The Cambrian Explosion, part 1: The burghers of Burgess

Burgher: A privileged city citizen in medieval Europe.

And now, for something completely different.
(Monty Python)

First, there was no life. There were layers of sediment rock; sandstone, shale, limestone, but there were no fossils in them.

Then, suddenly, life burst, with fossils of trilobites, shells, corals… Some familiar, some strange creatures. But even the strange ones had their place. Trilobites were clearly arthropods, the phylum that contains basically all animals with shells: Insects, crabs, spiders, scorpions… The shells were brachiopods, which now are rare, rather than bivalves, but shells nevertheless. Corals were different from today but still corals. It was a world very different from ours, but also familiar. It was, basically, life as we know life to be.

Later the first primitive armored fish came, and the first land plants. Fish threw off their armour and got jaws, and and lobe-finned fish evolved into the first amphibians. Amphibians evolved into reptiles, reptiles to dinosaurs, dinosaurs to birds. Other reptiles got fur and started to nurture their fetuses inside, eventually becoming mammals, monkeys – and us.

But: Before the trilobites, there was still nothing. Life seemed to come out of a void, ready assembled.

At least, that was what the early geologists found. But not anymore.

Mines, quarries, roadcuts, tunnels have made many scars in the Earth. We have found many, many times more fossils. These fossils give us glimpse also into the dark past before the trilobites and brachiopods.

It started with a railroad in Canada.

In 1867, the British relieved the provinces of Ontario, Quebec, New Brunswick and Nova Scotia, which became the nucleus of modern Canada. During the next years, vast areas joined the new country. When British Columbia joined in 1871, Canada stretched from the Atlantic coast to the Pacific.

To connect the coasts, construction of the Canadian Pacific Railway started in 1881. Only five years later, the first trains rolled across the continent. A big lift for a nation, of large area, but with only tree and a half million people.

Canadian Pacific Railroad Hotel and Mount Stephen, Field, British Columbia, c. 1908

The hotel at Mount Stephen in 1913.
(See page for author, Public domain, via Wikimedia Commons)

20130702 50 Canadian Pacific Rwy, Field, BC - Flickr - davidwilson1949

The modern Canadian Pacific Railroad in the Rockies.
(David Wilson from Oak Park, Illinois, USA, CC BY 2.0, via Wikimedia Commons)

Sports resorts, like Banff and Mount Stephen, were built along the line to draw traffic to the railroad. Near Mount Stephen, the line crosses the Burgess Pass. We will never know who found the first fossil there. Possibly, an anonymous railroad worker picked up a trilobite, sweating as he gnawed the railroad through the wild mountain sides. Soon, the workers discovered that “stone bugs” were abundant in the mountain sides around Mount Stephens, and the “trilobite beds” became advertised as a tourist attraction in their own right.

Mt Stephen and the Burgess Pass: Far away in the Rocky Mountains in British Columbia (Google Maps).

Ogygopsis klotzi fossil trilobite tail (Burgess Shale Formation, Middle Cambrian; Mt. Stephen, British Columbia, southwestern Canada) (15083634948)

The tail of the trilobite Ogyopsis klotzi from Mt Stephen.
(James St. John, CC BY 2.0, via Wikimedia Commons)

Words reached Richard McConnell, a geologist for the Geological Survey of Canada (GSC). In September 1886, he came to collect, and published his first finds the next year. The same summer in 1886, the astronomer Otto Klotz was sent out by the Department of the Interior to use astronomical observation to pinpoint the longitudes of the stations along the long railroad. Klotz received some fossils collected by his field cook, which he sent to Carl Rominger, a geology professor at the University of Michigan. Rominger also published a first description of the fossils in 1887. Klotz and McConnell never settled the dispute on which of them was first.

The first sign that this place was truly special came in 1892, when another GSC geologist, Joseph Whiteaves, described a fossil he called Anomalocaris canadensis – “strange shrimp from Canada”. It looked like the body of a shrimp, with spiky legs, but without the head and the tail – but it took nearly one hundred more years, for Anomalocaris to reveal its true nature.

