#protostome

Descriptions of what, exactly, these options mean in this Lorepost

Here are some example pictures of the options:

"Uzumaki", by Junji Ito

"Fuck this shit," *peels off my own skin*

Cambrian Explosion #57: Tuzoiida

What were tuzoiids?

We don't know.*

Despite hundreds of specimens having been found, and around 20 different species being described, these arthropods are an ongoing puzzle.

They're known from between about 518 and 505 million years ago, in deposits associated with tropical and subtropical regions all around the world. They had large spiny bivalved carapaces up to 18cm long (7"), shaped like an upside-down domed taco shell, with a distinctive reticulated net-like surface ornamentation – but the rest of their ecology and anatomy is very unclear.

Most fossils are just empty carapaces, which appear to have been made of unmineralized chitin. Rare examples of soft-part preservation show they had a pair of stalked eyes sticking out the front, and a pair of short simple antennae, but impressions of the rest of their bodies are fragmentary and indistinct enough to not be particularly helpful.

They may have been related to the similar-looking isoxyids, or they might have closer affinities to the pancrustaceans or possibly even the equally enigmatic thylacocephalans. 

Unless we find some better fossils with clearer anatomy, they're going to remain a mystery.

Tuzoia canadensis was one of the pointier-looking tuzoiids, known from the Canadian Burgess Shale deposits (~508 million years ago). About 10cm long (4"), its carapace was lined with many long spines, with a more prominent spiky side frill than most other species.

Its reticulated ornamentation had fairly large polygonal facets, and like in other tuzoiids this is thought to have had a strengthening function – allowing it to create a large but very thin carapace with enough structural integrity to hold its shape.

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* Edit: As of 2022, tuzoiid soft tissues have been found! Turns out they're early mandibulates and probably closely related to hymenocarines.

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You are a Protostome in the early Cambrian. Things have been changing rapidly in your environment. There has just been a mass-extinction event, of which you are one of the lucky few survivors. scientists are unsure of the exact cause or causes but the extinction did come after a long period of global glaciation, ending several million years prior, which could have had an effect.

The rapidly changing times demand your rapid evolution. The three cell-layers your ancestors had (like Kimberella from the previous poll) will no longer suffice. You need more complicated tissues with more complicated functions. Animals from other lineages are more suited to filter-feeding (like the suck tubes) and suspension hunting in the water column (like the murder assholes). Another lineage of note has bilateral symmetry and three tissue layers like you do, but is differentiated mainly by a softer body exterior but with a rod of harder tissue inside its body, protecting its nervous system. That and the fact that their embryos form their butt-hole before forming their mouth-hole. Disgusting. These "Butt-First Soft Speedsters" are probably irrelevant anyways. They may be faster and more mobile, but you brought protection. That protection may not be armor in the strictest sense but it does still protect you and enable you to form more complex internal organs.

Speaking of which, how would you like your protection? as you see it there are two options:

Carry it all with you, all the time, and only grow in one direction, or

when it comes time for you to grow bigger, ditch it and grow a new one.

The first option, we will call....

"Uzumaki", by Junji Ito

So called because your embryos develop in a spiral pattern, like in this diagram:

Not all of them even have "armor" so to speak. some of them ditch the idea of having a thick protective layer pretty quickly in favor of swimming in the open ocean because they were so jealous of the Butt-First Soft Speedsters. They looked a bit like this:

The ones that keep their armor generally wear it as a funny little hat or a backpack. this means that it can only cover part of their body, and they add to it as they grow. they kind of looked like this:

This may look like a snail but it's not, i promise you. Snails are a lot more complicated.

The second option is....

"Fuck this shit," *peels off my own skin*

This one is hopefully more self-explanatory. The drawback of this strategy being that you can only grow in bursts, it takes more energy to constantly make new skin which is generally weaker than permanent armor and when you grow you're vulnerable. The benefit is that you don't get heavier as you age and are generally more mobile and flexible. One big thing to note is that these all have some sort of hard mouth part for biting. Kind of like teeth. Some of them live their lives digging in the dirt like this ugly son of a gun:

.... yeugh....

