils."
Some of the fossils at the ROM actually belong to Parks Canada (which offers Burgess Shale tours, by the way). The ROM curates fossils for the parks, and the parks issue research and collection permits to the ROM, which makes its paleontological breakthroughs possible.
Paleontologists don't actually get to spend that much time in the quarry collecting fossils. The weather will cooperate for only so long, and only so much food and water can be easily delivered to a remote field site. So out of all the years that a paleontologist researches a fossil site, only a small proportion of that time is in the field. Much more time is spent in the lab and the office, prepping fossils and writing papers. Still, Caron recently tallied up the days that paleontologists spent in the Walcott Quarry actively collecting fossils. "It adds up to about two whole years, and most of that time has been spent by ROM scientists."
Caron wrote his master's thesis on Banffia constricta. Aiming to summarize what we know about Banffia, he summarized that we don't really know very much. "When you can't even be sure it's a deuterostome, you have a problem," he explained.
Nectocaris is another species hard to pin down. Caron told me that its original description was based on just one specimen. That's a bigger problem than you might think. Picture the tallest person and the shortest person you've ever met. Imagine visiting Earth as a future, alien scientist, and finding the remains of only those two individuals. Would you define them as belonging to the same species? With fossils from the Burgess Shale, that difficulty is compounded by the angle of preservation. If an individual is flattened and compressed from the top, that will leave a very different fossil than one flattened from the side. "But sometimes one fossil is all you get."
Some fossils from the Burgess Shale are so weird, you wouldn't be able to believe your eyes without multiple specimens. Take Herpetogaster collinsi. Seriously. Look at this thing. I asked Caron if he ever thought a Burgess Shale fossil simply couldn't be real. "When you see the first one, you think it can't be real, but then you find more specimens," he replied.
But while Nectocaris and Banffia still elude a solid understanding, and Herpetogaster is just plain weird, paleontologists have made great strides in understanding other Burgess Shale fossils and relating them to modern organisms. Caron's first paper in Nature, published in 2006, demonstrated that Odontogriphus omalus is actually a mollusk. The best clue was its radula, or toothy ribbon used by mollusks to feed. "It's a simplified form but still a radula," he said. The fossil species also had gill structure and a broad mollusk foot. "It's basically a shell-less mollusk." Meanwhile, the enigmatic Haplophrentis carinatus, long thought to be a mollusk or perhaps something in its very own phylum, is likely a relative of brachiopods.
Brachiopods, which were far more abundant and diverse millions of years ago than they are today, bore a superficial resemblance to clams, but had very different internal structures. For brachiopods that attached themselves to a particular place with a stalk-like pedicle, the soft sediments of the Burgess Shale provided few safe foundations. In the crowded Cambrian, the swimming larvae settled on any hard surface, often one provided by other animals: fellow brachiopods, sponges, even the punk rock spikes of Wiwaxia. (Wiwaxia was another Burgess Shale animal that long puzzled. It was initially identified as an annelid worm but Caron is confident it's a mollusk. The debate over annelid versus mollusk is understandable when you consider that those two groups may have diverged from a common ancestor.) Presumed to share a common ancestor with brachiopods, bryozoa don't have a superficial resemblance to clams. Instead, bryozoa have a superficial resemblance to moss. Bryozoa are tiny, colonial animals that today flourish in fresh and salty water the world over. Molecular clock data long suggested that they evolved in or before the Cambrian Period, though hard fossil evidence wasn't described until 2021.
Shortly before I visited him, Caron's work helped demonstrate that Hallucigenia sparsa — perhaps the principal icon of Burgess Shale weirdness — is less weird than originally thought, and actually related to modern velvet worms.
About 18 months after my visit, Caron described a likely relative of Hallucigenia. The new species was named Ovatiovermis cribratus, meaning "standing suspension-feeding worm," given its upright posture. The holotype specimen was collected by Collins in 1994. A second specimen of this rare creature was found, remarkably, by a participant in a guided hike to the Walcott Quarry.
Caron has remained productive. In August 2017, he coauthored a paper with Derek Briggs about a supersized version of today's planktonic arrow worms. Unlike modern arrow worms, this one was bigger, had more spines on its scary head, and likely swam near the seafloor (animals swimming higher in the water column likely would have escaped the underwater avalanches that buried and preserved the Burgess Shale fossils).
