bilateria – Nix Illustration

Strange Symmetries #07: Gastropods Do The Twist

Gastropods – snails and slugs – are a group of molluscs that originated sometime in the Cambrian Period, with the earliest definite stem-gastropods known from around 510 million years ago and the first true gastropods turning up in the early Ordovician.

The spiral-coiled shells of snails are their most familiar feature, giving them obvious external asymmetry, but gastropods are also defined by a specific type of internal asymmetry known as torsion.

Torsion is an anatomical process that occurs during larval development, and involves rotating their internal organs, mantle, and shell a full 180° relative to their head and muscular foot. This twists their gut into a U-shape, knots up their nervous system, and brings their respiratory organs and anus up close to their head.

And we still don’t really know why they do it.

One idea (the “rotation hypothesis”) is that it originated as a defensive function after early gastropods began developing their spiral shells. The shell opening may have originally been positioned at early gastropods’ rears, meaning they retracted their bodies back-end-first leaving their heads and sensory structures still vulnerable – but twisting the shell around would allow them to pull their front end in faster instead.

A competing idea (the “asymmetry hypothesis“) instead proposes that the shape of the coiled shell restricted the gills of early gastropods, which may have originally been positioned in mantle cavities on each side of their bodies. In response to this they developed a single larger gill cavity on just one side of their body, and then gradually expanded and rotated this asymmetric feature around to the front for better aeration.

In either case this resulted in some of the rest of their anatomy “coming along for the ride”. And regardless of whatever the original evolutionary advantage of torsion actually was, it made gastropods incredibly successful – they’re a massively diverse group, second only to the insects in terms of sheer number of species, and today they’re found all over the world in almost every habitat from deep sea trenches to high mountain elevations.

A colored line drawing of Spinyplatyceras, an extinct marine snail. It has a low coiling shell covered in very long thin pointed spines, and there are two short tentacles on its head. It's depicted with orange and black striped coloration on its shell, and a purplish body. — *Spinyplatyceras arkonense*

Spinyplatyceras arkonense lived in what is now Ontario during the mid-Devonian, about 391-385 million years ago. Around 5cm long (2″), it was part of a group of Paleozoic marine snails known as platyceratids, which were probably related to either modern limpets or neritomorphs.

Platyceratids seem to have had a unique parasitic relationship with crinoids, attaching themselves to the top of the host’s body and using their radula to drill into them, either robbing food directly from the crinoid’s gut or feeding on its other internal organs.

The long spines on Spinyplatyceras‘ shell probably helped to deter predators. In an interesting case of coevolution the crinoid hosts of some platyceratids developed their own defensive spines, too – and it seems this wasn’t to prevent the snails from infesting them, but to also discourage the snails’ predators. These crinoids may have been frequently indirectly injured during snail-eating predators’ attacks, and it might have actually “cost” them less to keep enduring an infestation than to deal with the collateral damage of the snails being removed.

Strange Symmetries #05: Enigmatic Eldoniids

Fossils of cambroernids were first discovered in the early 1900s, but these Paleozoic animals were so confusing that for a long time their evolutionary relationships were a mystery.

They had coiled bodies and fractal-branching feeding tentacles, and they ranged in shape from worm-like to cup-like to disc-like. Historically various species were classified as sea cucumbers, jellyfish, tunicates, gnathiferans, or lophophorates, but more recently they’ve been recognized as a single united lineage of ambulacrarians, closely related to modern echinoderms and hemichordates.

Discophyllum peltatum lived during the late Ordovician, about 458-448 million years ago, in what is now New York, USA. Up to around 11cm in diameter (~4.3″), it was one of the disc-shaped cambroernids – a lineage known as eldoniids – with a shallow domed body containing a clockwise-coiling sac and delicate feeding tentacles around its mouth.

Its disc would have been tough but flexible, containing numerous supporting radial structures that were probably part of a fluid-filled hydrostatic skeleton, giving it an almost-radially-symmetric body plan superficially resembling a jellyfish.

