#proarticulata

Most animals are bilaterally symmetric, having body plans with mirrored left and right sides – which also allows them to have a defined head end, rear end, top side, and underside.

It's not entirely clear what evolutionary advantage this type of symmetry gave to the first bilaterians, which would have been been small "simple" worm-like animals living sometime during the Ediacaran Period between 600 and 560 million years ago. The current generally accepted explanation is that it probably allowed for better active locomotion – clustering sense organs at the head end and directing body movement more efficiently towards food sources and away from threats.

However, this sort of symmetry is never completely perfect. Internal structures like organs are often arranged nonsymmetrically, and the realities of genetics, physical development, and environmental influences always result in external small deviations.

Zebra From The Back by Lynn Greyling | CC0 Public Domain

…But not every bilaterian has stayed roughly symmetrical.

Over the last half-billion years or so some bilaterians have abandoned their roughly-mirror-image body plans in favor of something distinctly wonkier. Asymmetry has evolved multiple times in various different lineages, and so every weekday this month we'll be looking at some examples.

And we might as well start way back near the beginning:

———

Strange Symmetries #01: Almost Bilateral

Living in the Ediacaran between about 567 and 550 million years ago, the proarticulatans were flattened rounded organisms with two rows of soft "quilted" rib-like segments (known as isomers) and sometimes a larger fused "head" section at the front. The left and right isomers weren't perfectly mirrored, instead being offset from each other in a glide reflection pattern – but the presence of a clear central body axis suggests these animals may have had some sort of relation to the earliest bilaterians, possibly even being a very early stem group that was experimenting with a not-quite-totally-bilateral body plan.

Discovered in what is now northwest Russia, and dating to around 555 million years ago, Vendia sokolovi was a small proarticulatan measuring about 1.1cm long (0.4"). It had a rather small number of isomers compared to some of its relatives, only 7 per side, and seems to have had a simple digestive tract that branched into each isomer.

(The superficial resemblance to trilobites was coincidental – while we might not be entirely sure what these things were, we do at least know they weren't closely related to early arthropods.)

Very little overall is known about these animals' lifestyles. Trace fossils suggest they were able to move around, feeding on microbial mats on the seafloor, and they may also have been able to firmly stick themselves onto the spots they were currently grazing.

———

Proati articu(with a proarticulata lusus) (his lusus covers him so protect him)

Namica Lathus (with her giant flesh palace lusus, a Namacalathus) (she hangs out here like in a treehouse once in a while)

Og animals under the cut

Thanks for reading or having a look. 

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.

———

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.

———

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.

———

Vendia sokolovi, a 1 cm-long enigmatic* organism from the upper Ediacaran of Russia (displaying the 'glide symmetry' characteristic of many Ediacaran forms), feeding (?) on the bacterial mats that used to be widespread before the Cambrian explosion.

*as Ediacaran organisms tend to be, alas

Some art I made a long time ago but still find kind of neat.

References: