Soft-bodied annelid worms only very rarely fossilize, so the group’s origins during the Cambrian Period are still rather poorly understood. So far about thirteen different species have been found in sites of exceptional preservation, showing that even very early on in their evolution these worms had already diversified into a wide range of ecologies including bottom-feeders, carnivores, swimmers, tube-builders, and even symbiotes sharing living space with early acorn worms.

Ursactis comosa here adds a fourteenth species to the list. Found in a newly-discovered outcrop of the 508-million-year-old Burgess Shale fossil deposits in western Canada, it’s known from nearly 600 specimens clustered together in several large groups, making it the current best-known and most numerous of all Cambrian annelids.

Up to about 1.5cm long (~0.6″), it was a polychaete-like worm bearing bundles of long bristles. There was a pair of large sensory palps on its head, and its body was made up of an unusually small number of segments – just 10, with larger individuals just increasing the size of their segments instead of adding on additional ones like most modern annelids.

Unlike other Cambrian annelids it also shows some evidence of basic tagmatization, differentiating some of the rear segments of its body with much longer bristles.

The large numbers of Ursactis found preserved in one place suggests these worms were exhibiting some sort of swarming behavior. Since ages from juveniles to fully-grown adults are represented together, and their anatomy indicates they were crawling detritivores, they were probably all taking advantage of a particularly nutrient-rich patch of seafloor at the time they were abruptly buried in a mudslide.

Strange Symmetries #20: The 16 Million Year Fiddler Crab Rave

Many decapod crustaceans have slightly asymmetrical pincers, often with one claw being chunkier and specialized for “crushing” while the other is more slender and used for “cutting”.

But fiddler crabs take this sort of asymmetry to the extreme as part of their sexual dimorphism – males have one massively oversized claw, which is used for both visual display to potential mates and for physical fights against rivals.

Some of the earliest fiddler crabs are known from the Miocene of what is now northern Brazil. Although the fossils have been given several different taxonomic names since their discovery in the 1970s (including Uca maracoani antiqua, Uca antiqua, and Uca inaciobritoi) they’re currently considered to be indistinguishable from the modern Brazilian fiddler crab, Uca maracoani, meaning that these crabs have remained externally unchanged for the last 16 million years.

Up to about 4cm in carapace width (~1.6″), modern Uca maracoani are found in coastal mangrove swamps and tidal mudflats around the northern and eastern coasts of South America – and some of these environments have also undergone little change since the Miocene. Males of the species can develop their enlarged pincer on either side of their bodies, with lefties and righties seeming to occur in equal numbers.

Strange Symmetries #16: Go Home Heteromorphs You’re Drunk

Most ammonites had spiral-coiling shells, but during the Cretaceous a group known as the heteromorphs evolved a much weirder range of forms. Some were straightened, some were hooked, some had helical snail-like shapes, and some even ended up bearing a strange resemblance to paperclips

But one of the most bizarre of all was the genus Nipponites, whose ribbed shell looked like a bundle of tangled asymmetrical coils.

Nipponites bacchus lived in what is now Hokkaido, Japan, during the late Cretaceous about 90 million years ago. Around 10cm long (~4″), its shell was less tightly coiled up than its better-known relative Nipponites mirabilis, but these looser whorls were formed in the same way via a series of U-bends in different directions during its growth.

Despite their irregular and ungainly appearance, the unique shape of these ammonites seems to have actually been very hydrodynamically stable. They weren’t fast-moving, but they didn’t need to be, probably spending most of their time floating suspended in the water column catching small planktonic prey from around themselves.

Strange Symmetries #13: The Hermit Crab Cycle

Hermit crabs are crustaceans that first appeared at the start of the Jurassic, about 201 million years ago. Despite their common name they aren’t actually true crabs, instead being a classic example of convergently evolving a crab-like body plan via carcinization.

They also have noticeably asymmetric bodies, with abdomens that coil to one side and differently-sized front claws.

And while modern hermit crabs are famous for inhabiting scavenged snail shells, their fossil record suggests this wasn’t always the case.

Originally, they seem to have lived in ammonite shells.

Palaeopagurus vandenengeli lived in what is now northern England during the Early Cretaceous, about 130 million years ago. Around 4-5cm long (~1.6-2″), it was found preserved inside the shell of the ammonite species Simbirskites gottschei.