While others specialized in swimming around in the water column once again, probably because all the cool kids were doing it and they wanted to feel included. These guys were usually WAY smaller than the other ones doing it.

Sometimes almost microscopic. Finally, Some of the skin-yeeters decided they wanted legs for walking on the sea-floor. Some of them also had spikes.

But at the same time there were "weird wonders" everywhere, things much harder to identify, with shapes so bizarre that their initial discovery was met with laughter.

Animal life was exploring so many different possibilities for body plans and ecologies, and one lineage in particular dominated this explosion of evolutionary experimentation: the arthropods.

Arthropods are represented today by the chelicerates (sea spiders, horseshoe crabs, and arachnids), myriapods (millipedes and centipedes), crustaceans, and insects, and together these groups make up over 80% of all known living animal species and are vital parts of almost every ecosystem on the planet.

We live on Bugworld, and this month we're going to see where that all began. Welcome back to the Cambrian Explosion series (only a couple of months later than usual) and the Earth of over half a billion years ago: the time of the Rise Of The Arthropods.

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Cambrian Explosion #32: More Ediacarans & Ecdysozoan Origins

Arthropods are part of a major branch of animal life known as ecdysozoans – characterized by growing via molting their outer covering. They're estimated to have last shared a common ancestor with their cousins the spiralians around 600 million years ago, and the protostome ancestors of both these groups split from the deuterostomes even earlier than that.

So ecdysozoans must have originated somewhere in the Ediacaran Period, but so far none of the enigmatic fossils from that time period have been definitively linked to this group.

Spriggina floundersi certainly looks like it could have been some sort of early arthropod, with a defined "head" and a segmented body covered in plates that made it resemble a trilobite – although its segments appear to be glide reflected rather than symmetrical. Up to 5cm long (2"), its known from the South Australian Ediacara Hills fossil site and dates to about 555 million years ago.

Praecambridium sigillum lived in the same time and place as Spriggina, but was much smaller at just 4mm long (0.15"). Its vaguely trilobite-like body had at least 5 segments, and this has also led to it being proposed as a potential arthropod-like animal.

Despite their superficial similarities, however, currently neither of these species are thought to be particularly closely related to ecdysozoans, with their trilobite-like shapes instead being the result of convergent evolution. Instead they may be part of a separate proposed phylum known as Proarticulata, a very early branch of bilaterian animals whose closest living relatives might be the weird little xenacoelamorphs.

The most likely Ediacaran arthropod-relative discovered so far is actually the segmented worm-like Yilingia spiciformis, which was originally featured back in part 1 of this series – and even then it could instead be closer related to annelids, or be another example of convergence, and so it can't be confidently classified as anything more specific than "an early bilaterian".

Isolated microfossil spines from the late Ediacaran may also be from early ecdysozoans, possibly originating from a scalidophoran-like animal.

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But outside of the weird Ediacarans, the closest thing we currently have to an example of what the ancestral ecdysozoans might have actually looked like is Acosmia maotiania.

Originally described as a priapulid worm, this species was recently reassessed as a "stem-ecdysozoan" – basically part of an ancient branch of the ecdysozoans that arose after the split from the spiralians but before the common ancestor of all other known forms existed, making it a evolutionary "cousin" to the entire rest of the group.

It's known from the Chinese Chengjiang fossil deposits (~518 million years ago) and measured up to about 10cm long (4"). It had a two-part worm-like body plan, with its mouth located at the tip of a bulbous proboscis region at the front, and a ringed trunk region further back.

