Eumetazoa is a proposed subkingdom or basal clade to the kingdom of Animalia that includes all true animals except sponges (Porifera or Parazoa). That means everything from jellyfish and corals to crustaceans, worms and backboned creatures like fish, reptiles, dinosaurs, birds and even us humans are eumetazoans.
But when did Eumetazoa first appear on Earth? And what makes them so super special?
We find out in a trip back in time to prehistoric Earth…
About Eumetazoa
Scientific Classification
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Time Frames
Molecular Clock: Proterozoic–Cryogenian, 748–604 million years ago (Petersen et al. 2005)
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!
Some Ediacaran creatures like Charnia Masoni (575 million years ago) have been proposed as stem-eumetazoans, meaning they are a lineage of eumetazoans that diverged before the bilaterian-cnidarian split and went extinct with no modern relatives.
Characteristics
Eumetazoans are thought to have neurons, and tissue organs (sometimes actual muscles) and have a more complex embryonic development than their closest metazoan relatives, sponges.
What are Eumetazoa Exactly?
Eumetazoa are all true animals except sponges. That means everything from bugs to fish to us humans are eumetazoans because we have neurons and tissue organs, which sponges don’t (even though they are still animals).
The biggest reason we would create Eumetazoa as a subkingdom to Animalia (animals) is to try and figure out what exactly the strange creatures in the Ediacaran were and how they possibly relate to all the animals that suddenly seem to appear in the Cambrian time.
What is the difference between Eumetazoa and Metazoa?
Metazoa is another word for true animals, which includes sponges. Eumetazoa are all the same animals excluding the sponges.
What is the difference between Bilateria and Eumetazoa?
Bilateria comes right after Eumetazoa, evolutionarily. Bilaterian animals are all those that have bilateral symmetry (a right and left side that are mirror images of each other) when they are embryos.
Of all the animals in Eumetazoa, cnidarians (jellyfish and corals) are the only ones that don’t have that bilateral symmetry, though. So Bilateria is all animals except cnidarians.
It’s as if we just take Eumetazoa and remove all the jellyfish and corals – what you’re left with are all Bilaterians.
9 Animal types that are Eumetazoans
1. Cnidarians
Cnidarians, including corals, sea anemones, and jellyfish, are fascinating marine creatures known for their stunning beauty and deadly sting. These organisms are equipped with nematocysts, specialised cells that deliver venom to their prey or predators.
Corals, a subset of cnidarians, play a crucial role in marine ecosystems by forming coral reefs, which are often referred to as the “rainforests of the sea” due to their rich biodiversity. Coral reefs provide habitat for thousands of marine species and are vital for marine life sustainability.
Although they first show up in the fossil record in the Ediacaran, cnidarians are believed to have split from bilaterian animals as early as the Cryogenian.
2. Echinoderms
Echinoderms, such as starfish, sea urchins, and sea cucumbers, are unique for their radial symmetry 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 time.
3. 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. Discover some of the earliest known chordates like stem-vertebrates Haikouichthys, Zhongjianichthys and Myllokunmingia.
4. Brachiopods
Brachiopods are protostomes, 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 time, Brachiopods are believed to have split from Annelids and Mollusks as early as the Cryogenian.
5. Annelids
Annelids, or segmented worms, including ragworms, earthworms, and leeches are protostomes and 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, Annelids are believed to have split from Brachiopods and Mollusks as early as the Cryogenian.
6. 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 Cambrian time, Mollusks are believed to have split from Brachiopods and Annelids as early as the Cryogenian.
7. Arthropods
Arthropods, the largest phylum in the animal kingdom, include crustaceans, insects, spiders, scorpions and a range of 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.
8. 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.
9. 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 protostome clade members such as Brachiopods, Annelids and Arthropods.
What are the first Eumetazoan fossils?
A trace fossil named Helminthoidichnites, dating back to 560–551 million years ago from Australia’s Ediacaran period, is now believed to be the earliest evidence of a bilaterian animal.
Furthermore, a study by SD Evans et al. in 2020 proposes that Ikaria Wariootia, a bilaterian organism identified from the same geological formations in Australia, represents the first physical fossil evidence and is likely responsible for creating these trace fossils.
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 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. But some forms of Charnia were also found here.
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.
