The alien world of the Burgess Shale



The Cambrian is remembered as a time when there was nothing on land and all life existed in the oceans. Gone were the days of simple one-celled organisms for an evolutionary explosion had just occurred and animal life had mushroomed into hundreds of new forms. It was a time of change, when the seas were ruled by creatures seemingly straight out of science fiction.

In the Canadian Rockies lies one of the best ancient fossil sites in the world. The Burgess Shale is a gigantic rock formation made up of black shale. It dates back to a time when the planet would have been almost unrecognizable. The globe was dominated by several continents, all of which were concentrated around the Southern Hemisphere. If we could have viewed it from space we may have seen brown and bare rock devoid of any greenery. Plants had not yet made the odyssey onto land and so the world was dominated by marine beasts unlike any seen by human eyes.

The Burgess Shale is one of the most famous areas for preserving these strange creatures. The site was discovered in 1886 by Richard McConnell of the GSC, the Geological Survey of Canada. Since then, the GSC had been prospecting the area for the fossils of Cambrian life forms and soon a whole environment began to emerge. While many fossil hunters have combed the shale for a long time, it was Stephen Jay Gould’s 1989 book Wonderful Life that truly brought it to the forefront. On the whole this is a biota which marks the middle of the Cambrian Period, and the animals here are roughly 508 million years old. Most of the fossils were either misidentified or not named right away although any taxonomic messes were soon to be cleaned up. Subsequent studies of the rocks here also showed that there was a number of quarries all over the formation. These were explored at different times over the past century plus or so and every new quarry has revealed something extraordinary. The animals here were preserved via a process that has allowed even the soft tissues of the animals to remain as they were, as perfect imprints in the rock.

All of the animals here are products of the Cambrian Explosion. This event was a boom of life that occurred somewhat earlier during this era and gave rise to some of the strangest creatures that have ever lived. Most of them belonged to families doomed to die out while others managed to pull through. First of all, nothing at this time had a backbone. For many millions of years, invertebrates would be the dominant form of life on the planet and nowhere is it more obvious than here in the Burgess Shale.

The biggest animal, and also top predator of the Burgess Shale is Anomalocaris. Like many of the other creatures here, it was an arthropod. Today’s arthropods include insects, spiders, crabs, centipedes and almost anything else with jointed limbs and an exoskeleton. Cambrian arthropods though were far from being so standard and Anomalocaris was just one of these. Its name means ‘odd shrimp’ and this is a complete misnomer.

Anomalocaris by Andrey Atuchin

Anomalocaris was anything but shrimp-like. It is a type of very ancient arthropod known as an anomalocaridid. These animals were carnivores that used grasping claws to snare their prey. Anomalocaris’ strange body design caused a few errors during its classification. Its rounded mouthparts were thought to be fossilized jellyfish while its graspers were assumed to be the tails of lobsters and similar crustaceans. It took a while but soon enough the creature began to take on its current form. It was ventrally flattened and used a set of flattened, undulating fins along its body to swim while a lobed tailfin helped it steer. The mouth of Anomalocaris echoes a segmented pineapple ring in shape and form, and is situated on its underside while two large eyes stare at the world above. This is the standard anomalocaridid design and is one that died out when the family went extinct.

Anomalocaris is the giant of the shale, at a meter long. It fed by grabbing little animals and holding them close to the underside of its head while moving the jointed graspers back and forth. This enabled the animal to shred its prey into smaller, more manageable pieces. It was thought that it was able to process other hard-shelled animals, including other arthropods like the ubiquitous trilobites. Evidence for this was first discovered in fossilized feces thought to come from Anomalocaris. However, later it was deduced that these might in fact be trilobite feces, bits and pieces of animals that they had encountered while scavenging on the seabed. Even if it just fed on softer prey, Anomalocaris would have been a formidable hunter, the world’s first large apex predator. Its massive eyes suggest that it hunted by sight. It had a pair of compound eyes, each with 30,000 lenses. These were positioned on stalks on top of its head and gave this armored beast an incredible field of vision.

There were still smaller hunters in the shadow of these huge animals. One of the weirder ones was Opabinia. It was a tenth the size of its massive neighbor and was also a distant relation. Opabinia’s body plan was similar to that of Anomalocaris, with the same swimming apparatus that helped it undulate its segmented body through the water. It lived close to the seabed, extending its clawed proboscis towards the sand to capture any tiny, soft-bodied invertebrates that swam its way. Opabinia’s eyes were also located on the top of its head, all five of them to be exact. It took a while to firmly ascertain its classification but as far as we know, this creature was a stem-arthropod. That means that Opabinia, like many of the Burgess Shale creatures, was related to the closest common ancestor of arthropods and annelids, or segmented worms.