However, it lacked the characteristic radial teeth and throat structures seen in other early ecdysozoans like scalidophoran and lobopodians. This suggests the common ancestor of the rest of the group may have been an Acosmia-like wormy animal with a combination of a ringed body and scalidophoran-like mouth structures – and maybe eventually some other Cambrian fossils currently classified as priapulid-relatives may turn out to also be stem-ecdysozoans instead.

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Cambrian Explosion #36: Phylum Loricifera

Even more obscure and poorly-understood than the mud dragons, loriciferans weren't even discovered until the 1970s. Over 40 living species of these tiny meiofaunal animals are currently known, but much like the kinorhynchs there are probably many more still to be described.

Less than 1mm long (0.04"), their most distinctive feature is the "lorica", a stiff corset-like casing surrounding their body. They're also the first multicellular organisms discovered able to live completely without oxygen in a deep basin in the Mediterranean Sea.

Traditionally loriciferans have been considered to be part of the scalidophorans, closely related to priapulids and kinorhynchs, but genetic studies have suggested they may actually be much closer related to nematoids and early panarthropods instead.

(The nematoids – nematode worms and horsehair worms – don't have a definite fossil record until the Devonian, and won't be making an appearance in this series. There's one possible larval form from Australia, but with all the disagreements about the mess of early ecdysozoan relationships it's unclear where exactly it actually fits in.)

Loriciferans have a very sparse fossil record – like the kinorhynchs and other meiofauna they're basically "paleontologically invisible" – but there are a few known Cambrian fossils of the group. The minute Eolorica from the late Cambrian of Canada (~497-485 million years ago) is incredibly similar in size and appearance to modern loriciferans, showing that by that time these animals had already taken up the same sort of ecological role as they have today.

But further back in the Cambrian there was something that may actually represent a very early or "stem" loriciferan.

Sirilorica carlsbergi is known from the Sirius Passet fossil deposits in Greenland (~518 million years ago). It was a worm-like animal with a tube of loricate plates around its body and spines around its head, and it was much bigger than modern species, up to about 8cm long (~3").

It suggests that modern-style loriciferans are actually highly miniaturized and neotenous, evolving to retain a tiny larva-like body plan and exploit a fully meiofaunal lifestyle, and lost their original larger worm-like adult forms in the process.

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Cambrian Explosion #42: Radiodonta – The Strangest Shrimps

Once the dinocaridids started exploring active swimming lifestyles, one branch of this group quickly became incredibly successful and diverse: the radiodonts. With muscular swimming flaps, head carapaces, stalked compound eyes, disc-like mouths, and large spiny front appendages, they occupied a wide range of ecological roles – and some of them went on to became giants, some of the largest animals of their time.

They were some of the closest relatives to the ancestors of the true arthropods (or "euarthropods"). And while the earliest radiodont fossils are known from about 518 million years ago, much like other panarthropods their actual evolutionary origins have to go back much deeper into the early Cambrian since they already lived alongside representatives of various early euarthropod groups. 

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The first radiodont ever discovered is also the most famous of the group, Anomalocaris canadensis from the Canadian Burgess Shale deposits (~508 million years ago).

Initially discovered in the late 1880s, only isolated front appendages were found and they were misidentified at the time as being parts of crustaceans. In the early 1910s an individual mouth cone was thought to represent a jellyfish, and a fairly complete body fossil was interpreted as a sea cucumber. It wasn't until 1985 that all these separate pieces were finally recognized as belonging to one type of surprisingly large Cambrian animal, which was given the earliest name assigned to any of the constituent fossils – Anomalocaris, meaning "strange shrimp" – and thankfully that tuned out to be a very fitting name for a bizarre-looking distant cousin of crustaceans and other arthropods.

Since then around 35 different species of radiodonts have been discovered, and Anomalocaris has turned out to represent just one branch of this lineage.