Anomalocaris, Burgess Shale - Redpath Museum - McGill University - Montreal, Canada - DSC07936 (cropped)

The fossil fangs of Anomalocaris.
(Daderot, CC0, via Wikimedia Commons)

Anomalocaris brine seep

Reconstruction of Anomalocaris, swimming in the Cabrian sea.
(PaleoEquii, CC BY-SA 4.0, via Wikimedia Commons)

Charles Doolittle Walcott is the great figure in the history of the Burgess Pass fossils. Despite his middle name, Walcott did an enormous work. Walcott was director of the United States Geological Survey from 1894 to 1907, and spent the rest of his career at the Smithsonian Institution in Washington.

Charles Doolittle Walcott Early 1900s

Charles Doolittle Walcott in his office, early 1900s. Note the geological map of the United States on the wall.
(See page for author, Public domain, via Wikimedia Commons)

Walcott got interested after reading Rominger’s description of trilobites, but it became 1907 before got there himself, at age 57. He wrote an article on the fossils the next year, and a guide to how tourists could go to collect for themselves:

“The best way to make a collection from the ‘fossil bed’ is to ride up the trail on a pony to about 2000 feet above the railroad, collect specimens, securely wrap them in paper, place them in a bag, tie the bag to the saddle, and lead the pony down the mountain. A fine lot can be secured in a long day’s trip, 6 a.m. to 6 p.m.”

In 1909, Walcott hit pay dirt: The Burgess Shale. Walcott quickly realized he had found a Mother Lode of fossils never seen before, some completely bizarre. The following years, he returned every summer and set up camp, together with his whole family, and published in both scientific journals, and in National Geographic under the headline “A Geologist’s Paradise”.

Charles Doolittle Walcott (1850-1927), Sidney Stevens Walcott (1892-1977), and Helen Breese Walcott (1894-1965)

Charles Walcott (left) working in the quarry together with his son Sidney and daughter Helen.
(Smithsonian Institution from United StatesUnidentified photographer, No restrictions, via Wikimedia Commons)

Tragedy struck when his wife Helena was killed in a train crash on July 11th 1911. Walcott buried his sorrows in work and spent five weeks digging fossils in what became a small quarry. The summers of 1912, 1913 and 1917 were also spent in the hillside, but then Walcott believed the quarry was exhausted. He only returned for brief collecting now and then until 1924, ending his remarkable fossil collecting career at the respectable age of 74. He went to the greatest outdoors three years later.


The Walcott quarry in 2009…
(Mark A. Wilson (Wilson44691) (Department of Geology, The College of Wooster).[1], Public domain, via Wikimedia Commons)

Charles Doolittle Walcott Excavating Burgess Shale

…and in its heyday.
(See page for author, Public domain, via Wikimedia Commons)

Field notebook of Charles D. Walcott from August 31 to September 3, 1909

Walcott’s field notebook from 1909, with the first sketches of the fossils.
(Charles D. Walcott (photo by Brian Boyle), Public domain, via Wikimedia Commons)

The fossils stayed safely in the Smithsonian Institution, but few bothered to do anything with them until the 1960s. Then, the Geological Survey of Canada started to collect more material, as part of a larger mapping project of the Rockies. At the same time, Harry Whittington, a trilobite professor at Harvard, had started to appreciate the strange beasts from Burgess. The two teams joined forces, and dug in the Walcott quarry during the summers of 1966 and 1967.

Walcott had named many fossils from Burgess, but had not got around to reconstruct them as live animals. He never had the time. It was Whittington & co who, using modern preparation methods and preparing the spcimens with tiny dentist drills, finally brough the Burgess animals back to life, and showed in glorious detail how bizarre they were.

(As a side note: One of the team members in 1967 did, thirty years later, become my teacher in paleontology at the University of Oslo: David Bruton was one of those people who live and breathe fossils, who just radiate passion for the life of long gone).

Whittington started with Marella splendens, “Marr’s splendid”, named by Walcott after his fellow paleontologist John Marr. Marella looked kind of like a trilobite without the shell: Only 2 cm long, it had a segmented body with limbs, antennae, and out from the head, four big spines. Two curved gently backwards, almost as wings, two rose above the back segments as protection. Marella was clearly an arthropod, but paleontologists still fight – i.e. publish papers few else read – on whether it was the direct ancestor of trilobites, or closer to the common root of arthropods. In any case, Marella was already a living fossil in Burgess.