The lifestyle of eldoniids is still uncertain, but they seem to have mostly sat on the seafloor, possibly extending their tentacles out from under their discs to grab nearby food.

Strange Symmetries #04: Even More Echinoderms

Early echinoderms seem to have gone through an asymmetrical phase before starting to evolving their characteristic radial symmetry. The first truly radial forms had three-way symmetry, but soon a group called the edrioasteroids upped that count to five.

First appearing in the fossil record around 525 million years ago in the early Cambrian, edrioasteroids were mostly shaped like discs or domes, and were immobile filter-feeders that lived permanently attached onto surfaces like the seafloor or the shells of other animals. Unlike most modern echinoderms their pentaradial symmetry was actually created by taking a tri-radial body plan and forking two of their arms near the bases to create a total of five.

A colored line drawing of Thresherodiscus, an extinct early echinoderm. It's a domed disc-shaped creature that looks like it has a starfish merged onto its upper surface – but the "arms" branch many more times than five, and not totally symmetrically, creating an erratic forking pattern. It's depicted with a dark purplish body and brighter orange arms. — *Thresherodiscus ramosus*

Thresherodiscus ramosus was an unusal edrioasteroid that lived in the shallow seas of what is now central Canada during the late Ordovician, around 460-450 million years ago. Up to about 4cm in diameter (~1.6″), its arms split additional times at irregular intervals, creating a complex asymmetrical branching pattern across its upper surface.

The tips of its arms protruded slightly over the rim of its body, and along with the erratic extra branching this may have been an adaptation to increase its food-gathering surface area.

Another group of early pentaradial echinoderms known as the blastozoans were characterized by erect feeding appendages called brachioles. But some blastozoans abandoned their five-way symmetry in favor of much stranger arrangements, sometimes having as few as two arms – and, in some cases, two mouths.

A colored line drawing of Amygdalocystites, an extinct early echinoderm. It has an oval body with a curving stem coming out from its right side, with the stem ending in a circular holdfast. Two "food grooves" run along its top edge, roughly in line with each other on each side of where its not-visible mouth is located. Each food groove has a single row of long tendril-like feeding appendages growing from its left edge. It's depicted with a red and yellow color scheme, with a striped stem and irregular stripes on its body giving a sort of flame-like pattern, and purple tips on its feeding appendages. — *Amygdalocystites radiatus*

Known from the same general area and time period as Thresherodiscus, Amygdalocystites radiatus was part of an Ordovician-to-early-Silurian lineage known as paracrinoids, which attached their irregularly-shaped bodies to the seafloor via a stem.

About 5cm long (~2″) Amygdalocystites had just two asymmetric arms forming “food grooves” along its upper edge, each lined with numerous long brachioles along just one of their sides. It probably orientated itself so its body was facing down-current, which would have created eddies that brought suspended food particles within easier reach of its brachioles.

Strange Symmetries #03: Eerie Early Echinoderms

Represented today by starfish, brittle stars, sea urchins, sea cucumbers, and crinoids, the echinoderms have a characteristic five-way radial symmetry that makes them barely even recognizable as bilaterians. Their true ancestry is only revealed by their genetics and their larvae, which still retain bilateral symmetry – and the way they metamorphose into adults is bizarre, essentially growing a whole new radial body from within the left side of their larval body.

(Sea cucumbers and sand dollars are superficially bilateral as adults, but evolved this secondarily on top of their existing radial symmetry. And some adult echinoderms like starfish also seem to retain a little bit of “behavioral bilaterism”, generally preferring to move with a specific arm always acting as their “front” end.)

The first known echinoderms appeared in the fossil record during the early Cambrian, about 525 million years ago, but the common ancestor of the whole group probably actually originated a few tens of millions of years earlier in the mid-to-late Ediacaran. Early echinoderms seem to have started off as flattened animals that sat on the seafloor filter-feeding, and with this largely immobile way of life their bodies started to shift into asymmetry, no longer constrained by the locomotory advantages of being bilaterally symmetric.

