The eugeneodonts were a group of cartilaginous fish that convergently resembled sharks but were actually much closer related to modern chimaeras. They had unique “tooth whorls” in their jaws, and the most famous member of the group is probably Helicoprion, whose bizarre buzzsaw-like tooth arrangement was only properly understood within the last decade.

Ornithoprion hertwigi here was one of the first eugeneodonts found with fossilized skull material, and helped with the early understanding of just how their weird jaw anatomy actually worked.

It lived during the Late Carboniferous, about 315-307 million years ago, in a shallow tropical sea that covered what is now southwestern Indiana, USA.

At only around 50cm long (~1’8″) it was one of the smaller eugeneodonts, and along with a small Helicoprion-like tooth whorl it also had a distinctive highly elongated chin. Similar to modern halfbeak fish this structure may have served a sensory function, helping Ornithoprion to detect prey in dark or murky waters.

Retro vs Modern #08: Helicoprion davisii

First discovered in Western Australia in the mid-1880s, the bizarre-toothed eugeneodont cartilaginous fish Helicoprion davisii was initially mistaken for a species of the equally weird Edestus. It was eventually recognized as part of a separate genus over a decade later, when similar fossils of its close relative Helicoprion bessonowi were found in the Ural Mountains.


With Helicoprion only known from strange buzzsaw-like spiral whorls, the function and location of this structure in the fish’s body was a huge source of confusion for over a century.

The earliest interpretation was a defensive structure curling upwards from the snout, then as part of the tail or dorsal fins. It was soon realized to probably be part of the lower jaw instead, but the exact arrangement was still a mystery.

A downward-curling position was popular in reconstructions for much of the 20th century. From the 1960s onwards, however, discoveries of preserved skull cartilage and soft-tissue body outlines of other eugeneodont species began to give a better idea of what these fish were and what they looked like. They were identified as being related to modern chimaeras, but with a very different appearance – they had streamlined shark-like bodies with large triangular pectoral fins, a single dorsal fin, no pelvic or anal fins at all, and broad keels along the sides of their tails.

A single tooth whorl sat in the middle of the lower jaw, with its sides covered by skin, and the largest and youngest teeth formed at the back before spiralling forwards, downwards, and inwards.

In the 1990s a “pizza-cutter” reconstruction gave Helicoprion long narrow jaws with the whorl positioned very far forwards, sawing and crushing prey against the underside of the snout. A version with the whorl very deep inside the throat was also proposed in 2008, but only a year later a new variant of the pizza-cutter saw-jaw model suggested the presence of a specialized “pocket” in the upper jaw lined with teeth.


Finally, in 2013, CT scanning of a Helicoprion specimen originally discovered in the 1960s revealed something incredibly special – an almost complete three-dimensionally preserved and articulated set of jaws. It showed narrow jaws that were shorter than the previous reconstructions, with the whorl occupying the entire lower jaw and braced by cartilage on each side.

We now know Helicoprion davisii was found worldwide during the early-to-mid Permian, about 272-268 million years ago, and based on some of the biggest known tooth whorls it may have reached sizes of up to 8m long (~26′), similar in size to modern basking sharks. It continuously added new and larger teeth to its whorl throughout its life, with the smaller older teeth being retained instead of shed, slowly pushed into a tight spiral deep inside the lower jaw.

The upper jaw formed a sheath-like pocket lined with a “pavement” of numerous tiny rounded teeth, and as Helicoprion closed its jaws the various parts of the whorl simultaneously grabbed, sliced, and pulled prey further into its mouth – a mechanism possibly specialized for efficiently de-shelling cephalopods like ammonites and nautiloids.


Echinochimaera meltoni here was a cartilaginous fish found in the Bear Gulch Limestone deposits in Montana, USA, dating to the Early Carboniferous about 326-318 million years ago.

It was an early member of the chimaera lineage, but unlike its mostly-scaleless modern relatives its body was covered in small shark-like placoid scales.

