Bilateria is a large clade or infrakingdom of Animalia, just below Eumetazoa that features organisms with bilateral symmetry – a clear right and left side that are a mirror image of each other. This body plan means bilaterians have a clear front “head” and back “tail”, with a right side and left side that look similar.
That means everything from insects and spiders to crabs, birds, dinosaurs, crocodiles, mammals and even us primates are bilaterians.
But when did Bilateria first show up, and why do they say bilaterians might have caused the “Cambrian Explosion”?
About Bilateria
Scientific Classification
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Infrakingdom: Bilateria
Time Frames
Molecular Clock: Cryogenian, 660–636 million years ago (Dos Reis et al. 2015)
First Trace Fossil: Ediacaran, 560–551 million years ago (Helminthoidichnites)
First Body Fossil: Ediacaran, 560–551 million years ago (Ikaria Wariootia, proposed maker of Helminthoidichnites trace fossil) – proposed!
Characteristics
Bilateria are animals that have bilateral symmetry (a clear left and right side that are mirror images of each other) during embryonic development. Bilaterians have a front (head) and back (tail), with a right-left symmetrical belly and back.
This body plan is extremely important for very physical animals who can move effectively through 3D space. And bilaterian evolution was very important in shaping the world we know today.
What are Bilaterians Exactly?
Bilaterians are all true animals and Eumetazoans except porifera (sponges) and cnidarians (corals and jellyfish). That means everything from bugs and crustaceans to fish, dinosaurs and mammals like us are all bilaterians. We have a right and left side that are mirror images of each other, as well as a clear “head” and “tail”.
Bilaterians are extremely ecologically important because our body plan helped us do something that no metazoans or eumetazoans before us could do – live in true 3D. Without bilateria, there might not have been a Cambrian “Explosion” as we know it, and the modern world would simply not have been possible.
How did Bilaterians Change the World?
You can’t overstate the ecological importance of Bilateria in our planet’s history. Before bilaterians, the other members of Animalia (incl. sponges) and Eumetazoa (incl. corals and Ediacaran biota like Charnia) were all sessile – they can’t move, they just sit in one place and filter-feed, more like a plant than an animal. That’s effectively living in 2D space only.
But the Bilaterian body plan is different. Think of spiders, crabs, wolves, tyrannosaurs, birds and us humans – we can move around, swim, fly and burrow into the ground.
Before bilaterians, there were no predators (there couldn’t be, no one could move to catch anyone else!). And the ocean floor was also different – it was covered in bio mats (thick mats of bacteria), with no loose soil. Meaning there was no oxygen in the mud, so nothing could have deep roots.
Bilaterians showed up and started burrowing into the ground, swimming up in the water column – they effectively started churning up the stagnant mud and oxygenated it, allowing other creatures to delve deeper into the soil, driving all sorts of evolution.
The World’s First Predators
And, since they could move, Bilaterians could become predators. And when you have predators, evolution kicks into overdrive.
What we think of as the “Cambrian Explosion” is probably only the fact that “hard parts” (shell, bone, armour etc.) first developed as a direct response to predation by bilaterians. In short: Bilaterians probably caused the Cambrian time radiation, which led to the world we know today.
Which two groups make up the bilaterians?
Bilaterians split into protostomes (arthropods, mollusks etc.) and deuterostomes (echinoderms and chordates) whose trace fossils appear in the Ediacaran and Cambrian time, but probably split sometime around the Cryogenian–Ediacaran boundary (Blair et al. 2005).
Are humans bilaterians?
Oh yes, we humans are definitely bilaterians! See, chordates are bilaterians, and chordates are what become vertebrates – everything with a backbone. And, since you have a spine (not to mention a head and “tail” end and symmetrical left and right sides, you are most certainly a part of Bilateria.
You’ll be quite surprised to learn that we humans are deuterostomes, too.
What are Non-Bilaterian Animals?
