Cambrian explosion: the biological big bang in the history of the Earth

Welcome to one of the strangest times in Earth's natural history.
Christopher McFadden
Opabinia regalis on the left and Anomalocaris on the right.1, 2

The Cambrian explosion, which occurred around 542 million years ago, was one of the most "productive" for want of a better word, in the history of life on this planet. This period was the "Wild West" of speciation where everything and anything went.

Of course, many deadends evolved, but a very selected few made it through the period to lay the genetic foundations for many modern phyla. Some of the creatures that we've found from this part of Earth's history are so strange that you'd be forgiven for thinking they might be the work of some insane surrealist artist.

Marking the beginning of the Palaeozoic Era (542 to 250 million years ago), this is one of the most interesting and least understood periods in natural history. While many different body forms were put through the wringer during this time, the most successful modern phyla (such as the chordates, which include vertebrates) were actually rare at the time. 

While the Cambrian explosion was undoubtedly an important time in natural history (it has even been called "Life's Big Bang"), we can't really be sure if it was unique or not in terms of speciation. Since this event marks the point when creatures began to evolve hard body parts like shells, the chances of their preservation were greatly increased from those of their soft-bodied ancestors, and the fossil record becomes much more complete from this time onward.

Unless we find evidence of exceptionally well-preserved soft-bodied organisms that predate the Cambrian explosion, like those from the even stranger Ediacaran Period, we can only surmise what might have come before. 

The Cambrian–Ordovician mass extinction event at the end of the Cambrian (approximately 488 million years ago) would put to rest this era of experimentation, with a relatively small number of organisms living to fight another day. 

What are some of the strangest creatures from the Cambrian explosion?

All of them are very interesting, but you are probably anxious to see some of the weird organisms that evolved during this period? So, let's get stuck in.

There are so many strange creatures that evolved during this period that it is difficult to make a shortlist. But here we present those species that we believe are probably the oddest things you've ever seen. 

Hold on tight, this is about to get very strange. 

1. Hallucigenia is aptly named

cambrian explosion hallucigenia
Modern reconstruction of Hallucigenia sparsa. Source: Jose manual canete/Wikimedia Commons

Let's kick things off with perhaps the strange life form to ever roam our planet. Looking like a cross between a pair of combs and a bandy-legged worm, this creature truly is as alien today as it must have been in its day.

It was first discovered by Charles Doolittle Walcott over a century ago, who is said to have been so confused by its appearance that he thought he must be "hallucinating". From that moment, the creature's name was never in doubt. 

Most specimens range in size from between 3/16 and 2 and 3/16ths of an inch (0.5cm - 5.5cm). The creature lived roughly 505 million years ago

Believed to be an early invertebrate, it is characterized by having between seven or eight spindly "legs" with a matching number of sharp spikes along its "back." It is anyone's guess which end of the animal was its head or tail.

Specimens of the creature have been found in Cambrian-era Burgess Shales of Canada and China either more or less complete or with just their spines remaining. The Burgess Shale is one of the most important geological deposits around the world and has proven to be a literal treasure trove for paleontologists over the years. 

Early reconstructions of the creature had it using the spikes as some form of stilts, but paleontologists now believe the spikes were held erect from its back. 

For decades, naturalists pondered whether Hallucigenia represented a completely new (and completely extinct) animal phylum of the Cambrian period, often simply placing it into the holdall taxon of "problematica." Today, however, it's believed to have been a member of the extinct lobopodian phylum, and/or a remote ancestor of onychophorans, or velvet worms.

At least we think so. 

2. Anomalocaris is something else

cambrian explosion anomalocaris
A modern reconstruction of Anomalocaris. Source: Murray Foubister/Wikimedia Commons

Another of the strangest organisms to evolve during this period was the enigmatic Anomalocaris. Roughly translated as meaning "abnormal or strange shrimp," this creature would have been an apex predator in its day over half a billion years ago.

A member of the now-extinct genus radiodont (meaning radial or circular tooth, an order closely related to modern-day arthropods), it was first discovered by Joseph Whiteaves in 1892. Other examples have also been found including in the aforementioned Burgess Shale. 

Like most Cambrian explosion fauna we'll discuss here, the creature has a very odd body shape and design, but does share some features we are not too unfamiliar with modern animals. For example, its segmented body, and prehensile limbs are not too dissimilar to modern arthropods. 

