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.

It Came From The Wastebasket #09: Hammering Away At Hamites

Ancyloceratines were a lineage of distinctive-looking ammonites, commonly known as “heteromorphs“, which had unusual uncoiled shells that ranged in shape from near-straight to hooked to helical to paperclip-like to “balls of string“.

Heteromorphs’ strange shells would have created a lot of drag in the water, and they may not have been especially agile swimmers, but they were very hydrodynamically stable and easily maintained neutral buoyancy. Their paleobiology has only just started to be properly understood in recent years, and now most species of these ammonites are thought to have floated suspended in the photic zone and twilight zone of the open ocean, catching small zooplankton from the water around themselves.

What these ammonites were doing obviously worked very well for them, because they were incredibly diverse and successful during the Cretaceous period. They were also the only type of ammonite to persist for a short time after the end-Cretaceous mass extinction, existing as a “dead clade walking” for another half a million years or so before finally disappearing entirely.

An illustration of Hamites attenuatus, an unusually-shaped extinct ammonite. It has a mostly-uncoiled shell shaped like a long sideways U, almost paperclip-like. At the bottom end of the shell it has a squid-like body, with large eyes and ten short webbed arms that are lined with speculative fringes for filter-feeding. It has a pinkish color scheme with a faint iridescent sheen.
Hamites attenuatus

The hamitids were a group of heteromorphs from the mid-Cretaceous (~110-90 million years ago), with their namesake genus Hamites traditionally being used as a wastebasket taxon for anything that that didn’t neatly fit into any other group of similar heteromorph ammonites.

By the late 1990s Hamites had become a mess of multiple different diverse lineages, with over 20 species all lumped together – and this was a problem because the hamitids were the ancestors of several other heteromorph ammonite lineages, and having the taxon in such disarray made studying the evolutionary origins of all those other groups very difficult.

So in the early 2000s attempts were made to clean this all up, figuring out the relationships between the different Hamites species and dividing the genus into multiple new genera.

There hasn’t been much more detailed research on the relationships of hamitids since then – and other groups of heteromorphs are still in need of revision – but it’s a start at clearing the wastebasket, at least.

It Came From The Wastebasket #04: Breaking Up Bellerophon

Bellerophonts were small snail-like marine molluscs that were either early gastropods or very close relatives of them. They had symmetrically-coiled shells superficially shaped like those of nautiluses, with about half of the shell covered by their mantle similarly to some modern sea snails, and some fossil shells also preserve hints of banded color patterns.

First appearing in the late Cambrian (~500 million years ago), these molluscs existed all the way until the early Triassic, surviving the Great Dying mass extinction (~252 million years ago) only to go extinct just a short time later (~249 million years ago) – a phenomenon known as “dead clade walking”, when a group just barely scrapes through a mass extinction event but doesn’t manage to actually recover afterwards.

The whole group is something of a wastebasket of similar-looking shells, and might actually be more of an “evolutionary grade” made up of various early gastropods and gastropod-relatives than a single defined lineage.

But there’s also another wastebasket nestled inside this wastebasket: the namesake of them all, the genus Bellerophon.

An illustration of Bellerophon, an extinct sea-snail-like mollusc. It has a banded shell that coils vertically like a nautilus, with a ridge along the midline. It has a wide flat foot, its mantle covers about halfway up the sides of its shell, and it has a pair of snail-like head tentacles and a siphon.
Bellerophon tenuifascia

Originally named in 1808, this genus has had a huge number of species assigned to it over the last couple of centuries. This gives a false impression that Bellerophon-like molluscs didn’t change for hundreds of millions of years, and it makes figuring out their actual long-term patterns of evolution and extinction much more difficult.

In the last few decades some mollusc paleontologists have been gradually chipping away at Bellerophon, and multiple new genera have been broken off from it. But even today it remains a very bloated mess – there are still well over a hundred named species spanning about 230 million years of geologic time.

Studies do indicate the whole genus is highly polyphyletic, made up of a tangle of multiple different lineages that all really need to be revised and renamed – but there’s a lot of work still needing to be done to clean up this particular wastebasket.

Retro vs Modern #17: Ammonites

Ammonites (or ammonoids) are highly distinctive and instantly recognizable fossils that have been found all around the world for thousands of years, and have been associated with a wide range of folkloric and mythologic interpretations – including snakestones, buffalo stones, shaligrams, and the horns of Ammon, with the latter eventually inspiring the scientific name for this group of ancient molluscs.

(Unlike the other entries in this series the reconstructions shown here are somewhat generalized ammonites. They’re not intended to depict a specific species, but the shell shape is mostly based on Asteroceras obtusum.)