The original reconstruction of Hallucigenia, done by Conway Morris and highlighted by Gould, showed a bulbous-headed animal tottering on spiky legs with tubes projecting from its back. That reconstruction turned out to be both upside down and backwards. The animal was flipped right side up years ago, but just several weeks before my visit, Nature published a paper by Caron and Martin Smith showing the animal's cute little face, and reporting that what had long been mistaken for its head was really the remains of decay fluids squeezed out of its posterior. (If I were Hallucigenia, I'd feel very grateful for the rehabilitation of my anatomy and reputation.)
And many of the animals found in the Walcott Quarry are arthropods.
Arthropod fossils dominate the quarry, Caron noted, and the animals were already diverse by the time those biota fossilized. The arthropod Opabinia regalis was small but bizarre, with a Hoover-like extension projecting from the front of its head, and five eyes on the top of its head. Yes, five.
Much bigger than Opabinia was Anomalocaris canadensis. Anomalocaris and similar animals, known as anomalocaridids, probably swam at the top of the food chain at the time of the Burgess Shale. Anomalocaridids puzzled paleontologists for years, and for a long time, their various pieces were attributed to different animals. Laggania cambria's mouth was initially identified as a jellyfish, and Anomalocaris's grasping tentacles were mistaken for shrimp.
Another anomalocaridid, Hurdia victoria, was equipped with a pointy-ended battering ram for a head. (Maybe it just used its pointy head to rifle through sediments on the ocean floor, but a battering ram sounds more dramatic.)
Creepy to be sure, but all these animals were arthropods. (For perspective, just consider modern-day arthropods that lack the charm of extinction: spiders, centipedes, cockroaches, or scorpions. They're horrifying. And yellow jackets are mean.) One charismatic extinct arthropod species from the Burgess Shale bears a resemblance to the Millennium Falcon of Star Wars fame, and Caron and a coauthor duly named it Cambroraster falcatus in 2019.
Potential ancestors of modern-day vertebrates (including people) also appear among Burgess Shale fossils. Gould made much of the simple chordate Pikaia gracilens, but Caron suspects that Metaspriggina walcotti might be even more significant to chordate evolution.
Metaspriggina shows up in the Walcott Quarry, but many specimens are much better preserved at a new location. Caron and his team devoted many hours to assembling the Burgess Shale Virtual Museum (http://burgess-shale.rom.on.ca) but, just as opening the new Human Origins hall at the Smithsonian pretty much guaranteed a Murphy's Law announcement of never-before-heard-of Denisovans weeks later, not too long after the Burgess Shale site was launched, Caron and crew found a new quarry 40 kilometers from the Walcott Quarry: Marble Canyon.
Three years after finding Marble Canyon, Caron still couldn't hide his excitement about it. "We've only done a little collecting there so far, but considering the short time spent there, Marble Canyon could be at least as productive as the Walcott Quarry," he said. The new site shows slightly different communities from the same time as the Walcott Quarry.
In fact, his enthusiasm would prove well justified. In 2017, he coauthored, with his grad student Cédric Aria, a description of one of the earliest known mandible-mouthed arthropod species: 508-million-year-old Tokummia katalepsis. An old arthropod with a mandible mouth might not sound like such a big deal, but mandibulates are now some of the most abundant animals on Earth, comprising roughly 80 percent of named species. Insects, crustaceans and millipedes are all mandibulates. A 2018 feature in Science focused on Caron's research at Marble Canyon, including a new find: a hand-sized fossil resembling a helmet or, on a much smaller scale, a sci-fi ship. Nicknamed the "mothership," the fossil was, at the time of the feature story, the only fossil of its kind.
In 2018, Caron published another paper, based on several specimens from the original Burgess Shale site — and more than 500 specimens from Marble Canyon. The new species, Kootenayscolex barbarensis, is described as an annelid, the phylum that includes earthworms. Plenty of annelids were (and are) fancier than nightcrawlers, and the species Caron described bore stiff bristles. But even with tough bristles, annelid fossils are rare, making Marble Canyon's abundance even better.
