eumetazoa – Nix Illustration

Lobodiscus

Trilobozoans (also known as triradialomorphs) are some of the more enigmatic members of the Ediacaran biota. In the past their unique three-way-symmetrical body plan was interpreted as linking them to groups like sponges, cnidarians, or echinoderms, but currently they’re considered to be their own weird little phylum with uncertain evolutionary affinities, classified no more specifically than “probably some sort of early eumetazoan animal“.

Lobodiscus tribrachialis is a newly-described member of this mysterious lineage. It lived in warm shallow marine waters covering what is now Southwestern China, and with an age of around 546 million years it’s currently the youngest known trilobozoan, extending the group’s time range by several million years.

About 3.7cm in diameter (~1.5″), it had the characteristic trilobozoan disc-shaped shield-like body, with a central depression surrounded by three triradially-symmetric lobes with branching ridges and grooves.

Its body would have been soft but fairly rigid, and it’s not clear if it was capable of moving over the seafloor or if it had a more static lifestyle. Like its relative Tribrachidium it was probably a filter feeder, with the grooves on its surface directing water flow towards the central depression – and this surface ornamentation may also have been covered with cilia that actively caught and transported suspended food particles.

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.

Auroralumina

Cnidarians – a group of animals that includes modern corals, sea anemones, sea pens, jellyfish, hydra, and a couple of parasitic forms – are one of the most ancient animal lineages, originating at least 580 million years ago in the Ediacaran period.

Actual identifiable fossils of cnidarians that old are incredibly rare, however, and until now there was only one example – the small polyp-like Haootia from Canada.

But a second definite Ediacaran cnidarian has now been described: Auroralumina attenboroughii.

It was discovered in Charnwood Forest, England, in the very same site where the first recognized Precambrian fossils were found in the 1950s. About 20cm tall (~8″) it dates to around 560 million years ago and was made up of a pair of forking stiff-walled tubes which expanded into wide four-sided goblet-like shapes full of stubby tentacles. These densely-tentacled crowns would have been used to capture tiny planktonic organisms from the water around it, making it the current earliest known example of a predatory animal.

The one known fossil specimen has an incomplete base, so it’s uncertain if this was actually the full life appearance of Auroralumina or if it was even larger with more branches and goblets. And although it was preserved in deep-water sediments, it appears to have originated from much shallower waters, being swept down into the depths during a volcanic eruption.

While it superficially resembled a sea anemone, details of its anatomy suggest it was actually much closer related to medusozoans, having similar traits to the immobile polyp stage of the jellyfish life cycle. Its four-way symmetry and boxy shape may also link it to the enigmatic conulariids.

It’s not clear if it was able to bud off swimming medusa stages like its modern relatives – that might be an evolutionary innovation that came along later – but it at least shows that a basic medusozoan body plan was already in place around 20 million years earlier than previously thought.

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.

Cambrian Explosion #58: Hymenocarina

The pancrustaceans are a grouping of mandibulates that contains all of the crustaceans and hexapods (insects and their closest relatives) along with their various stem-relatives.

They’re critical components of most ecosystems on the planet, and are major parts of the nutrient cycle. In aquatic environments the crustaceans dominate, with modern copepods and krill being some of the most abundant living animals and making up enormous amounts of biomass providing vital food sources for larger animals. On the land springtails and ants are especially numerous, and the air is full of flying insects, the only invertebrates to ever develop powered flight. Some groups of insects have also co-evolved complex mutualistic partnerships with flowering plants and fungi.

Hexapods and insects don’t appear in the fossil record until the early Devonian, but they’re estimated to have first diverged from the crustaceans* in the early Silurian (~440 million years ago), around the same time that vascular plants were colonizing the land.

(* Hexapods are crustaceans in the same sort of way that birds are dinosaurs. They originated from within one of the major crustacean lineages with their closest living relatives possibly being the enigmatic remipedes.)

But crustaceans and their pancrustacean ancestors go back much further into the Cambrian, and we’ll be finishing off this month and this series with some of those early representatives.

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.

Cambrian Explosion #56: Euthycarcinoidea

The euthycarcinoids were a group of euarthropods known from the mid-Cambrian to the mid-Triassic (~500-254 million years ago), surviving through multiple mass extinctions including the devastating “Great Dying” at the end of the Permian that finished off the trilobites. But despite an evolutionary history spanning around 250 million years they have a very sparse fossil record, extremely rare and known from less than 20 species across their entire time range.

For a long time their affinities were uncertain, and they’ve been variously suggested to have been crustaceans, trilobites, or chelicerates, or even to have been a lineage of earlier stem-euarthropods. But since the early 2010s better understanding of their anatomy has placed them in the mandibulates, probably as the closest relatives of the myriapods and helping to close the gap between the aquatic ancestors of that group and their earliest known terrestrial forms.