Walcott Cambrian Geology and Paleontology II plate 26

Walcott’s reconstructions of Marella splendens.
(Charles Doolittle Walcott, Public domain, via Wikimedia Commons)

Opabinia regalis became the animal that made the Burgess Shale famous – and infamous. Walcott named it after the Opabin pass in the Candian rockies. When Whittington presented it to a conference in Oxford in 1972, the audience broke into a loud laughter! Opabinia had a segmented body, but no legs, no antennae, but instead a long trunk with a sort of jaw on the end – and five segmented eyes, each on a pedestal! Opabinia is today also regarded as a kind of early arthropod, but the lack of legs is still confusing.

Opabinia smithsonian

Fossil of Opabinia, with the long “trunk” in the front (left) and “flaps” along the side.
(Jstuby at English Wikipedia, Public domain, via Wikimedia Commons)

Burgess Shale reconstruction

Reconstruction of Opabinia sitting on the sea bottom. The threatening shadow of an anmomalocarid appears from behind.
(PaleoEquii, CC BY-SA 4.0, via Wikimedia Commons)

Anomalocaris canadensis, Josepth Whiteaves’ headless and shortlegged “strange shrimp from Canada” took a long time before it revealed its true nature. Whittington & co’s painstaking preparation of the fossils showed it to be a fierce predator, an arthropod with gilled swimming flaps. The “strange shrimps” were fangs on the head, which would catch and feed prey to a circular mouth with diaphgram-like teeth. Anomalocarids are now known from around 30 species, and fossils range in size from two cm to nearly two meters. The large anomalocarids were most likely the top predators of the Cambrian sea – but only to soft-bodied animals, as their diaphragm mouth was not mineralized.

Bizarre as they are, Opabinia and the anomalocarids are at least kind of recognizable, as segmented, shelled arthropod-lookalikes. But some of the Burgess-burghers really beat imagination. Meet Hallucigenia sparsa. Yes, the species name comes from the sparse fossils and the genus name from, well, the bad trip evolution or God apparently was on when designing it.

Imagine an up to 30 cm long, slightly thick earthworm. On its back, place two rows of sharp spikes, likely for defense. Underneath, one tube-like leg for each spine, with some extra tube-legs as tiny arms in the front. At one end, a slight thickening, the head, with a grinning mouth with tiny teeth and eye dots.

NMNH-USNM83935 Hallucigeniasp (cropped)

The fossil of Hallucigenia…
(Michael Brett-Surman, CC0, via Wikimedia Commons)

H. sparsa

…and a live reconstruction, with tube-like legs and spikes on the back.
(Jose manuel canete, CC BY-SA 4.0, via Wikimedia Commons)

Walcott was the first to note also Hallucigenia, but it was not until 1977 that Simon Conway Morris described it as a worm walking on stiff spine legs, with a row of feeding tentacles along the back. If you can say a worm has a back. Morris put the head at one end, where there seemed to be a thicker blob. Then, in 1991, Hallucigenia finally revealed its true nature – in China.

The Burgess Shale was the first place to host this Cambrian bizarro-fauna, but far from the last one. Today, “Burgess shale fauna” is used about similar organisms from many places. They are spread around the world, and also in time.

Geologists roamed the Yunnan province in southwest China during the 1930s and 40s, mapping and hunting for phosporite deposits for fertilizers. Trilobites and typical Cambrian fossils were well known, but on the 1st of July 1984, Xianguang Hou’s eyes fell on soft-body fossil in Chengjiang County near Kunming. Hou immediately recognized it as a Burgess-lookalike.

Since then, thousands of fossils of at least 120 different species have appeared across an area over 100 km wide. Chengjiang county hosts a fauna equally spectacular to the Burgess fauna.

Location of Chengjiang near the city of Kunming, the Yunnan province, southwest China.(Google maps).

Eoredlichia intermedia Chengjiang

The trilobite Eoredlichia intermedia from Chengjiang.
(Dwergenpaartje, CC BY-SA 3.0, via Wikimedia Commons)

Among them are several species of Hallucigenia and its relatives. One of these was a fossil that in 1991 revealed their true nature: Microdictyon had only hard plates where the spikes sit on Hallucigenia, but the tubes were still there. They literally turned the animal upside-down. Clearly, the tubes were a kind of legs, because nothing else could be legs. Finally, in 2015 electron microscope studies of Hallucigenia found the teeth and eyes – and moved the head to the other end of the animal.