Cnidarians are our closest non-bilaterian relatives. This includes corals, sea pens and even jellyfish – even though jellies can kinda move up and down in the water column, they still don’t have bilateral symmetry. They have radial symmetry – like a circle, with no clear left or right.
And you can clearly see in a jellyfish’s larval stage that it’s non-bilaterian – they are sessile polyps that sit still on rocks and filter-feed for half of their lives, like corals.
What are the first Bilaterian fossils?
Helminthoidichnites, a trace fossil from the Ediacaran period in Australia, estimated to be 560–551 million years old, is currently considered the oldest known evidence of a bilaterian animal.
Additionally, research conducted by SD Evans et al. 2020 introduces Ikaria Wariootia, a bilaterian found within the same Australian strata, as the first bilaterian body fossil, presumably the creature that made these trace fossils.
After this, more evidence wouldn’t appear until the Cambrian with arthropods, echinoderms and even the stem vertebrate Myllokunmingiidae family as well as the slightly larger stem vertebrate, Metaspriggina.
8 Animal types that are Bilaterians
1. Echinoderms
Echinoderms, such as starfish, sea urchins, and sea cucumbers, are unique for their radial symmetry (later in life, as embryos they have bilateral symmetry like the rest of us) and remarkable regenerative abilities.
Starfish, for instance, can regrow lost arms; in some cases, a single arm can regenerate into an entirely new starfish. This group plays a pivotal role in the ocean’s ecosystems, often acting as keystone species that significantly influence the ecological balance.
Echinoderms are deuterostomes that first show up in the fossil record in the Cambrian.
2. Chordates
Chordates encompass a broad range of animals, including lancelets, tunicates, and all vertebrates (fish, amphibians, reptiles, dinosaurs, birds, and mammals like you!).
This group is characterised by having a notochord at some stage in their development. Vertebrates, a subset of chordates, are particularly notable for their complex nervous systems and adaptive capabilities, which have allowed them to inhabit nearly every ecosystem on Earth, from deep oceans to high mountains.
Chordates are deuterostomes that first show up in the fossil record in the Cambrian time. Discover some of the earliest known chordates.
3. Brachiopods
Brachiopods are protostomes and shelled marine creatures that resemble bivalve molluscs but are distinguished by their unique anatomy and lophophore, a specialised feeding structure.
These ancient organisms have been inhabiting Earth’s oceans for over 500 million years and serve as important indicators of environmental change and water quality in marine ecosystems.
Although they first show up in the fossil record in the Cambrian, Brachiopods are believed to have split from Annelids and Mollusks as early as the Cryogenian.
4. Annelids
Annelids, or segmented worms, including ragworms, earthworms, and leeches, play essential roles in their ecosystems. Earthworms, for example, are crucial for soil health as they aerate the soil and break down organic matter, enhancing nutrient cycling and soil structure.
Although they first show up in the fossil record in the Cambrian time, Annelids are believed to have split from Brachiopods and Mollusks as early as the Cryogenian.
5. Mollusks
Mollusks, a diverse group including snails, gastropods, and cephalopods like octopuses and cuttlefish, are protostomes celebrated for their intelligence, especially cephalopods.
Octopuses, in particular, display remarkable problem-solving abilities and can navigate complex mazes, demonstrating advanced learning and memory capabilities.
Although they first show up in the fossil record in the Ediacaran, Mollusks are believed to have split from Brachiopods and Annelids as early as the Cryogenian.
6. Arthropods
Arthropods, the largest phylum in the animal kingdom, include crustaceans, insects, spiders, scorpions and other protostomes.
This incredibly diverse group is characterized by their exoskeleton, segmented bodies, and jointed limbs. Insects, a subgroup of arthropods, play vital roles in ecosystems as pollinators, decomposers, and as food sources for other animals.
Although they first show up in the fossil record in the Cambrian, Arthropods are believed to have split from the Brachiopods, Annelids and Mollusks line as early as the Cryogenian.
