3 Chinese finds and the evolutionary puzzle piece overlooked by Walcott. Stem vertebrates are a very special branch from the main vertebrate lineage that diverged long before anyone would have thought vertebrates existed – in the Cambrian.
In fact, they split off just before the emergence of the crown group of Vertebrata, which includes all of today’s living vertebrates.
But who were these sten vertebrates? How did they live? And how do they ultimately teach us more about prehistoric life such as fish, dinosaurs and ultimately, ourselves?
About Stem Vertebrates
Stem Vertebrates Class & Scientific Classification
Domain: Eukaryota
Kingdom: Animalia (Metazoa)
Subkingdom: Eumetazoa
Infrakingdom: Bilateria
Superphylum: Deuterostomia
Phylum: Chordata
Paleo Classification: Stem Vertebrate
(in Biology: Craniate)
Stem vertebrates are basal vertebrates (craniates) from the Cambrian that diverged from chordates right before the emergence of crown-group vertebrates.
A Phylogeny of Early Chordate-Vertebrate Divergence
Stem Vertebrate Time-Frame: When they Appear on Earth
Myllokunmingiidae family members such as Haikouichthys and Myllokunmingia’s fossils from China date back to 518 million years ago. The much larger Burgess Shale stem vertebrate, Metaspriggina, is from 508 million years ago. As far as we know, none of these actually survive beyond the Cambrian period, though the crown group of Vertebrata appears shortly after.
Basal Vertebrate Size Comparison with a Human Diver
Stem Vertebrate Characteristics (Synapomorphies)
All these basal vertebrates (craniates) share the ancestral chordate characteristics of a notochord, dorsal nerve chord and post-anal tail. But, in addition, they also show transitional vertebrate characteristics, such as:
- Vertebral elements hint at the possible evolution of cartilaginous structures around the notochord.
- Possible lobe-like extensions in the head, with potential eyes and cavities that could be interpreted as nasal and otic (hearing) function, which is the possible development of a primitive tri-partite brain.
- A clearly defined skull, albeit likely cartilaginous.
Notably, those are all vertebrate characteristics, placing all of these stem vertebrates as transitional forms between chordates and vertebrates, falling just outside of the crown group of Vertebrata.
What Are Stem Vertebrates Exactly?
Stem vertebrates, or basal vertebrates (craniates), are a group of early chordates that branched off from the main lineage leading to modern vertebrates before the development of the crown group Vertebrata (in that time, fishes).
Unlike crown vertebrates, which start with fishes in the Cambrian and Ordovician but include all living vertebrates and their closest extinct relatives, stem vertebrates possess features that are primitive yet unmistakably indicative of their place in the vertebrates family tree.
They showcase the evolutionary beginnings of characteristics that define vertebrates, such as a notochord, rudimentary vertebrae, and in some cases, basic sensory systems (Shu et al. 2008).
Why Are Stem Vertebrates Important?
The study of stem vertebrates is crucial for understanding the evolutionary innovations (steps) that were necessary to transition invertebrates into vertebrates – backboned creatures like Dunkleosteos, Velociraptor, Smilodon and us humans.
This includes the development of a more complex nervous system and the emergence of a rudimentary skeletal structure, particularly the vertebrae around our dorsal nerve chords and notochords.
Additionally, stem vertebrates help scientists understand the ecological dynamics of the Cambrian period, a time when the foundations of modern ecosystems were being laid. Through the lens of stem vertebrates, researchers can infer patterns of predator-prey relationships, ecological niches, and evolutionary pressures that shaped the course of vertebrate evolution (Brazeau & Friedman, 2015).
When and Where Were the First Stem Vertebrate Fossils Found?
The first stem vertebrate fossils that were recognised for what they are – basal vertebrates – came from early Cambrian deposits of the Chengjiang biota in Yunnan Province, China in the late 1990s. Haikouichthys probably provided the most evidence that these creatures were a little more than mere chordates.
Yet, technically, Metaspriggina was discovered first – in the very early 1900s, at Canada’s Burgess Shale. It merely sat unstudied and unclassified for decades before first being misinterpreted and then later re-interpreted after the Chinese “cousins” were correctly identified as stem vertebrates.
4 Stem Vertebrates You Should Know
1. Myllokunmingia
This small, fish-like member of the Myllokunmingiidae family is characterised by chordate features such as a notochord, dorsal nerve chord and post-anal tail. But it also has the beginnings of vertebrate anatomy – a clearly defined skull, lobe-like extensions in the head (which could be a primitive brain) and vertebral elements, hinting at the beginnings of a backbone.
Remarkable, since it lived almost 518 million years ago in the Middle Cambrian, long before anyone thought we’d find any vertebrates. Discover more fascinating insights into Myllokunmingia.
2. Haikouichthys
Possibly the most famous Myllokunmingiidae family member and stem vertebrate of all! Its initial find in China showed it was a chordate. But a remarkable follow-up discovery of some 500 new specimens revealed way more details – including vertebral elements and possible sensory organs and the beginnings of a tri-partite brain.