It also showed a large degree of sexual dimorphism, with males and females almost looking like different species entirely. Males are identified by the presence of claspers and were up to 15cm long (6″), with four pairs of spiny “horns” on their heads, larger more pointed dorsal fins, and rows of spines along their tails. Females were less than half the size of males at just 7cm long (2.75″), with only one pair of smaller “horns” and none of the additional spines.

The rounded bodies and relatively small paddle-like tail fins of both sexes suggest they weren’t very strong swimmers, probably relying on their large dorsal fin spines to defend themselves – which may have been venomous much like those of modern chimaeras.


Discovered in the late 1820s by pioneering paleontologist Mary Anning, the odd-looking fossil of the cartilaginous fish Squaloraja polyspondyla seemed to have characteristics of both sharks and rays.

It was initially thought to be a “missing link” transitional form between those two groups, but later it was identified as being something else entirely – it was actually part of the chimaera lineage, much closer related to modern ratfish, and its ray-like features were due to convergent evolution for a bottom-feeding lifestyle.

Living during the early Jurassic period, about 200-195 million years ago, Squaloraja fossils are now known from the south coast of England, southern Belguim, and northern Italy. Around 30cm long (1’), this weird fish had a massive wide flat snout that looked like an even more extreme version of the long noses seen in some of its modern relatives, and this enormous snoot would have been absolutely packed with sensory receptors to help it locate small aquatic prey hidden in the muddy seafloor.

Some specimens also have a distinctive long horn-like spine on their foreheads, and since these individuals also have claspers it seems like this was a sexually dimorphic feature. Much like the smaller head claspers on modern chimaeras, male Squaloraja probably used this “horn” to hang onto females’ pectoral fins during mating – and with it being such a large elaborate structure it may also have been used for visual display purposes, too.

Weird Heads Month #11: Scissor-Toothed “Sharks”

The eugeneodontidans were a group of cartilaginous fish which convergently evolved to resemble sharks but were much closer related to modern chimaeras. Due to their cartilage skeletons usually little more than their teeth are found as fossils, and for a long time their ecology and life appearance has been poorly understood because of just how weird those teeth were.

These fish had unique “tooth whorls” in their lower jaws, and the most famous member of the group is probably Helicoprion, with the exact anatomical placement of its buzzsaw-whorl only being properly figured out in 2013.

But another eugeneodontidan named Edestus was equally strange.

Living during the late Carboniferous, about 306-299 million years ago, Edestus giganteus was the largest species in the genus, reaching estimated lengths of up to 6m (19’8″), similar in size to a modern orca or a particularly large white shark.

Let’s take a closer peek at that mouth.

A close up drawing of the head of the extinct shark-like fish Edestus. It has a single central row of large teeth in its upper and lower jaws.

Yes, that’s a single central row of teeth in both its upper and lower jaws.

Edestus‘ whorls grew in curving “banana-shaped” brackets that resembled an enormous pair of pinking shears, with new teeth being added on at the back and the oldest teeth occasionally being ejected off from the front. How this jaw arrangement worked was a longstanding paleontological mystery, with various bizarre ideas being proposed over the years – until a particularly well-preserved skull was analyzed in early 2019, revealing a two-jointed system in its lower jaw that allowed it to move its tooth brackets quickly back and forth, using a “snap-and-slice” motion to grab hold of prey like fish and soft-bodied cephalopods and cut them in half.

Along with body impressions from other related eugeneodontidans like Fadenia, showing a shark-like tail and a complete lack of rear fins, we now have a much better picture of what this bizarre fish probably looked like.


Falcatus falcatus, a 30cm long (12″) cartilaginous fish from the mid-Carboniferous of Montana, USA (~326-318 mya).

Although it looked very shark-like it was actually much more closely related to modern chimaeras, and its most distinctive feature was the forward-pointing “unicorn horn” spine just behind its head – a sexually dimorphic structure formed from a highly modified dorsal fin, found only on mature males.

The spine’s function is unknown for certain, but it may have been a sort of clasper involved in courtship and mating, since one fossil seems to preserve a female in the act of biting onto it. Some of its close relatives like Damocles and Stethacanthus also had similarly weird dorsal fins, so whatever these fish were actually doing with these structures it must have been a fairly successful strategy.