For its day, Anomalocaris was a true giant measuring in at up to 6 feet (183 cm) in length. The creature appears to have been a very good swimmer and would have propelled itself along by undulating its flexible flaps or winglets along the length of its body.

Its relatively large front limbs are thought to have been used for predation and could be used to scope its prey into its unusual disk-like mouth on the underside of its head. For its time, its eyesight would have been excellent, using its complex compound eyes with some believing it may even have rivaled modern-day dragonflies. 

Anomalocaris’ mouth was made up of 32 overlapping plates which researchers believe could easily crush prey. Some specimens have been found with fossilized remains of trilobite carapace in its stomach, showing a clear indication of the kind of things it evolved to kill and eat. 

3. Kerygmachela is Anomalocaris' "prettier" cousin

cambrian explosion Kerygmachela
Source: Tae-Yoon S. Park et al.

Beauty, as they say, is in the eye of the beholder, but in a world where everything else is weird-looking, it might not take much to be considered "beautiful". This is certainly the case for Kerygmachela.

While it may look very intimidating, this creature, at least specimens found thus far, rarely exceeds 7 and 3/32ths of an inch (18 cm) in length. Kerygmachela lived roughly 520 million years ago. 

A gilled lobopodian found in the Buen Formation of Sirius Passet Lagerstätte, in Greenland, this creature was first described in 1993. From its body form, you can see why it has been classified as a relative of other creatures like Anomalocaris, and perhaps a distant ancestor of modern-day true arthropods. 

Its name comes from its rather elegant and exaggerated front appendages and means, roughly translated, "proclaimed claw". These appendages terminate in a series of rather nasty-looking spines which were probably used for capturing its prey. 

In life, it would have had a pair of sessile, or slit-like, compound eyes that were located just behind the point where its very large appendages join its "head". A small forward-facing mouth was located below its head and bore a pair of probe-like structures.

The creature probably swam in a similar fashion to Anomalocaris. Another key feature of this animal is its elongated tail spine. Long thought to be some kind of sensory organ, called cerci, there is some debate as to what this organ was actually used for.  

4. Marrella is another freak of nature

cambrian explosion marrella
Source: 10 Tons/Twitter

Another of the Cambrian-era's oddest experiments is a creature called Marrella. Living roughly 520 million years ago, it is a long-extinct genus of arthropod.

Marrella is one of the most common creatures found in the aforementioned Burgess Shale Lagerstätte

The creature was first discovered in 1909, again in Burgess Shale by Charles Walcott, who initially believed it to be some enigmatic variety of trilobite. Trilobites, in case you are not aware, are some of the most common early lifeforms and are often used to date old rocks, as they are so common and appeared very rapidly in geological time

If you've ever visited a natural history museum, chances are you've seen plenty of them. 

Marrella was a tiny beast, measuring roughly 25/32 inches (2 cm) long, and is one of the more "beautiful" preserved animals from this period. 

This creature, or rather its remains, is characterized by its paired antennae, rear-facing head spikes, and 25 or so body segments. Each of these segments also has its own pair of "legs".

Marrella is thought to have been fed by scavenging for organic debris on the ocean floor. They are one of the few genera from the Cambrian explosion to survive the large extinction event that marked the end of the Cambrian, and survive into the Devonian period (between about 419 million and 359 million years ago).

5. Opabinia looks like some kind of weird organic vacuum cleaner

cambrian explosion opabinia
Various reconstructions of Opabinia through time. Source: The Royal Society

With five eyes, a long forked frontal trunk, and a segmented body, Opabinia might be one of the strangest evolutionary experiments of the Cambrian explosion. First discovered in the Burgess Shale in 1912 (we'll let you guess by whom), this creature would have been roughly 2 and 3/4 inches (7 cm) long and lived around 505 million years ago. 

Opabinia is thought to be closely related to the previously mentioned Anomalocaris, it is believed the creature would pass food to its backward-facing mouth using its strange frontal proboscis. 

It is one of the less common creatures preserved in the Burgess Shale, and to date only around 20 or so identifiable specimens exist. Scientists are not entirely sure about Opabinia's mode of life, but it is likely that it roamed the seafloor hunting and grabbing creatures smaller than itself to consume. 

It is also possible that Opabinia may have been a scavenger

6. Habelia is an early ancestor of mighty sea scorpions

cambrian explosion Habelia
Artist's reconstruction of Habelia. Source: Royal Ontario Museum

Another interesting specimen from the Cambrian explosion is the weird-looking HabeliaFirst discovered in 1912 by you know who, more than fifty specimens have been collected. 