In 2021, Caron published a description of yet another weird fossil found at Marble Canyon. Estimated at half a meter in length, Titanokorys gainesi belonged to the same group as Anomalocaris and Hurdia. While the new find wasn't the biggest specimen ever found of these animals, it was a giant for the time, with the biggest head carapace so far found for Burgess Shale fauna. In relation to the size of its body, its head was so big that Joe Moysiuk, Caron's coauthor, described the animals as "little more than swimming heads."
When we talked in 2015, Caron pointed out the presence of good Cambrian fossil sites in China. And he lamented that there are surely other good sites, but they're simply not accessible.
"A quarry is artificial in a way," he said. Quarries can provide good data on past environments, but a quarry only allows scientists access to certain rock layers. "There are different windows into the past and some are smaller than others." He elaborated, "Paleontology is challenging because sometimes it's like trying to understand all boreal forests by looking at a 10-meter by 10-meter patch and extrapolating that to all of Canada."
Caron's understanding of the Cambrian has come a long way since he first read Gould's book in the early 1990s. Gould's argument in Wonderful Life (based on the film starring Jimmy Stewart) is about contingency, the role of chance in the history of life on Earth. Gould contended that if you rewound the tape and started over, one little change could completely rewrite the history of life. If Pikaia had gone extinct, for instance, vertebrates would never have arisen. Further, Gould argued that the "weird wonders" of the Cambrian were evolutionary experiments, and that many more animal body plans existed half a billion years ago than exist now. Conway Morris, who was profiled and praised extensively in the book, later criticized it (a little too harshly, according to trilobite expert Richard Fortey, and I have to agree; the weird-wonders narrative Gould presented in 1989 largely matched Conway Morris's interpretations at the time). But what I wanted to know in August 2015 was what Caron thought of the Cambrian explosion.
For starters, he's no fan of the term "explosion." He considers the term fairly worthless, preferring "radiation" or "diversification." A big reason is that, since Gould's book was published, new research has shown that many of the Burgess Shale's "weird wonders" really are members of modern phyla, and Caron's own research has contributed to the improved understanding. His assessment of Wonderful Life now is that it's "probably wrong for the most part because it emphasized the differences, not the commonalities."
But that's not to say the Cambrian has lost its mystery. Mystery certainly persists.
Caron explained that the Cambrian Period started 542 million years ago. The oldest trilobites date back 521 million years, and the oldest soft-body preservation so far found happened in China 513 million years ago (a newer fossil-locality find in South China has since pushed that date back to about 518 million years ago). The Burgess Shale fossils are 508 million years old. He said, "I'm not so puzzled by the period of time between 521 and 513 million years ago, but I'm very puzzled by what happened 542 to 521 million years ago. Is the fossil record largely missing because soft tissue didn't preserve well, or is there some other reason?" There are well-preserved trace fossils from the beginning of the Cambrian, but Caron expressed skepticism of many interpretations. "We don't understand trace fossils that well. The same animal can leave more than one kind of trace."
I asked about potential triggers of the Cambrian diversification. I first heard of the Neoproterozoic Snowball Earth theory around the year 2000, and besides dramatic images of a planet enrobed in ice, it offered a potential explanation for the evolution of animal life, after billions of years of microbes. Another candidate for a Cambrian trigger was the rise of oxygen. Both explanations have their drawbacks, though. Neoproterozoic glaciers likely receded about 90 million years before the Cambrian, and there's not much evidence of changes in oxygen levels across the Ediacaran-Cambrian boundary.
Some researchers have posited a predator-prey arms race, but Caron thinks this raises a chicken-and-egg conundrum. He thinks the answer is in the genes. "The developmental aspect is probably the most important, and it doesn't necessarily need environmental changes to happen."
Evolution isn't random; natural selection proceeds by using traits that give organisms an advantage. But evolution can be driven by random changes in genes. The Cambrian Period likely witnessed tweaks to Hox genes. French naturalist Etienne Geoffroy Saint-Hilaire discerned a similarity of body plans between vertebrates and invertebrates in the early 19th century, and he was onto something.
Hox genes operate across bilaterally symmetrical animals, and control an animal's head-to-tail body plan. In fact, Hox genes, which drive positioning along the nervous system, are just a subset of a larger group of genes known as ANTP-class genes that perform similar functions across bilaterians. In a 2015 paper, Oxford zoologist Peter Holland described the functions of multiple ANTP-class gene groups. Besides Hox genes, he described ParaHox genes, which focus specifically on the mouth, gut and anus; and NK genes, which focus on the mesoderm (the middle layer of embryonic cells that develop into muscle, bone and circulatory-system tissues).