In fact, for these early sedentary filter-feeders being radial was actually much more advantageous, able to distribute sense organs all around their bodies and grab food from any direction without having to reposition themselves, converging on the lifestyle of non-bilaterian cnidarian polyps. The evolutionary transition from bilateral to asymmetrical to pentaradial seems to have happened incredibly quickly during the Cambrian Explosion, and all modern echinoderms probably evolved from a group called the edrioasteroids, maintaining their new base body plan even when they later began taking up more mobile lifestyles again.

But during the process of all that some very alien-looking lineages split off at various stages of anatomical weirdness.

Stylophorans had asymmetrical bodies with a single feeding arm at the front, and varied from irregular boot-like shapes to almost bilateral heart shapes depending on their specific ecologies. The highly asymmetrical forms were probably spreading their weight out over soft soupy mud in quiet waters, while the more bilateral forms may have been more streamlined to deal with stronger water currents.

A colored line drawing of Sokkaejaecystis, an extinct early echinoderm. It has a body shaped roughly like a flattened boot, with spikes and flanges growing from around its margin. What looks like a long tail-like appendage growing from the sole of the boot shape is actually a starfish-like feeding arm at the animals' "front" end. It's depicted with orange-brown coloration with brighter yellow on the spikes and flanges, and darker brown irregular stripes over its body. — *Sokkaejaecystis serrata*

Sokkaejaecystis serrata was a stylophoran that lived during the late Cambrian, about 501-488 million years ago, in what is now South Korea. It was tiny, only about 1cm long (~0.4″), and its boot-shaped body was surrounded by spines and flanges that spread out its surface area and probably also made it much more awkward for small predators to attempt to eat.

Meanwhile the solutes started off as immobile animals living attached to the seafloor via a stalk-like appendage. But fairly early in their evolution they switched to a more active mode of life, modifying their stems into tail-like “steles” that were used to push themselves along.

A colored line drawing of Maennilia, an extinct early echinoderm. It has a flattened body shaped like a lumpy trapezoid, with a single short starfish-like arm growing from its left side and a long thin segmented tail-like appendage growing from the right side of its back end. It's depicted with mottled red and yellow coloration like a camouflage pattern. — *Maennilia estonica*

Maennilia estonica lived in what is now Estonia during the late Ordovician, about 450 million years ago. It was quite large for a solute at about 12cm long (~4.7″), with a sort of vaguely-trapezoidal body, a short feeding arm, and a long thin stele.

Both of these strange early echinoderm lineages were surprisingly successful, surviving for a good chunk of the Paleozoic Era alongside their more familiar radial relatives. The solutes lasted until the early Devonian about 400 million years ago, and the stylophorans continued all the way into the late Carboniferous about 310 million years ago.

Strange Symmetries #02: Oh Worm

Living during the Cambrian Period about 518 million years ago, Wufengella bengtsonii was discovered in the Chinese Chengjiang fossil deposits and was recently named and described in late 2022.

It was a small worm-like animal about 1.6cm long (~0.6″), with bundles of long bristles along its sides and flap-like structures on its underside. Its back was also covered with sclerite armor arranged in a strangely asymmetrical fashion, with larger overlapping plates in the middle and numerous smaller cap-like sclerites distributed unevenly along each side.

Although its bristles and appendages resemble those of annelid worms, the distinctive structure of the sclerites identifies Wufengella as being a member of the tommotiids – early relatives of modern lophophorates (bryozoans, brachiopods, and horseshoe worms).

Its discovery actually confirms an old prediction that lophophorates probably originated from armored worm-like animals, representing an evolutionary link between earlier free-living annelid-like forms and later immobile filter-feeding tommotiids.