Thought to have been an ocean-roaming predator, it lived around 508 million years ago in what is today the Canadian Rockies in, yes you've guessed it, the Burgess Shale. 

For many years, Habelia represented one of the most difficult specimens to categorize until recently

Measuring in at around 25/32 inches (2 cm) long, this creature was heavily armored and well-equipped for a hunter's life. One of its standout features is its large jaws, technically called gnathobases, situated underneath its large head shield. It also has a set of long dorsal spines on its thorax and a long spine on its tail. 

Recent studies and reconstructions of this creature appear to reveal that it had a Swiss Army Knife-like set of jaws that appeared to provide a range of tasks much like the different teeth in a mammal's jaw. It is believed these different "tools" were an adaptation for its apparent diet of eating hard carapaced and shelled prey. 

Studied specimens also appear to have well-developed limbs on their thorax, perfectly adapted for rapid locomotion on the seafloor. It is believed that these creatures would hunt down and kill small or immature prey like trilobites or other small, hard-shell organisms. 

7. Meet your very, very distant ancestor (possibly), Pikaia 

cambrian explosion pikaia
Pikaia might be a very distant relative of yours. Source: Nobu Tamura/Wikimedia Commons

Less of an odd-looking creature than the others listed above, this creature is incredibly interesting, as it is the most primitive known vertebrate - making it a very early ancestor of all living vertebrate animals alive today. Pikaia, first discovered in the Burgess Shale in 1911, is one of the least common preserved species within the fossil-loaded rock formation. 

Existing around 530 million years ago, this creature is one of the best-studied of all preserved fossils from the period. 

A primitive chordate (the phylum that includes vertebrates), this creature resembles a modern-day lancelet and probably swam in a very similar fashion to eels. It measured in at around 1 and 1/2 inches (3.8 cm) long and had an elongated, fish-like body, and a poorly defined head structure. 

It had two elongated antenna-like tentacles that protruded from the front of its "head", and a series of short appendages underneath that may have acted as primitive gills. 

Although primitive, Pikaia shows the essential prerequisites for vertebrates, including a very primitive notochord – a flexible "rod" found in chordates and which develops into part of the vertebrate backbone. A nerve chord and vascular system, other key vertebrate structures, have also been identified in Pikaia specimens.

When alive, Pikaia would have resembled a compressed, leaf-shaped animal with an expanded tail fin. Its flattened body was divided into pairs of segmented muscle blocks, that would have been seen as faint vertical lines. 

An approximate proxy for what Pikaia may have looked like in life is the modern-day Branchiostoma.

8. Wiwaxia looked like a half-coconut with spikes

cambrian explosion wiwaxia
CGI reconstruction of Wiwaxia. Source: Martin T. Smith/Wikimedia Commons

Wiwaxia is another of this period's strangest creatures. Effectively a plate-armored dome with sword-like protruding blades, this might be one of the oddest animals from the Cambrian explosion. 

Specimens range in size from 5/64ths of an inch (2 mm) to 2-inches (5 cm) long, scientists are now pretty confident they have examples from most of the life stages of this animal. 

Little else beyond its impressive defensive structures has been preserved, so its classification is tricky. The opinion is split between it being a kind of ancient worm or a kind of mollusk.  

9. Aysheaia looks like some kind of terrible parasite

cambrian explosion Aysheaia
Reconstruction of Aysheaia in life. Source: PaleoEquii/Wikimedia Commons

And finally, we present our last amazing creature from this period - Aysheaia, which is one of the rare animals from the Burgess Shale (and Wheeler Formation in Utah). Living between 570 and 500 million years ago, this creature ranged in size from 1 to 6 cm long. 

A soft-bodied lobopod, it shares some features with velvet worms and tardigrades. It had ten pairs of spiked limbs with claws, and a ring of finger-like appendages around their mouths, as well as two grasping appendages on their heads. Specimens of this animal are commonly associated with ancient sponges, indicating that it may have fed off the sponges in life or avoided predators by living in sponge colonies (or both).

Its mouth and mouthparts are also ostensibly similar to modern tardigrades, which are omnivores. 

And that, ancient lifeform fanatics, is your lot for today. 

There is no doubt that the Cambrian explosion was a fascinating time in Earth's history and it lay the foundations (literally and figuratively) for many of the main phyla of animals alive today. 

However, there is also no doubt that many of these creatures are, to our modern eyes, as alien to us as aliens can be. 

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