Falcatus lived out in the open ocean, with proportionally big eyes giving it good vision in deep dark water, and its large symmetrical tail fin suggests it was a fast maneuverable swimmer that actively chased after small prey. Numerous fossils have been found together, which may also indicate schooling behavior.

Although definite fossils of falcatids are only known from the Carboniferous, recently there’s been some possible evidence of them surviving for much much longer. A few isolated fossil teeth from Europe suggest that some of these fish may have persisted for at least another 180 million years into the Early Cretaceous, living in isolated deep water refugia environments in a similar situation to the modern coelacanth – making them fossils of what would have been “living fossils” at the time!

Almost-Living Fossils Month #27 – Those Giant Sharks

For the final entry this month, let’s look at a particularly famous lineage: the megatooth sharks.

More formally known as the otodontids, the megatooths were a group of sharks that first appeared in the Early Cretaceous, about 115 million years ago. They were a branch of the mackerel shark lineage – making them evolutionary cousins to a variety of modern species like the great white shark, basking shark, and goblin shark – and had a near-worldwide distribution, with fossils known from every continent except Antarctica.

Early otodontids in the Cretaceous were usually small-to-medium sized, around 2-3m long (6′6″-9′10″), but after surviving through the end-Cretaceous extinction they took over the marine apex predator niches left vacant by the vanished mosasaurs and plesiosaurs and began to get very big. Species of Otodus in the Paleocene and early Eocene may have reached sizes of at least 9m long (29′6″), twice the size of an average great white.

Their teeth gradually became proportionally larger in their jaws, losing their side cusplets and taking on a chunky triangular shape with finely serrated edges. This gave them an incredibly powerful bite force, and they would have probably fed on pretty much any other large marine vertebrates they could catch, including bony fish, smaller sharks, turtles, and early penguins – and then when marine mammals like early whales and sirenians appeared in the mid-Eocene, they adapted to this new food source too.

By the Late Eocene (~35 mya) the Otudus lineage was still developing even chunkier and more serrated teeth, and by the Early Oligocene (~28 mya) Otodus chubutensis reached even larger sizes rivaling the modern whale shark at around 12m long (39′4″).

But the most well-known member of the group evolved just a few million years later in the Early Miocene (~23 mya) – the absolutely enormous “megalodon”.

There’s some debate about what genus name megalodon should be assigned to – at the moment its formal name is usually considered to be Otodus megalodon, but some paleontologists place it in Carcharocles or Megaselachus or Procarcharodon instead. Whatever you want to call it, it was a ridiculously big shark – size estimates range up to about 18m (59′), which would make it potentially the largest fish to have ever lived.

Since these huge sharks are all known mostly from just their fossilized teeth (and occasionally a few exceptionally preserved cartilaginous vertebrae), it’s hard to tell what they actually would have looked like in life. Megalodon is frequently depicted as simply a scaled-up great white, but it’s unclear how accurate that really is – it may have convergently resembled a giant great white due to their similar predatory habits, or it could have had a build more like the larger basking shark or whale shark.

A preserved megalodon skull has actually been found, but no studies of it have been published yet. It might give us some important clues about the head shape of this giant shark, but until there’s some official information all we can do is continue to speculate.

Megalodon was a highly successful species, living all around the world in warm and temperate ocean waters for around 20 million years. Its teeth have been found in association with the bones of many different smaller whale species, suggesting it frequently ate marine mammals, and the patterns of the bite marks indicate it probably used different hunting strategies than modern great whites. Some whales seem to have been heavily rammed and then had their ribcages bitten into, targeting their hearts and lungs, while others had their flippers ripped off to immobilize them.

During the Pliocene (~5-2.6 mya), however, megalodon began to struggle. Cooling oceans and changes in the abundance of the marine mammals it ate began to restrict its available prey. Baleen whales started to grow too large for it to effectively hunt, since it preferred to target smaller species, and they also shifted their ranges towards the cold polar waters that megalodon didn’t seem to be able to survive in. In addition, dropping sea levels may have destroyed most of its shallow warm-water nursery sites, making it harder for newborn young to survive into adulthood.