Changes in these developmental genes make sense to Caron as a potential trigger of Cambrian radiation because so much of the diversification in the Cambrian is at the phylum level.
"If you read the fossil record, that must have happened," he mused. After the Cambrian Period, the fossil record suggests, body plans were somehow locked in place, a process sometimes referred to as the canalization of body plans. (Most or all of the Cambrian body plans remain today. "Later extinction events didn't eliminate any phyla. They just trimmed the upper branches," he observed.)
Caron's logic is hard to ignore. In the half a billion years that have elapsed since the end of the Cambrian Period, our planet has experienced no shortage of drastic environmental changes: the formation and breakup of Pangaea, India slamming into Eurasia, at least one asteroid strike big enough to cause a massive extinction, rising and falling seas, rising and eroding mountain ranges, the Eocene hothouse, the Pleistocene Ice Age, and Deccan Traps volcanism spewing enough lava to swallow a Texas twice and giving Earth's atmosphere a makeover. Where they haven't driven species extinct, these changes have provided plentiful fuel to natural selection. Yet only the Cambrian Period brought such widespread diversification of animal body plans.
Caron pointed out in his 2018 paper on Kootenayscolex barbarensis that the Cambrian's apparent plasticity of body plans has implications for paleontologists: They can't really anticipate what fossils they'll find based on what kinds of animals lived earlier. All they can reasonably expect is that they'll be surprised.
So while he came to differ with the Wonderful Life "weird wonders" hypothesis that initially drew him to the period, Caron is confident that the Cambrian Period was unique. "It marked the emergence of phyla." Yet he remained cautious about understanding triggers, genetic or otherwise. "If someone claims to know the answer, don't believe them."
After our talk, he gave me a tour of the collections, showing his students at work cutting rock slabs and photographic specimens. (Photography with polarized light has proven especially promising.) He also showed me his fossil preparation area. As a volunteer and technician, Caron spent years prepping fossils, honing a skill that serves him well. "But I could use a technician of my own now," he grinned. He also showed my flats and drawers full of fossils, including a specimen from C.D. Walcott himself.
Before we left his office for the tour, I asked Caron how Cambrian fossils fared compared to dinosaurs. I expected a Rodney Dangerfield-style lament about Cambrian animals getting no respect, but to my surprise, he jumped out of his chair, grabbed a Japanese catalog, and showed me pictures of toys popular in Japan. Toy after toy featured a Burgess Shale fossil.
He usually gives popular lectures to adults but not long before my trip, gave a talk to about 700 school children in British Columbia. "After my talk, the kids went wild. They couldn't believe this was in their own back yard," he said.
Caron thought for a moment and continued, "There's a bigger story than dinosaurs. This is the story of where we came from."
To give myself a little latitude in catching Dr. Caron, I scheduled a couple days in Toronto, and found Ripley's Aquarium easily reached from my hotel via subway and SkyWalk. It was also situated next to the landmark CN Tower.
I suspect the aquarium's Dangerous Lagoon is its most popular attraction, but I focused instead on arguably less flashy inhabitants.
One phylum present in the Cambrian Period, and probably one that evolved long before the period started, is that of the cnidaria, including sea anemones, corals and jellyfish. Cnidaria might count among the first animals capable of sensation and movement. Besides sea anemones, the aquarium featured a nice display devoted to jellyfish.
The other fish I focused on were seahorses. Who doesn't like these peaceful little fish? And that's the thing to remember about them. They are fish. Back in 1997, while taking a DMNS class in invertebrate paleontology, I marveled at a statement the instructor made: Even after hundreds of millions of years of terrestrial evolution, the most diverse vertebrates alive today are still fish. At first, I thought the statement was ridiculous. Fish are all, you know, fish-shaped. But I forgot about the seahorses. Remember: A leafy sea dragon is a fish. A fellow member of our very own phylum.
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Narrative text and graphic design © 2015-2023 by Michon Scott - Updated June 4, 2022. All photographs by Michon Scott unless otherwise credited.