Protostomes are all bilaterians, and thus eumetazoan animals, who fall into the superphylum of Protostomia (meaning “first mouth”), characterised by the mouth forming first in embryonic development.
This includes all animals on the Mollusca (snails, clams, octopuses) line, as well as all annelids (worms), brachiopods and arthropods – amazing creatures such as prehistoric trilobites, Anomalocaris, Opabinia, Hallucinageina etc. and modern-day insects, spiders and scorpions etc.
But Protostomes generally predate deuterostomes by many millions of years, making them one of the first animal groups to appear. When do they first show up on Earth and where can you go to see their fossils?
About Protostomes
Scientific Classification
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Infrakingdom: Bilateria
Superphylum: Protostomia
Time Frames
Molecular Clock: Most say Cryogenian, 670 million years ago (Ayala et al. 1998) but some contend it may have been earlier: circa 900 million years ago (Blair et al. 2005)
First Body Fossils: Ediacaran, Kimberella, 555 million years ago.
Characteristics (Synapomorphies)
1. Protostomes are unified by their unique embryonic development; unlike deuterostome animals, their mouth forms from the first opening in the embryo, and the anus develops later.
2. They tend to have a ventral nervous system – a nervous system primarily located in the belly region.
What are Protostomes Exactly?
Protostomia is not something you can clearly see in the fossil record per se, but rather a series of evolutionary traits that biologists noticed while studying the embryos of modern animals.
In the 1800s already, scientists looked at the embryos of various animals, noticing that bilaterians (who have bilateral symmetry as embryos) develop in one of two very distinct ways.
In Protostomes (meaning “first mouth”) the cells that become the intestinal tract form, and then two openings at either end – the first opening that forms is the mouth; the anus comes later. Protostomes’ mouths form first as embryos, thus “first mouth”.
Protostomes also have ventral nervous systems – nerves concentrated in their bellies, rather than the dorsal (back) of most deuterostomes. Many of the creatures we would think of as invertebrates, from cuttlefish, orthocones and octopuses to crabs, lobsters, worms and insects are all protostomes.
Are Arthropods Protostomes?
Oh, yes, arthropods are definitely protostomes. From Anomalocaris, Opabinia and trilobites to monsters like Arthropleura and Meganeura. Even today’s spiders, scorpions, and their ancient eurypterid relatives are all protosomes.
Are Annelids Protostomes?
Yes, annelids (worms), even though they don’t have a great fossil record, are today easily distinguished as protostomes because we can watch their modern embryos develop and confirm them.
Are Humans Protostomes or Deuterostomes?
Humans are vertebrates, and all vertebrates are deuterostomes – “second mouth” creatures, who have pharyngeal gill arches as embryos. So humans are definitely deuterostomes and NOT protostomes.
What Are The First Protostome Fossils?
The first bilaterian fossil is Iakria Wariootia (SD Evans et al. 2020) from the Ediacaran, 650 million years ago. But we can’t determine deuterostomia or protostomia in that organism just yet – even though it looks like a small worm, we can’t for sure say it is one yet.
One that’s more easily placed is Kimberella, a mollusk-like organism from the Ediacaran, at around 555 million years ago (Fedonkin et al. 2007). Although it was discovered in the 1950s and reinterpreted and redescribed a few times, most scientists seem to think it does resemble a mollusk, which could make it a protostome.
It’s still somewhat debated, though.
What we do know is that by the Cambrian (541 to 485 million years ago), you have loads of confirmed protostomes – Anomalocaris to Opabinia, trilobites to Wiwaxia, Hallucigenia and so many more wonderful prehistoric creatures.
4 Animal types that are Protostomes
1. Brachiopods
Brachiopods are 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.
2. 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, Annelids are believed to have split from Brachiopods and Mollusks as early as the Cryogenian.
3. Mollusks
Mollusks, a diverse group including snails, gastropods, and cephalopods like octopuses and cuttlefish, are 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, Mollusks are believed to have split from Brachiopods and Annelids as early as the Cryogenian.
4. Arthropods
Arthropods, the largest phylum in the animal kingdom, include crustaceans, insects, spiders, and scorpions.
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.
Fossil locations: Where can you Go to See Early Protostome 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 and deuterostomes, 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. Fossils for Free at the Smithsonian
The US’s Smithsonian National Museum of Natural History 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.
Documentaries Featuring some of the First Deutoretomes
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. 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. 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!
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 bits. It’s also available via Amazon.
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