Microdictyon Chengjiang

The fossil of Microdictyon, which finally revealed also Hallucigenia’s true nature.
(Smith609, CC BY-SA 4.0, via Wikimedia Commons)

Today, Hallucigenia and its relatives are lumped into the “sack-phyla”Lobopodia, containing fossils that look more or less like caterpillars or worms with legs. Lobopodia is a convenient one-size-fits-all phylum, but it may be a bit too convenient, because we do not know how these animals were related in real life.

The Burgess shale is from the middle Cambrian, around 510-505 million years ago – usually shortened to m.a. The Chengjiang fauna is clearly older than the Burgess fauna, but how much older is uncertain. There are radiometric dates only for layers a bit above and beneath it. But the age is early Cambrian, likely ca 518 m.a..

There are, of course, many more animals to describe from both Burgess Pass and Chengjiang. The big discussion will continue, on which of these belong to which phyla, and which animals may be ancestors to later animal phyla.

But, for now, instead of blowing your fuses with more latin names, we will look at these fossils as parts of ecosystems, rather than collections of individual species. It’s like the dino-loving kid who grows up and goes to college and discovers that they need to know more than the names of Triceratops and Stegosaurus to impress the teacher.

Fossils are biology. Long time dead biology, but biology, in which predators ate prey in the big web of food and dependency that all ecosystems have.

What did these ecosystems look like? How did they work? Who ate who? And, because Chengjiang is around ten million years older than Burgess Pass, how are the two ecosystems different?

The two Burgess faunas share some striking characteristics, although the species differ across space and time: Anomalocarids were the top predators both places. Below them were a wide range of small arthropods, like the common Marella at Burgess pass and Naraoia, a small shelled arthropod which dominates in Chengjiang.

Then follow a variety of more-or-less strange, but recognizable arthropods, mostly small. In the Walcott quarry, arthropods are one third of the species, but a more than half of the specimens. Swamps follow next; one fifth of the species, but one seventh of the specimens. Swamps are something for themselves, their own phylum.

Algae are also one seventh of the species, but only one in twenty of the specimens at Burgess, which is not strange since they are soft even among the soft bodied. But algae are important because they show that the bottom of the food chain was in place; they were the food for the vegetarian animals.

Notably, molluscs and brachiopods are altogether only one twelfth of species and one-sixteenth of specimens at Burgess. This is important: Shells are among the most common of fossils in general, but the Burgess fauna suggests that is not because they are the most abundant animals, but because the hard shells are those who most easy become fossils.

The same goes for the arthropods; trilobites rule the fossil arthropod world, but they are in a minority in the Burgess faunas. Only five trilobite species are known from Chengjiang and 19 from the later Burgess. Trilobites had started to spread, but were still a minority.

At Burgess, around one in seven of the species and one in ten of the fossils belong to the category “others”. These are the fossils that either are very rare, or so strange that we just don’t know which phylum to put them into – if they belong to a phylum we know, or are one of their own.

Finally, we come to the last category, rare, but important: Chordata. The animals not with shells, but with a notochord, a stiff string along their backs. These are the first ancestors of vertebrates. Usually seen as distinct dark imprints in the shale, they are tiny, jawless proto-fish, surely often hiding in the shadows to avoid the greedy jaws of anomalocarids. Later, the notochord would develop into cartilage or bone, and then these animals to jawed fish, amphibians, reptiles, mammals, apes – and us.

Pikaia gracilens is the most common at Burgess, known from over a hundred fossils. It is named after the Pike, a cousin of rabbits living in the mountains near Burgess and the fossils’ gracile, thin, simple shape. Pikaia looks like a tinly lancelet fish, jawless, no real head, but fins along the back and base, clearly defined muscle segments and tentacles or antennae in front.