Its left claw was much larger than its right, and together they would have been used to block the shell opening when it was hiding away inside. And while the exact shape of its abdomen isn’t known, it probably asymmetrically coiled to the side to accomodate the spiralling shape of the host shell.

Hermit crabs seem to have switched over to using gastropod shells by the Late Cretaceous, around 90-80 million years ago, possibly due to marine snails developing much stronger sturdier shells during this period in response to the increasing prevalence of specialized shell-crushing predators. The more upright snail shells would also have been much easier to drag around the seafloor than ammonite shells – and meant that they were ultimately less affected by the total disappearance of ammonites during end-Cretaceous mass extinction.

Strange Symmetries #10: Shellraiser

Brachiopods (also known as “lamp shells”) superficially look very much like bivalves, but these two groups aren’t very closely related to each other – although they’re both lophotrochozoans, their last common ancestor probably lived sometime in the Ediacaran at least 560 million years ago, and their similarities in appearance are due to convergent evolution.

The two valves of their shells are also arranged differently. Bivalve shells grow on their left and right sides and are usually symmetrical, but brachiopods form their shells from the upper and lower surfaces of their bodies.

As a result brachiopod shells are usually unequal in size and shape but have their own plane of bilateral symmetry down the center – but some of them still managed to become asymmetrical anyway.

Torquirhynchia inconstans lived during the Late Jurassic, about 161-145 million years ago, in the warm shallow seas that covered what is now Europe and Iran. Around 3cm across (~1.2″) it had a strongly ridged shell with an asymmetrical closing edge, positioned high on one side and low on the other.

This unusual uneven arrangment is thought to be an adaptation to living on soft sediments. Asymmetrical brachiopods like Torquirhynchia may have lived with one side of their body mostly buried into the seafloor, and twisted their shell edges so the still-exposed half was raised up to better function for water circulation and filter-feeding.

Strange Symmetries #09: Not Toeday, Satan

The shells of bivalve molluscs are formed on the left and rights sides of their bodies, and so are usually roughly bilaterally symmetric.

But some bivalves break that arrangement, developing asymmetrical valves that can be massively different in size and shape.

Gryphaea arcuata was an oyster that lived during the Early Jurassic, about 200-174 million years ago, in the warm shallow seas that covered what is now Europe and eastern Greenland. Around 6cm long (~2.4″), its left valve was thick and strongly convex and curled, while the right valve was relatively thin and slightly concave forming a “lid”.

The gnarled curled claw-like shape of Gryphaea fossils led to them being colloquially known as “devil’s toenails” in some of the regions where they’re commonly found, with folk beliefs that they had the power to prevent joint pain.

Their shape was actually an adaptation to living on very soft seafloor sediments. The larger curled valve acted sort of like a boat on the soupy mud, supporting the Gryphaea‘s weight and preventing it from sinking.

Strange Symmetries #06: Trilobite Tridents

The genus Walliserops was one of the weirdest-looking trilobites, covered in numerous pointy spines and sporting a large three-pronged “trident” on the front of its face.

They also had some degree of asymmetry in their bodies. Their tridents often didn’t fork evenly, and their long forehead spines curved off to one side – possibly so they could lift their heads up without stabbing themselves in the back.

Walliserops hammii lived in what is now Morocco during the early-to-mid Devonian, about 403-392 million years ago. Around 5cm long (~2″) It was one of the “short trident” species of Walliserops, and its chunky forehead spine curved particularly strongly to the right.

The function of these trilobites’ elaborate tridents is still poorly understood. But an unusual individual of the long-tridented species Walliserops trifurcatushas been found with a lopsided four-pronged trident, and since it was able to grow to full maturity the shape of the structure probably wasn’t absolutely vital for survival, suggesting it wasn’t used for feeding or sensory purposes.

The tridents may instead have been used for combat with each other similar to the horns of some modern beetles. However, these sorts of features are usually only seen in males, and there’s currently no definite evidence for any significant sexual dimorphism in trilobites.

(Although perhaps like ceratopsid dinosaurs their ornaments were just present in both males and females, being also useful for species recognition, visual display, and defense against predators.)


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.

