proarticulata – Nix Illustration

Strange Symmetries #01

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.

A photograph of a grazing zebra, viewed from the back so the focus of the shot its its stripy rump. The stripes are not perfectly symmetrical. — 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.

A colored line art drawing of Vendia, an extinct enigmatic Precambrian animal. It's shaped like a flattened oval and vaguely resembles an eyeless limbless gummy trilobite, with several rib-like segments on each side of its midline. The segments alternate instead of lining up symmetrically, and the larger front pair are fused into a head-like structure. — *Vendia sokolovi*

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.

Cambrian Explosion #32: Rise of the Arthropods

The world of the Cambrian Period was a strange combination of both familiar and alien. The land would have seemed rather barren, populated mainly by microbes and algae, yet the oceans teemed with creatures already identifiable as sponges, comb jellies, jellyfish, acorn worms, vertebrates, echinoderms, arrow worms, annelids, molluscs, and brachiopods – small and primitive-looking in some cases but still recognizable enough.

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.

Unsolved Paleo Mysteries Month #17 – Enigmatic Ediacarans

Although Precambrian fossils have been known since the mid-1800s, the overwhelming belief among 19th and early 20th century scientists that complex life couldn’t have originated that early meant such discoveries either weren’t taken seriously or were forcibly assigned to a Cambrian age. It wasn’t until the discovery of Charnia in the 1950s that views began to change.

(Or, rather, the second discovery of Charnia, since the schoolgirl who first found it wasn’t taken seriously either.)

Since then, a wide variety of strange soft-bodied fossils have been identified from over 30 different localities around the world, on every continent except Antarctica, dating to ages from over 600 to 542 million years ago. They’re now known as the Ediacaran biota, after the Ediacara Hills in Australia where some of the most famous examples have been found.

A few show possible similarities to known groups, but we still don’t know what sort of lifeforms most of them they actually were. Animals, fungi, algae, foraminifera, microbial colonies, or lichens have all been proposed – but they might also belong to a completely unique kingdom or phylum, a “failed experiment” in multicellular life with no living descendants.

And they’re gradually turning out to be not nearly as “simple” as once thought, showing evidence of their own thriving ecosystems and evolutionary specializations – which makes their sudden disappearance at the end of the Ediacaran Period all the more mysterious.

Tribrachidium has been found in Australia, Ukraine, and Russia (558-555 mya), grew up to 5cm in diameter (2″), and shows unusual tri-radial symmetry. Affinities to both cnidarians and echinoderms have been suggested, but no classification has really stuck. Recent 3D modelling and fluid dynamic studies reveal its shape was adapted to direct water currents into the nooks between its “arms”, allowing it to feed on suspended organic particles.

Yorgia is known from Australia and Russia (~555 mya), and appears to be a transitional form between two other ediacarans, Dickinsonia and Spriggina. It has what appears to be a “head” end with an asymmetrical lobe, and a segmented body in a glide reflection pattern, growing up to 25cm long (10″). Trace fossils from its feeding strategy have also been found – chains of imprints over seafloor microbial mats, where it moved from spot to spot and “grazed” with the entire underside of its body.

Fractofusus is somewhat older (575-560 mya) and perhaps even stranger. Discovered in Canada in 1967, it was known only as “the spindle organism” for 40 years before being finally named in 2007. Its 40cm long form (16″) shows fractal self-similarity, made up of frond-like elements that branch even further again and again and again, creating a large surface area relative to its internal volume that may have been used to directly absorb nutrients from the surrounding seawater. It also seems to have been capable of reproducing in two different ways – producing both water-borne offspring and stolon-like clones of itself.