It’s not known why the armor on Wufengella‘s back was so unevenly organized – but some of the later tube-like tommotiids also had weird symmetry going on, with forms like Eccentrotheca having irregular sclerites arranged in a spiral around their bodies.

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.

Typhloesus

Typhloesus wellsi has been a mystery for a long time.

First discovered in the early 1970s, in the mid-Carboniferous Bear Gulch Limestone deposits (~324 million years ago) of Montana, USA, it was initially mistaken for the long-sought-after “conodont animal” due to the presence of numerous conodont teeth inside its body. But just a few years later well-preserved eel-like conodont animals were found elsewhere, and it became apparent that the conodont teeth inside Typhloesus had actually just been part of its last meal.

But if it wasn’t a conodont… then what was it?

Up to about 10cm long (4″), Typhloesus had a streamlined body with a vertical tail fin and paired “keels” along its sides. It had a mouth and a gut cavity, but no apparent anus, and it also didn’t seem to have any eyes or other sensory structures. And in the middle of its body there was something very weird – a pair of “ferrodiscus” organs, disc-shaped structures which contained high concentrations of iron but whose function was completely unknown.

This anatomy just didn’t match any other known animals, so much so that it gained the nickname of “alien goldfish”.

For the next few decades it remained a bizarre enigma, at best tentatively considered to represent an unknown lineage of some sort of metazoan that left almost no other fossil record due to being entirely soft-bodied.

But now, 50 years after its initial discovery, we might just finally have a clue about Typhloesus’ true identity.

Recently something new was discovered in some Typhloesus specimens – a radula-like feeding structure that was probably part of an eversible proboscis. This would mean that Typhloesus was a mollusc, possibly a gastropod that convergently evolved a swimming predatory lifestyle similar to modern pterotracheoids.

It’s not a definite identification yet, and even if it was a mollusc it was an incredibly strange one, with features like the ferrodiscus still lacking any explanation. But this discovery at least shows that there are still new details waiting to be found in the “alien goldfish” fossils, and gives us a start towards bringing its classification back down to earth.

Cambrian Explosion #61: Crustacea – Little Wigglers

We’re finally at the end of this series, and to finish off let’s look at one of the few types of Cambrian true crustaceans that are known only from fully mature adults: the skaracarids.

These tiny soft-bodied meiofaunal animals are known from late Cambrian areas of “Orsten-type preservation” in Sweden and South China, with a possible additional fragmentary occurrence in Poland – suggesting that they had a global distribution.

Cambrian Explosion #60: Crustacea – Larvae Larvae Everywhere

One of the characteristic features of the crustacean lineage are their larval forms, passing through various tiny larval stages. They often look nothing like their eventual adult forms and historically weren’t even recognized as being the same species, with their complex lifecycles not being properly recognized until the late 1800s.

A lot of Cambrian crustaceans are only known from their larvae, preserved in exquisite microscopic detail in sites of “Orsten-type preservation”. Only disarticulated fragments of larger-bodied forms have been found in a few places, and it isn’t until much later in the Paleozoic that fossil crustaceans actually seem to become abundant in marine ecosystems.

It’s not clear why there’s such a bias in their early fossil record compared to most other arthropods, but possibly they were just very very rare animals early on. Adult forms may have mostly lived in places where they just didn’t fossilize, while their tiny larvae sometimes dispersed into different environments with a better chance of preservation.

Cambrian Explosion #59: Stem-Crustacea – Actual Ancient Aliens & Bivalved Buddies

The majority of known fossils of Cambrian crustaceans are in the form of minuscule microfossils with “Orsten-type preservation” – formed in oxygen-poor seafloor mud and exceptionally well-preserved in three-dimensional detail. They can only be discovered and studied after dissolving away the rock around them with acid and picking through the residue under a microscope, then they’re scanned with an electron microscope to see their fine details.

And it turns out some of these tiny early crustaceans looked really weird.