It had 13 pairs of large body flaps and 3 smaller pairs on its neck region, along with a vertical three-part tail fan and rows of hair-like gills attached to each body segment along its back. Its stalked compound eyes were made up of around 16,000 lenses each, and it would have had incredibly sharp vision comparable to modern dragonflies. Several sclerites formed a carapace around its head, with a tri-radial mouth underneath and a pair of large jointed spiny appendages at the front.

While some past estimates of its size are now known to be rather exaggerated (or based on different radiodont species entirely), it was still a relatively huge animal for the time at around 50cm long (1'8"). Its raptorial front appendages suggest it was an apex predator, with a lack of wear on its mouthparts indicating it probably mostly fed on soft-bodied animals.

The closely-related Lenisicaris and several other still-unnamed species of Anomalocaris are known from various mid-Cambrian fossil sites that at the time were tropical or subtropical latitudes, showing that anomalocaridids were widespread in warm waters around the world. Ultimately these predatory radiodonts lasted for at least 20 million years during the mid-Cambrian, with the last known anomalocaridid fossils coming from the Weeks Formation in Utah, USA (~499 million years ago).

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Related to the anomalocaridids, the amplectobeluid radiodonts had front appendages with a highly enlarged first spine, forming a specialized grasping pincer-like structure.

Amplectobelua symbrachiata is known from the Chinese Chengjiang fossil deposits (~518 million years ago) and was around the same size as Anomalocaris at about 50cm long (1'8"), making it one of the largest known Chengjiang animals.

It had 11 pairs of large swimming flaps with strengthening rays along their front edges, and while it's unclear if it had a tail fan it did have a pair of long streamer-like appendages at its rear end. Underneath the smaller pairs of flaps on its neck there were at least 3 pairs of "gnathobase-like structures" that would have been used to hold and shred up prey after transferring it from its pincer-like front appendages, chewing it up before passing it up to its mouth.

Several other amplectobeluids have been identified, including Lyrarapax and Ramskoeldia, and they also seem to have occurred mostly in tropical and subtropical waters during the mid-Cambrian. The latest known fossils of this family come from the Burgess Shale, about 508 million years ago, and much like their cousins the anomalocaridids they may have been wiped out around the time of the late Cambrian "SPICE" event, unable to cope with cooling oceans and rising anoxic conditions.

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Cambrian Explosion #46: Megacheira

The megacheirans were a grouping of Cambrian stem-euarthropods that had a distinctive pair of "short great appendages" on their heads with long finger-like spines. Their bodies were fully arthrodized into multiple hard-shelled segments, and their double-branched biramous limbs featured both legs and gill-fringed paddle-like flaps.

As small predators and scavengers they were important elements of some Cambrian ecosystems, and probably swam around just above the sea floor using their arm-like great appendages to grab and tear apart food items.

They weren't really a distinct lineage, more of an "evolutionary grade", usually considered to be in the "higer stem" close to the common ancestor of all the major euarthropod lineages, or possibly the very earliest actual euarthropods – but some studies instead place them as more "advanced" euarthropods closely related to chelicerates.

Part of this classification disagreement comes down to whether their great appendages were anatomically equivalent to the front appendages of radiodonts or whether they were convergently evolved, and whether one or both of those structures are also related to the development of antennae and chelicerae in later euarthropods or if they became highly modified into the labrum instead. It's a subject of longstanding and ongoing debate so complicated that it's known as the "arthropod head problem" – or sometimes "the endless dispute".

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Fortiforceps foliosa was a fairly rare megacheiran known from the Chinese Chengjiang fossil deposits (~518 million years ago).

About 4cm long (~1.5"), it had stalked eyes, 20 body segments, and a fluke-like tail. There was a pair of club-tipped antenna-like sensory structures on its head, and its stout great appendages featured three long scissor-like spines.

A recent analysis of its anatomy supports the idea that megacheirans were stem-euarthropods, and that their great appendages were derived from the front appendages of radiodont-like ancestors.

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Leanchoilia superlata was a rather common megachairan in the Canadian Burgess Shale deposits (~508 million years ago), known from over a thousand specimens.