Meet the family: Pikaia, the fossils of the first vertebrate – our earliest ancestor?
(Bruce Martin, CC0, via Wikimedia Commons)

Pikaia BW

Live reconstruction of Pikaia.
(Nobu Tamura (, CC BY 3.0, via Wikimedia Commons)

Only one other vertebrate comes from Burgess, Metasprigginga walcotti. Metaspriggina was originally thought to be related to the Ediacaran fossil Spriggina – more on that in a later post!– but is really a very primitive fish with possible beginning cartilage bones, gill slits (unlike Pikaia) and eyes. The gill slits and eyes suggest that Metaspriggina is closer to be our direct ancestor, and that Pikaia was a dead end.


Fossil Metaspriggina – a more direct ancestor than Pikaia?
(Stefan Walkowski, CC BY-SA 4.0, via Wikimedia Commons)

Metaspriggina NT small

Live reconstruction of Metaspriggina. Note the eyes.
(Nobu Tamura, CC BY-SA 4.0, via Wikimedia Commons)

Vertebrates occur also in the Chengjiang fauna, including the agnathan fish Myllokunmingia fengjiaoa, which was up to 3 cm long. But Myllokunmingoia is rare – perhaps because vertebrates actually were rarer earlier in the Cambrian?

Myllokunmingia big

Myllokunmingoia fossil from Chengjiang.
(Degan Shu, Northwest University, Xi'an, China, Attribution, via Wikimedia Commons)

Why did the Burgess fauna evolve and why did it disappear? What does it mean for evolution?

We noted earlier that a striking feature at both Burgess and Chengjiang is that that our old friends, the trilobites, are in a minority. Trilobites are otherwise the fossils of the lower Paleozoic, which everyone know. But at Burgess pass, only 2% of the fossil specimens have hard shells of chalk, and these encompass trilobites as well as molluscs, brachiopods and “small shellies” – more on them here! All the rest are soft-bodied or have their shells made of chitin, which is hard, but breaks down fast.

The dominance of hard shells in most places is thus a star case of survivorship bias and a major reason that Cambrian life, trilobites and shells, appeared to early geologists to come out of nowhere.It seems that Burgess fauna “Lagerstätten” are the ones that give a representative image of what Cambrian life really looked like.

Burgess fauna localities have now come to light around the world. They are still rare, but less than before, and give a picture of the Burgessians in space and time. Some of these are special because the fauna clearly differs from Burgess Pass and Chengjiang, suggesting different species in different places and environments. Some are notable because they show the evolution of Burgess organisms through time – and that they persisted longer that we believed previously.

In Canada, a string of localities now stretch out around 80 km southeastwards from Mt Stephen. Nearly all of them sit next to the Cathedral Escarpment, which was once a 100 to 300 m high cliff beneath the sea, on the edge of a large, flat platform. The Burgess faunas lived in its shadow, in the deeper water below the edge. This explains how the delicate animals could be preserved: Now and then, flows of mud ran down from the escarpment and buried the animals so fast, that they did not have a chance to rot or be eaten by others.

The most interesting of these new localities appeared in 2012, when the Royal Ontario Museum found spectacular site in the Kootenay National Park. Notably, it is around 200 000 years younger than the Walcott quarry. Although a mere blip on the time scale of geology, it was enough to make the fauna quite different, with a full one-fifth new species. One of these was the Metaspriggina fish mentioned above, but also many organisms similar to Chengjiang. It not only shows that these survived into the middle Cambrian, but also that they spread out from China.

Tit for tat, the Chinese also have a Burgess type fauna from the Middle Cambrian, near the city of Kaili in the Guizhou province in south China. It is around 506-510 million years old, likely in between the Burgess and Chengjiang faunas. Overall, the fauna the familiar one, but it also has fossil invertebrate eggs!

Other places with Burgess fauna fossils now include the Wheeler shale in Utah. and Emu Bay on Kangaroo Island in South Australia. Emu Bay is now known for trilobites and anomalocarids with preserved eyes. The anomalocarids had typical arthropod or insect compound eyes, up to 3 cm in diameter and with 16 000 lenses. It shows that good vision already was established, and that the anomalocarids were just as well equipped with vision as trilobites.

The most remote and inaccessible of the Burgess type localities is at Siriuspasset, on north Greenland. At 82 degrees northern latitude, it can be reached only by ship and helicopter in summer and dog sled in winter. Siriuspasset is 518-515 m.a., and thus of the same age as the Chengjiang fauna.

