#yilingia

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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(…Part 1, because this is a topic I can't possibly fit comfortably into just 31 days. Part 2 will happen later this year.)

The start of the Paleozoic Era 541 million years ago is marked by what's become known as the "Cambrian explosion" – a time of rapid evolutionary diversification when representatives of most known animal phyla first appear in the fossil record.

For a long time some of the creatures from this period were seen as mysterious alien-looking fossil weirdos, barely seeming to make anatomical sense and unable to be classified into any known animal groups. Some paleontologists even proposed they represented an entirely separate branch of the animal evolutionary tree, regarding them as "failed experiments" that had left no living relatives at all.

But, slowly, we've been piecing together the evidence and figuring out their relationships, studying exceptionally well-preserved fossils and the genetic connections between modern-day animals. And gradually many of the bizarre fossil species are starting to find their places in the tree of life as evolutionary "cousins" of modern groups, novel early branches of more familiar lineages that are giving us more insight into their origins.

It isn't clear what actually prompted the sudden innovation of the Cambrian Period, but it was probably a combination of various environmental and ecological factors – including changes in oxygen levels, the formation of the ozone layer, early burrowing animals altering the aeration and chemistry of the seafloor, the development of biomineralization, and increasingly complex "arms races" between predators and prey.

It's also looking increasingly likely that it wasn't really an isolated "explosion" at all, instead being one of multiple pulses of rapid diversification that started further back in the Ediacaran Period.

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Cambrian Explosion Month #01: The Explosion Before The Explosion

An evolutionary burst known as the Avalon explosion around 575 million years ago produced a range of weird unidentifiable fossil creatures – the strange Ediacaran biota – many of which seem to defy classification and have often been considered to be early "failed experiments" in multicellular life.

…That sounds awfully familiar, doesn't it?

Our understanding of the Ediacaran has recently been echoing our history of understanding the Cambrian, and we've even been starting to figure out that some of these bizarre creatures weren't quite so alien as they first seemed. A few of them are now revealing their evolutionary connections to early animal life, and showing that some groups go back much further than previously thought.

Ikaria wariootia was a tiny worm-like creature the size of a grain of rice, up to 7mm long (0.25"). Discovered in the South Australian Ediacara Hills fossil site that gives the entire time period its name, and dating to 555 million years ago, it was probably an early bilaterian animal that burrowed around in thin layers of sand on the seafloor.

Kimberella quadrata was another early bilaterian known from Australia and northwest Russia, living around 558-55 million years ago. It was about 15cm long (6") and vaguely resembled a slug, and it may have actually been a very early member of the mollusc lineage – some interpretations consider it to have had a radula-like feeding structure and a leathery "shell".

Namacalathus hermanastes is known from Namibia 549-542 million years ago. About 5cm tall (2"), it was an odd-looking creature shaped like a "cup on a stalk", lightly calcified and covered in spines, with six "holes" around the outside of its body. While in the past it's been compared to cnidarian polyps, more recent studies suggest it was actually an early lophotrochozoan – related to the ancestors of annelid worms, molluscs, bryozoans, and brachiopods.

Yilingia spiciformis was a segmented worm-like bilaterian that may have been related to either annelids or panarthropods. Living in South China during the late Ediacaran and earliest Cambrian, between 551-539 million years ago, this animal reached up to 27cm long (10.5") and was found fossilized at the end of its own trackway.

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