This indicates a significant advancement in the evolutionary narrative towards more sophisticated vertebrate organisms. See the fascinating story of Haikouichthys.
3. Zhongjianichthys
The most elusive member of the Myllokunmingiidae family. Characterised by its distinctive features, such as a thicker skin covering and strange lobe-like extension from the head (which makes it look like it had a “big nose”!), add another layer to our understanding of vertebrate evolution.
Despite its small size, it plays a big role in piecing together the evolutionary puzzle of early vertebrates. Help unlock the mysteries of Zhongjianichthys.
4. Metaspriggina
Another big celebrity among the Cambrian stem vertebrates. Metaspriggina was famously discovered by none other than Burgess Shale discoverer Charles Doolittle Walcott himself – who set the specimen aside for further study but never got round to it.
It took over 60 years for someone else to try, but he misinterpreted it as a relative of the Ediacaran biota, Spriggina (hence the name). It took some research and a few extra specimens found in Canada to reveal the truth – though it’s almost double the size of any of the others on this list, it’s a basal vertebrate.
Uncover the story and mysteries of Metaspriggina.
Are There Any Living Stem Vertebrates Today?
No, unfortunately not. The moniker “stem” means they are a transitionary form between one node on the vertebrate evolutionary tree (the phylum Chordata) and another (the subphylum Vertebratea).
And, normally, the “stem” groups fall just outside the “crown” groups (which are the groups that have modern relatives).
So by definition stem vertebrates shouldn’t have any direct descendants; they are offshoots from the main vertebrate line that we haven’t connected to any subsequent animals.
We can only assume they lived and went extinct without living relatives.
But that’s our present understanding, though. There’s no reason why you can’t go and discover a missing evolutionary link that changes everything!
They might not be around anymore but stem vertebrates like these show us how our vertebrate body plan formed over time. Important and exciting, because the vertebrate form unites all of prehistory’s most exciting creatures – from Dimetrodon and Triceratops to Stegasaurus, T Rex and all of us humans.
Fossil Locations: Where Are Cambrian Fossils Like Metaspriggina Found?
- Metaspriggina itself is known from the Burgess Shale and Marble Canyon sites in British Columbia, Canada.
- All the Cambrian Myllokunmingiidae family members are from the Chengjiang fossil site in Yunnan Province, China.
- The Wheeler Shale in the House Range of western Utah, USA
- The Sirius Passet site in North Greenland is another critical location for understanding Cambrian life.
Museum Fossils: Where can you Go to See Early Chordate and other Cambrian 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 and eumetazoans, including bilaterians, protostomes, deuterostomes and chordates, for yourself…
1. The Burgess Shale
Did you know you can actually go to the Burgess Shale fossil sites in the Canadian Rockies yourself? You can book a guided hike with The Burgess Shale Geoscience Foundation and visit the Walcott quarry, Mt. Stephen – the works.
Phone 1 (250) 343-6006, email info@burgess-shale.bc.ca or visit their website.
2. Royal Ontario Museum
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
3. Smithsonian National Museum of Natural History
The Smithsonian has quite a large Cambrian fossil collection. And it’s completely free! So, if you’re ever in Washington DC, it’s one of the coolest places to go check out Cambrian fossils.
4. Chengjiang Fossil Site Natural History Museum
Being in China, it’s a bit hard to track down English info, but the Chinese government says there is a Chengjiang Fossil Site Natural History Museum, with over 60’000 fossil specimens at its location in Xincun Road, Chengjiang county, Yuxi City in Yunnan province.
5. The Natural History Museum, London, UK
With a rich collection of fossils and exhibitions that trace the history of life on Earth, the London Natural History Museum’s displays on Cambrian and Precambrian life provide context for the evolution of complex organisms.
Phone +44 (0)20 7942 5000 or visit their website.
6. Field Museum of Natural History, Chicago, USA
Boasting a broad collection of fossils and active in paleontological research, its Evolving Planet exhibition takes visitors through the history of life on Earth, including the Cambrian time.
Phone +1 312 922 9410 or visit their website.
Documentaries Featuring some Awesome Cambrian Creatures
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. Fossil Wonderlands: Nature’s Hidden Treasures
BBC, 2014
Dr Richard Fortey, a British palaeontologist and trilobite specialist you might recognise from a few David Attenborough documentaries, explores some of the most amazing Lagerstätte fossil sites in the world.
2. 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 bits. It’s also available via Amazon.
3. Lost Worlds, Vanished Lives, Episode 4
BBC, 1989
Definitely old and dated, film-wise. But, apart from the almost impossible-to-track-down First Life series, this is the only one where David actually goes to Walcott’s original quarry in the Burgess Shale itself. Old as it is, the entire series is still remarkable to watch.
4. Mankind Rising
2012
The 2012 TV movie production of Mankind Rising is a single animated journey from the earliest vertebrates ancestors right through to present-day humans – it’s pretty cool, maybe a bit dated, but still a lot of fun!
5. 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.
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