1830s

It was only in the 1700s that ammonites began to be recognized as the remains of cephalopod shells, but the lack of soft part impressions made the rest of their anatomy a mystery. The very first known life reconstruction was part of the Duria Antiquior scene painted in 1830, but to modern eyes it probably isn’t immediately obvious as even being an ammonite, depicted as a strange little boat-like thing to the right of the battling ichthyosaur and plesiosaur.

The argonaut octopus, or “paper nautilus”, was considered to be the closest living model for ammonites at the time due to superficial similarities in its “shell” shape, but these modern animals were also rather poorly understood. They were commonly inaccurately illustrated as floating around on the ocean surface using the expanded surfaces on two of their tentacles as “sails” – and so ammonites were initially reconstructed in the same way.


1860s

While increasing scientific knowledge of the chambered nautilus led to it being proposed as a better model for ammonites in the mid-1830s, the argonaut-style depictions continued for several decades.

Interestingly the earliest known non-argonaut reconstruction of an ammonite, in the first edition of La Terre Avant Le Déluge in 1863, actually showed a very squid-like animal inside an ammonite shell, with eight arms and two longer tentacles. But this was quickly “corrected” in later editions to a much more nautilus-like version with numerous cirri-like tentacles and a large hood.

The nautilus model for ammonites eventually became the standard by the end of the 19th century, although they continued to be reconstructed as surface-floaters. Bottom-dwelling ammonite interpretations were also popular for a while in the early 20th century, being shown as creeping animals with nautilus-like anatomy and numerous octopus-like tentacles, before open water active swimmers eventually became the standard representation.


2020s

During the 20th century opinions on the closest living relatives of ammonites began to shift away from nautiluses and towards the coleoids (squid, cuttlefish, and octopuses). The consensus by the 1990s was that both ammonites and coleoids had a common ancestry within the bactridids, and ammonites were considered to have likely had ten arms (at least ancestrally) and were probably much more squid-like after all.

Little was still actually known about these cephalopods’ soft parts, but some internal anatomy had at least been figured out by the early 21st century. Enigmatic fossils known as aptychi had been found preserved in position within ammonite shell cavities, and were initially thought to be an operculum closing off the shell against predators – but are currently considered to instead be part of the jaw apparatus along with a radula

Tentative ink sac traces were also found in some specimens (although these are now disputed), and what were thought to be poorly-preserved digestive organs, but the actual external life appearance of ammonites was still basically unknown. By the mid-2010s the best guess reconstructions were based on muscle attachment sites that suggested the presence of a large squid-like siphon.

Possible evidence of banded color patterns were also sometimes found preserved on shells, while others showed iridescent patterns that might have been visible on the surface in life.

In the late 2010s the continued scarcity of ammonite soft tissue was potentially explained as being the same reason true squid fossils are so incredibly rare – their biochemistry may have simply been incompatible with the vast majority of preservation conditions.

But then something amazing happened.

In early 2021 a “naked” ammonite missing its shell was described, preserving most of the body in exceptional detail – although frustratingly the arms were missing, giving no clarification to their possible number or arrangement. But then just a few months later another study focusing on mysterious hook-like structures in some ammonite fossils concluded that they came from the clubbed tips of a pair of long squid-like tentacles – the first direct evidence of any ammonite appendages!

A third soft-tissue study at the end of the year added in some further confirmation that ammonites were much more coleoid-like than nautilus-like, with more evidence of a squid-style siphon, along with evidece of powerful muscles that retracted the ammonite’s body deep inside its shell cavity for protection.

Since ammonites existed for over 340 million years in a wide range of habitats and ecological roles, and came in a massive variety of shapes and sizes, it’s extremely likely that their soft anatomy was just as diverse as their shells – so there’s no single “one reconstruction fits all” for their life appearances. Still, at least we now have something less speculative to work with for restorations, even if it’s a bit generalized and composite, and now that we’re finally starting to find that elusive soft tissue there’s the potential for us to discover so much more about these iconic fossil animals.

Cambrian Explosion Month #26: Phylum Mollusca – Tentacle Time

Cephalopods‘ highly distinctive body plan and incredible intelligence make them some of the most charismatic marine animals. Today they’re mainly represented by the soft-bodied coleoids (octopuses, squid, and cuttlefish), with the modern giant squid and colossal squid being the largest living invertebrates. In comparison the shelled nautiluses seem like weird oddballs, but they’re actually far more typical examples of the group than their squishier cousins – as part of the conchiferan lineage the ancestors of all modern cephalopods were also shell-bearing molluscs, and for much of their evolutionary history shelled forms like ammonites and orthoceridans were extremely abundant.

The exact evolutionary relationships of cephalopods within the conchiferan family tree aren’t clear, but their closest relatives might be modern monoplacophorans and they probably descended from limpet-like “monoplacophoran-grade” ancestors in the early Cambrian. The current oldest potential cephalopod fossils come from about 522 million years ago, but the first definite cephalopods in the fossil record come from much later in the period.