Not all the arthropods here were giants. One of the more delicate and certainly spectacular fossils here was Marrella. This is the commonest animal in the Shale, with as much as 25,000 fossil specimens in existence. Marrella was related to the stem-arthropods, the common ancestor from which all other members of the family evolved. Other than this, not much is known about its exact classification. It was half as big as a human finger, and was beautiful in its construction. Marrella’s head was crowned by a pair of backward-curving spikes and its shell had a matching pair as well. The animal had two pairs of massive antennae that swung through the water when it swam. Marrella must have moved somewhat like a larval crab or a shrimp, by beating its many pairs of limbs.

While many Burgess Shale animals can be put into at least the arthropod group, there are plenty of others that defy classification. One such creature is the armored Wiwaxia, an animal so bizarre that you would be forgiven if you thought it didn’t exist. Outwardly it looks like a scaly cushion with spikes jutting out of its back. Its discovery turned scientists’ beliefs about the Cambrian on their heads as they raced to classify this unearthly beast. It has been bouncing around families, from a stay with mollusks to polychaete worms. Today it is known to have some affinities to the mollusks. It moved in a similar way, by contracting a soft and muscular foot on its underside. The little creature was a grazer that fed on microbial mats of marine bacteria on the seabed. For this it had a rasping tongue that helped it scrape off its food. Wiwaxia’s scales are known as sclerites. Eight rows of these pinecone-like scales cover its little body, and as the creature grew, these sclerites became more and more numerous. It would shed these to grow, and the spikes on its back would grow in during adolescence.

Hallucigenia by Andrey Atuchin

Anomalocaris and Wiwaxia are not the only animals to have been given a complete makeover recently. The centimeter-long Hallucigenia went through so many alterations that one might imagine the paleontological community to have given up after the first two tries. It has been known since at least 1977. It superficially resembles a worm with fourteen spines on its back and a matching set of tentacles. For a long time, it was reconstructed walking on its spines while the tentacles waved through the water above it. More recently it was finally righted. The spines were for defence and its walked along the seabed on a set of soft limbs. Hallucigenia’s head extended farther than its legs and pointed towards the seafloor to search for food through the sand. Long and thin tentacles extended from its neck, waving against the current while a pair of tiny eyes looked upward into the water above it. For a long time it was unknown just what Hallucigenia was. It is now thought to be a relative of velvet worms. Today, the velvet worms are a group of small invertebrates that look like soft-bodied centipedes. Hallucigenia of course was completely shape and form, other than the fact that it had no shell or any hard body parts. It has managed to be preserved in the rocks due to a unique process that allowed almost all organic compounds to be fossilized. This extends to even the soft parts and innards of the invertebrates of the shale.

One such soft-bodied terror was Ottoia. It was a type of marine worm called a priapulid, known to some as a penis worm. This rather ribald moniker comes from its strange shape which can only be described visually. These odd little monsters are still alive today, living buried in the seafloor and functioning as passive hunters. Ottoia was probably doing something quite similar. The fossils of this creature are found in the same position as when they died in their burrows. Only the tip of their spiny proboscis protruded from the sand. These spines were razor-sharp in life and allowed the worm to capture any passing prey that were foolish enough to swim close by. It could retract this organ back into its burrow to consume its victims. Fossils even show us other worms of its kind preserved in its gut. Like penis worms today, it was a cannibal. Ottoia specimens are also incredibly common in the Burgess Shale, with around 1000 specimens on record.

The Cambrian was a time in Earth’s history when the evolutionary arms race had begun at last. Hard-bodied predators were going after their hapless prey, and often the prey fought back with all manner of spiky defenses. The soft-bodied animals though, had one more card to play. Not only were these still diverse, but they positively thrived as sessile hunters and grazers, hardly straying from their refuge on the seabed. The reign of these spineless monstrosities though, was not to last.

Pikaia by Andrey Atuchin

One revolutionary animal from the Burgess Shale would change the course of life on Earth for the next half a billion years to come. Its name is Pikaia. It looked somewhat innocuous – a finger-sized swimmer with a flattened and transparent body. It must have looked like a small ribbon as it undulated through the water. Despite this rather standard appearance, Pikaia’s uniqueness lay on the inside. For a while, it had been classified as a marine worm, a polychaete, but fossils of relatives in China have revealed something extraordinary. It is one of the first animals to actually carry a notochord. It was a stiffening structure that held up a number of powerful blocks of muscle in its tiny body. These are muscle blocks which would one day set the stage for all vertebrates.

On the whole, Pikaia resembles a lancelet. These little creatures are still alive, and are about as unnoticeable as their extinct relative. They are, however, relatives of the progenitor of all vertebrates. Pikaia and its Cambrian cousins prove just how ancient this family is. The alien beasts of the Burgess Shale, while weird, were not always as unearthly as we think. They were merely setting the stage for other animals to come, and these future creatures would rise to dominate the seas and finally the land, in a very big way indeed.

Image Credit: Lucas Lima and Earth Archives