It was about 5cm long (2") and had a hook-like "horn" at the front of its head, along with a fairly slender body with wide flaring lobes on each segment, and relatively long limbs. It had one or two pairs of downwards-facing eyes (the preservation of them is unclear) and its great appendages featured long whip-like feelers extending from the spines.

A recent study of a closely related species from China, Leanchoilia illecebrosa, shows it possessed a reduced labrum in front of its mouth, and supports the idea that megacheirans were very close relatives of chelicerates with their great appendages convergently evolved from a different set of front limbs.

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Cambrian Explosion #43: Radiodonta – Splash Of The Titans

The most famous radiodont is the classic charismatic Anomalocaris, but there were plenty of other members of the group who explored very different lifestyles. Instead of big apex predators, some of them became equally large filter feeders – the whales of the Cambrian.

Tamisiocaris borealis was discovered in the Sirius Passet fossil deposits in Greenland (~518 million years ago). Known only from its front appendages and head shield, its full life appearance and size is unknown but it's estimated to have been between 35 and 70cm long (1'2"-2'4").

Amazingly the existence of this radiodont was actually predicted by a piece of speculative evolution artwork about a year before the discovery of its filter-feeding habits was published.

Its front appendages bore long pairs of spines, each covered in densely-packed bristles that would have functioned like sweeping nets, capturing numerous small planktonic animals from the water and bringing them close to its mouth so it could suck them in.

The eyes in this reconstruction are based on "Anomalocaris" briggsi, a closely related species that wasn't actually an Anomalocaris but hasn't been given its own genus name yet. Surprisingly for a radiodont it had unstalked eyes with vision adapted for low-light conditions, suggesting that it may have vertically migrated, spending daytime in the deeper twilight zone and moving up near the surface to feed on swarms of zooplankton at night.

Along with the recently-named Houcaris these unusual radiodonts make up the tamisiocaridid family, and like their cousins the anomalocaridids and amplectobeluids they seem to have been primarily restricted to the warm waters of tropical and subtropical regions. They were also a much shorter-lived lineage, only known from the "series 2" division of the mid-Cambrian between about 518 and 512 million years ago – and they were probably among the casualties of the end-Botomian mass extinction, when widespread anoxic conditions would have severely disrupted the complex planktonic food webs they depended on.

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The final major branch of the radiodonts were the hurdiids, with fossils of this group known from all around the world.

They were characterized by their enormous head carapaces, sometimes so big compared to the rest of their bodies that they were little more than "swimming heads", and some species had two pairs of flaps on each body segment. Their front appendages were also uniquely shaped, with only a few of the segments each bearing a single massive spine lined with bristles. These spines usually curved inwards, creating a large "basket" that would have been used to capture small prey.

Titanokorys gainesi was a hurdiid recently described from the Canadian Burgess Shale deposits (~508 million years ago). Known from 12 specimens, it was one of the largest animals in that ecosystem rivalling its cousin Anomalocaris at around 50cm long (1'8").

Its huge carapace's streamlined spaceship-like shape led to it being nicknamed "the mothership", and the huge structure may have been used like a plough, shoving through seafloor sediment and sifting around with its front appendages to rake up all the small soft-bodied burrowing animals it uncovered.

Hurdiids were far more tolerant of low temperatures than their tropical relatives, found throughout the seas of the Cambrian southern hemisphere all the way into cold-temperate and subpolar waters. And this ultimately allowed them to outlast all the other radiodonts by far and survive throughout the first half of the Paleozoic Era – in the early Ordovician (~480 million years ago) the gigantic filter-feeding Aegirocassis reached sizes of around 2m long (6'6"), making it both one of the biggest animals in the world at the time and one of the largest panarthropods to ever live, while the last known hurdiid (and the last ever known radiodont) was Schinderhannes from the early Devonian of Germany (~400 million years ago).

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