7. Nematodes
Nematodes, or roundworms, are a vast and mostly parasitic group affecting plants, animals, and humans, with significant implications for agriculture, health, and ecosystems. Despite their parasitic nature, they are essential for soil health and fertility, contributing to the decomposition process and nutrient cycling.
Although they first show up in the fossil record in the Silurian and Devonian, Nematodes are believed to be almost as old as Brachiopods, Annelids and Arthropods.
8. Platyhelminthes
Platyhelminthes, or flatworms, including the notorious tapeworm, are simple yet fascinating due to their flat bodies and lack of a body cavity. Some species are free-living, while others are parasitic, affecting a wide range of hosts.
Although they have practically no fossil record, contentiously only showing up around the Permian, Platyhelminthes are believed to be almost as old as its protostomes clade members such as Brachiopods, Annelids and Arthropods.
Fossil locations: Where can you go to see First Animal fossils?
If you live nearby or can make the trip, there are a few great places to go and see some of the first animals, including bilaterians, for yourself…
1. Mistaken Point, Newfoundland Canada
Although it’s on the other side of the world, the Mistaken Point Ecological Reserve in Newfoundland’s Avalon Peninsula in Canada is of the same Avalon Assemblage as the original Charnia specimen. And it has loads more examples of Charnia and other Ediacaran biota.
It’s a UNESCO World Heritage site. And there’s an area of 5.7km of rocks with various Ediacran fossils all over the place. Not to mention it’s a beautiful place just on its own.
Email MistakenPointTour@gov.nl.ca to book a tour | or visit their website for more info
2. Ediacaran Hills, Australia
Although not part of the same assemblage, the Ediacaran Hills in the Flinders Ranges of mountains in Southern Australia (about 650km from Adelaide) are extremely important. This is where the first Ediacaran trace fossils were discovered. It’s where the Ediacaran gets its name.
The White Sea Assemblage here is both younger and from shallower water than where Charnia was first found in the UK. But some forms of Charnia were also found here, as was the first bilaterian, Ikaria Wariootia.
Email hello@ediacarafoundation.org to book a tour | or visit their website for more info
3. Royal Ontario Museum’s Burgess Shale (Cambrian) Experiences
The Royal Ontario Museum in Toronto, Canada is famous for its connection to the world-famous Cambrian site, the Burgess Shale. They have both an amazing fossil collection if you visit in-person or even here online and a cool virtual sea odyssey you can experience online right now.
Phone: 416 586 8000 (Canada) | or visit their website
Documentaries featuring the First Animals
They’re not always 100% accurate because we make new scientific discoveries all the time. But documentaries at least deliver the gist of the information in a fun and engaging way. Just click play, hit full screen and enjoy hours’ worth of awesome documentaries on first animals, right here…
1. First Life
David Attenborough, BBC, 2010
Episode 1 of David Attenborough’s First Life features the discovery of Charnia and the evolution of the first complex life. It was produced by the BBC in 2010. You can also buy it on DVD or Blu-ray via Amazon. And it is available to watch in some regions via Prime Video.
2. David Attenborough’s Rise of Animals: Triumph of the Vertebrates
BBC, 2013
Although more focused on the evolution of vertebrates, it’s still an awesome documentary to watch. It’s available to purchase from Amazon.
3. Australia: The Time Traveller’s Guide
ABC, 2012
Despite focusing more on the geology of early Earth, later episodes in the series unpack some wonderfully bizarre first animals and plants from down under. You can also buy it via Amazon.
4. Walking with Monsters
BBC, 2005
A bit older now and flashing past early animals a bit too quickly, this one in the “Walking With” series is still pretty cool for its Cambrian time bits. It’s also available via Amazon.
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Enjoy the journey of discovery through Earth’s ancient past, geology, big ideas and, of course, awesome prehistoric life.