It Came From The Wastebasket #19: The Pterrible Fate Of Ptychopariida

The Ptychopariida were some of the earliest known trilobites, first appearing in the early Cambrian about 521 million years ago and surviving until the end of the Ordovician about 444 million years ago. They included some of the most numerous and common trilobite species, and were probably ancestral to multiple other major lineages – including the very last trilobites at the end of the Permian – making them incredibly important in understanding the overall evolution of trilobites as a whole.

…But this group is also one of the biggest wastebaskets in paleontology.

First established in the early 20th century, the ptychopariids seemed to have some fairly good defining characteristics based on their facial sutures, large thoraxes, and relatively small pygidia. But the group quickly became a dumping ground for a massive amount of Cambrian trilobites, eventually containing numerous different families, hundreds of genera, and many more individual species.

Actually figuring out their internal evolutionary relationships also turned out to be extremely difficult – so much so that some paleontologists working on them just gave up trying and arranged the genera names alphabetically instead!

Even cladistic studies from the 1970s onward struggled to make sense of these highly “problematic” trilobites, and any larger-scale analysis was a daunting task due to how huge and diverse the ptychopariid wastebasket had become over the years. Worse, some of the anatomical features the group had been based around were starting to look more like the result of a lot of convergent evolution across multiple lineages than any actual shared ancestry.

Efforts were still made at breaking up the mess, however, with better-understood sub-groups like the Proetida, Harpida, Asaphida, Trinucleida, and Olenida being gradually split off into their own separate orders over the course of the last few decades.

An illustration of Ptychoparia, an extinct trilobite from the Cambrian period. It has a large semicircular head with a pair of antennae, small eyes, and a bulbous "forehead" region. Its body has 13 segements, and its "tail" is small and shaped like a blunted triangle.
Ptychoparia striata

But even by the early 2010s what remained of the Ptychopariida was still paraphyletic at best, more of an “evolutionary grade” of early trilobites than a single lineage, with most of its constituent families also rather poorly defined. There was even a proposal to abandon the group entirely, stating that “it serves no scientific purpose” and that its orphaned contents should be considered “order uncertain” until their actual relationships can be untangled.

Today the “ptychopariids” are in dire need of a full revision – since they were the ancestors of many other major groups they’re still crucial for understanding early trilobite evolution. There may be a salvageable single lineage somewhere in the remains of this wastebasket, even if it’s restricted to just close relatives of the genus Ptychoparia, but until somebody tackles them properly they’re stuck in taxonomic limbo with their name only being used in a loose sense.

It Came From The Wastebasket #14: The Protorthoptera Puzzle

Protorthoptera was a group of fossil insects created in the early 20th century to categorize “primitive” neopterans – some of the earliest insects to have evolved the ability to fold their wings down over their backs. Known mostly from just fossilized forewings, they first appeared around 320 million years ago in the late Carboniferous, and after heavy losses during the Great Dying mass extinction they eventually disappeared in the mid-Triassic about 240 million years ago.

And this group was a massive wastebasket taxon.

As early as the mid-20th century the protorthopterans were recognized as being a general taxonomic dumping ground, containing a mixture of early members of multiple different “orthopteroid” insect lineages. But invertebrate paleontologists at the time considered this collection of “primitive” insects to lack enough distinctive features to confidently separate them out from each other, and so the highly paraphyletic grouping continued to be used well into the 1990s.

An illustration of Ctenoptilus elongatus, an extinct insect. It has long thin antennae, a small grasshopper-like head, six legs each ending in two small claws, a cylindrical abdomen, and two pairs of large wings folded over its back. It's colored red, tan, and dark brown, with striped markings on its wings.
Ctenoptilus elongatus

But in the early 2000s this situation finally changed. Proper cladistic analysis of protorthopteran fossils identified defining features of the wing vein patterns, and many species were reclassified into various lineages within the Archaeorthoptera – which includes modern grasshoppers, crickets, and locusts along with several closely related fossil groups like the titanopterans and caloneurodeans.

“Protorthoptera” is still sometimes used in a loose sense for fossil neopteran insects that still can’t be confidently classified anywhere else, so the wastebasket isn’t entirely cleared here.

And there are some alternate classification systems (mainly proposed by Russian paleontologists) that instead consider many protorthopterans to be notopterans closely related to modern ice-crawlers, and place others as part of other modern neopteran lineages such as webspinners and true bugs.

Hopefully better fossil discoveries and future studies will eventually help clear things up, and give us a better overall picture of the evolution of these insects.