A notochord, streamlined body, gill slits and fin-like structures on the tail. When Charles Doolittle Walcott first discovered the fossils of the metazoan Pikaia Gracilens in Canada’s Burgess Shale in 1911, he thought it was a worm.
While it was reclassified by Conway Morris et al. as a primitive chordate, the evolutionary precursor to vertebrates (backboned animals like dinosaurs, fish and we mammals), inhabiting the ancient Cambrian seas of 541 to 485 million years ago, was Pikaia a fish?
Is Pikaia a fish?
Although Pikaia is a chordate, the precursor to all vertebrates, Pikaia Gracilens itself is not a fish. While it shares some characteristics with modern fish, such as a notochord (a primitive backbone), it lacks definitive fish features such as fins, scales, and a complex skeletal structure.
Pikaia is more accurately described as an early chordate. This group includes fish and vertebrates but also encompasses a broader range of organisms that don’t have spinal cords like us vertebrates do.
Discover all about the earliest Cambrian chordates.
Also discover whether Pikaia had bones and if humans evolved from Pikaia.
Did Pikaia have gills?
This is extremely interesting: As an early chordate and, in fact, a deuterostome, Pikaia definitely had the presence of pharyngeal gill slits – structures akin to a fish’s gill slits. But, in chordates, these structures are not always used for respiration like in fish.
One of Pikaia’s closest living chordate relatives, the lancelet, whom it resembles uncannily even after 500 million years of evolution, also has these gill slits. But they are not used for breathing, they are used for filter-feeding.
So, unlike modern fish gills, Piakia’s gill slits were likely not as specialized or efficient in oxygen exchange – they may also have been feeding structures. They do identify Piakia as a chordate, though, a key step in the evolution of vertebrates (Morris & Caron 2012).
Where did Pikaia live?
Pikaia gracilens lived in marine environments, specifically in the shallow seas of the Cambrian period.
Its fossils have been found in the Burgess Shale, a famous fossil site in British Columbia, Canada, that was once part of an ancient sea.
This indicates that Pikaia was adapted to a life swimming or possibly burrowing in the mud of these early seas (Briggs, Erwin & Collier 1994).
Also learn: When did Pikaia go extinct?
What did Pikaia eat?
We don’t know the exact diet of Piakia, but scientists have speculated that it might have been a filter feeder like its closest living chordate relatives, the cephalochordates (lancelets).
Alternatively, Pikaia gracilens might have been a detritivore, consuming small particles of food or organic debris suspended in the water.
The structure of its body suggests it could swim, possibly aiding in feeding by moving through water to filter out nutrients.
Learn more about how and what Pikaia ate.
What was Pikaia’s size?
Pikaia gracilens was relatively small, with fossil specimens measuring only up to approximately 5 centimetres in length.
Its size and streamlined body shape would have facilitated its lifestyle as a swimmer in the Cambrian seas. Discover more about Pikaia size.
Also see: Did Pikaia have bones?
What did Pikaia evolve from?
Pikaia likely evolved from earlier, simpler non-chordate ancestors, but still deuterostomes.
These precursors would have been soft-bodied organisms, lacking the distinctive notochord and other complex structures that characterise chordates.
The evolution of Pikaia represents a significant step in the diversification of early animal life during the Cambrian Explosion.
Discover more: When did Pikaia go extinct?
Is Pikaia a vertebrate?
No, Pikaia is not considered a vertebrate. While it is an early chordate. Vertebrates are distinguished by having a well-developed spinal column, a feature Pikaia lacks.
Its discovery has been crucial in understanding the evolutionary progression from simple chordates to more complex vertebrates, such as fish, though.
Also learn: Did humans evolve from Pikaia? And: What is so special about Pikaia?
Fossil Locations: Where Are Cambrian Fossils Like Pikaia Found?
- Pikaia itself is only known from the Burgess Shale in British Columbia, Canada
- Yet other Cambrian sites include 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 period.
Phone +1 312 922 9410 or visit their website.
Documentaries Featuring some of the First Chordates
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. 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.
2. 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!
3. 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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