Sirius Passet in the Cambrian

Geological map with the remote location of Sirius Passset.
(David Harper and colleagues, CC BY-SA 4.0, via Wikimedia Commons)

But the fauna is as far out as the location.

Think of the small, segmented shelly insects you sometimes find in your basement. Those, which signals trouble if there are too many of them and are why you should love spiders. Give them long antennae and fangs, scale up to the size of a hand or two and release them swimming in the sea. Voila, you have, almost, a gilled lobopod.

Gilled lobopods were likely a distinct branch of arthropods. They had a long, segmented body, with a head typically with big fangs, and round, cone shaped mouths with a circle of pointed teeth. Clearly, they were carnivores. Along the body, they had gilled flaps, similar to the anomalocarids, and they were probably closely related.

At Siriuspasset, the gilled lobopods are among the most prominent fossils, and they have also been found in other locations, including Chengjiang.

Diania specimen

The gilled lobopod fossil Diania, from Chengjiang in China.
(Qiang Ou and Georg Mayer, CC BY-SA 4.0, via Wikimedia Commons)

Other local strangers there were Halkiierids, which basically looked like big slugs in chain mail. May be they were slug-likes in a kind of armor, may be their own phylum.


Halkiierid fossil from Sirius Passet, looking like abig slug in chainmail.
(derivative work: Martin (talk)Halkieria2.jpg: Photo taken by Jakob Vinther, CC BY-SA 3.0, via Wikimedia Commons)

For long, it was assumed that the Burgess fauna disappeared as swift as it came, at the end of the Cambrian. Outcompeted by more sensible, useful constructions like trilobites, shells and sea scorpions, not to mention the emerging vertebrates. Animals we know. Survival of the best constructions.

Not so fast.

A big surprise came in 2010: Burgess fauna animals were found in the early Ordovician, ca 477 m.a. . The place was Fezuoata near Zagora in the dry desert in southeast Morocco, near the border to Algeria. Fezuoata is a fossil mother lode, including our friends the anomalocarids, marrellas, lobopods, sponges, and a truckload of trilobites. It shows without doubt that the Burgess fauna survived into the Ordovician.

To nail the Ordovician burgessians home, another site with Burgess fossils appeared in the town of Salta in northwestern Argentina in 2014. It is slightly older than Fezuoata, around 480 m.a. and so far not much investigated. And then there is the Llanfallteg Formation in Wales, a deep marine deposit also with a few Burgess fossils, and which is in fact Middle Ordovician, ca 460 m.a.

The Burgess fauna continuing into the Ordovician raises the questions: When did it end? And why? What if we find the right lagerstätte of the right age to see also a flourishing Burgess fauna through the Ordovician? May be the Burgess fauna really died out in the severe extinction that ended the Ordovician 443 million years ago?

The cause for that extinction was probably an ice age. That ice age, just like the current one, which I wrote about in this blog post, probably happened because a big landmass over the south pole accumulated a big ice sheet. With sea water tied up in ice, sea level fell. In the Ordovician, there were wide, shallow continental seas, which then dried out, taking with it much of the shallow water fauna.

Life itself may also have contributed to the ice age, because of the first emergence of land plants and a big spreading of micro-phytoplankton. As the name suggests, phytoplankton are important contributors to photosynthesis, and these developments thus pulled CO2 out of the air, reducing the greenhouse effect.

The cooling and the extinction is thus an example of how life itself changes both climate and through that the path of evolution, like we saw in this earlier post on the Carboniferous ice ages.

Only the future will, may be, tell for sure if the Burgessians slowly died out, or if they left the building with a bang in the end-Ordovician extinction. But, in any case, it is clear that the Burgess fauna was not just some bizarre evolutionary misunderstanding at the dawn of the Cambrian. The Burgess fauna was the normal fauna of the Cambrian and at least into the beginning of the Ordovician.

Which brings us back to the question that haunted Darwin: Sure, the Burgess fauna shows that there are likely precursors to trilobites living among them, like our precursors, apes and monkeys still are with us. But still, where did it all come from? What happened in the big, black void before the Cambrian?

Quite a lot, which we will see in the next blog post.

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