Continue reading “Cambrian Explosion Month #26: Phylum Mollusca – Tentacle Time”

Cambrian Explosion Month #25: Phylum Mollusca – Shelling Out

The exact evolutionary relationships of the main groups of modern molluscs are rather debated, with several different proposed family trees. But one of the main possibilities is that there are two major lineages: the aculiferans and the conchiferans.

Modern conchiferans include slugs and snails, cephalopods, bivalves, tusk shells, and monoplacophorans – all groups that ancestrally have either a single-part shell or a two-part bivalved shell, with some lineages later becoming secondarily shell-less.

The ancestral conchiferans are thought to have been monoplacophoran-like molluscs, limpet-like with a cap-shaped shell, and likely diverged from a common ancestor with the aculiferans around the end of the Ediacaran. (But modern monoplacophorans probably aren’t “living fossil” descendants of early Cambrian conchiferans, and may instead be close relatives of cephalopods that have convergently become similar in appearance to their ancestors.)

Some of the earliest conchiferans were the helcionelloids, a lineage of superficially snail-like molluscs with coiled cone-shaped mineralized shells. They appeared in the fossil record at the start of the Cambrian (~540-530 million years ago) and lasted until the early Ordovician (~480 million years ago), and have been found all around the world as components of the “small shelly fauna“.

And while they’re usually tiny, only a couple of millimeters in size, they may actually represent juveniles or larvae – there’s evidence that at least some species grew up into much larger 2cm (0.8″) limpet-like adult forms.

Continue reading “Cambrian Explosion Month #25: Phylum Mollusca – Shelling Out”

Cambrian Explosion Month #24: Phylum Mollusca – Coats of Mail

Much like Odontogriphus and Wiwaxia, the evolutionary relationships of a group called the halkieriids have been debated for a long time. These animals looked like “slugs in chain mail“, covered in thousands of tiny overlapping mineralized armor plates along with a larger shell plate at each end.

In the past they’ve been assigned to different parts of the lophotrochozoan family tree, sometimes being placed closer to annelids or brachiopods, but at this point they’re generally accepted to be molluscs. The spiny species Orthrozanclus may link halkieriids with wiwaxiids in a larger “halwaxiid” lineage of early molluscs – or they might instead be early members of a group called aculiferans

Aculiferans are represented in modern times by chitons and aplacophorans, and they’re distinguished from all other molluscs by having either eight shell valves (chitons) or no shell at all and a worm-like body covered with tiny calcareous spines (aplacophorans).

(Also chitons are especially weird, with magnetite teeth and thousands of eyes in their armor plates.)

A related fossil species called Calvapilosa kroegeri from the early Ordovician of Morocco (~480 million years ago) seems to link halkieriids with aculiferans, placing the chain-mail-slugs as a stem lineage close to the common ancestor of modern forms.

Continue reading “Cambrian Explosion Month #24: Phylum Mollusca – Coats of Mail”

Cambrian Explosion Month #23: Phylum Mollusca – The Stem Weirdos

Molluscs are one of the largest animal phylums, second only to the arthropods, and are also hugely diverse, found in marine, freshwater, and terrestrial environments all over the world. Not only are familiar modern animals like bivalves, slugs and snails, and squid and octopuses included in this huge lineage, but also nautiloids, chitons, tusk shells, monoplacophorans, worm-like aplacophorans, and the extinct ammonites and orthocerids.

Like the annelids they’re lophotrochozoan spiralians, and their exact evolutionary relationships within that group are a bit uncertain. But their fossil history seems to go back at least 558 million years with the “mollusc-like” Ediacaran Kimberella, and the earliest members of most major mollusc lineages had probably already diverged from each other before the start of the Cambrian.

The common ancestor of all molluscs probably had features like an unsegmented body, a muscular foot on their underside, a mantle and mantle cavity, a radula, and possibly a tough but non-mineralized leathery “shell” – and Odontogriphus omalus may represent an early stem lineage retaining that basic body plan into the mid-Cambrian.

Continue reading “Cambrian Explosion Month #23: Phylum Mollusca – The Stem Weirdos”

Kulindroplax

Kulindroplax perissokomos, a mollusc from the mid-Silurian of England (~428-423 mya).

About 4cm long (1.6″), it had a wormlike body covered in spicules (tiny spines) which suggests it was a member of the aplacophoran molluscs – but it also had a row of seven larger shells along its back resembling those of chitons.

Modern aplacophorans are all shell-less and were traditionally thought to be a very early branch of the mollusc lineage that retained a “primitive” ancestral body plan. More recently, however, a combination of genetic evidence and fossil discoveries of animals like Kulindroplax have revealed that they’re actually close relatives of the chitons and instead lost their shells much more recently during the course of their evolution.