By the end of the Pliocene, somewhere between 3.6 and 2.6 million years ago, megalodon went completely extinct. Despite some very pseudoscientific claims, there’s definitely no living “Meg” out there anymore – if there was, we’d be constantly finding freshly-shed teeth and whales with giant bite marks on their bodies!

Almost-Living Fossils Month #21 – More Sharks

First appearing in the Early Permian, about 290 million years ago, the synechodontiformes were an early branch of the neoselachian lineage of cartilaginous fish, slightly closer related to living sharks and rays than to the hybodontiformes featured earlier this month.

They originated in the Paleo-Tethys Ocean and survived through the devastating end-Permian “Great Dying” mass extinction (~252 mya), then went on to quickly spread around most of the world and also survive through the Triassic-Jurassic extinction (~201 mya). During the Jurassic and Cretaceous they became quite common and diverse, taking over some of the niches previously occupied by the hybodontiformes and adapting to a range of marine environments from shallow coastal waters to open ocean.

Most known synechodontiform fossil remains are just their teeth, since cartilage skeletons don’t preserve very often, but there are a few rare body fossils that show they were varied in appearance with differing arrangements of dorsal fins and spines.

Paraorthacodus jurensis here was one of the species known from the Late Jurassic of Germany (~155-150 mya). Reaching lengths of at least 1.3m (4′2″), it had only one dorsal fin far back on its body, along with large pectoral fins and a low asymmetrical tail that gave it a superficial resemblance to the modern sixgill sharks.

Its teeth were close in shape to those of sand tiger sharks, and it may have had a similar lifestyle opportunistically hunting prey just above the sea floor in the waters around the continental shelf and slope. Remains of a chimaera in the mouth and gut contents of a couple of Paraorthacodus jurensis fossils suggest that smaller cartilaginous fish were fairly common elements of its diet.

A few synechodontiformes managed to survive the end-Cretaceous extinction 66 million years ago – but while the ancestors of moderns sharks thrived in the Cenozoic, the synechodontiformes never recovered anything close to their Mesozoic levels of success and instead began to decline.

The last known synechodontiforme was a currently-unnamed member of the Paraorthacodus genus, hanging on in the waters around Antarctica in the Late Eocene (~37 mya). If they managed to survive past that time it probably wasn’t for very much longer, and it’s likely they finally disappeared during another extinction event at the Eocene-Oligocene boundary.

Almost-Living Fossils Month #09 – Horned Sharks

All modern species of sharks and rays are part of a single lineage of cartilaginous fish known as neoselachians, and the closest evolutionary “cousins” to all of them were the hybodontiformes.

First appearing way back during the Devonian, about 400 million years ago, these early sharks were widespread around the world and incredibly successful as a group, living in both marine and freshwater environments.

Although due to their cartilaginous skeletons hybodontiformes are mostly known from fossilized teeth, there are still some complete specimens known that show us their overall body shape. They had two dorsal fins, each with a long spine in front, and an asymmetrically-shaped tail. Some of them also had small horn-like spines on their heads – this seems to be a sexually dimorphic trait, since the ones with “horns” also have claspers which show they were males – and they generally had powerful jaws with teeth specialized for crushing.

They were probably fairly slow swimmers most of the time, but would have still been capable of occasional bursts of higher speed, and various species were adapted to a wide range of food sources. Some had wider flatter teeth for cracking open hard-shelled seafloor invertebrates, and others were more opportunistic hunters that would have crunched on pretty much anything they could fit in their mouths.

Hybodontiformes were the dominant type of shark around the world before the end-Permian “Great Dying” mass extinction (~252 mya), and then went on to recover and flourish once again up until the mid-Jurassic.

Hybodus hauffianus was one of the Early Jurassic species, living around 183 million years ago in Europe. About 2m long (6′6″), it had two different types of teeth in its mouth – sharper ones in the front and flatter ones in the back – suggesting it was a generalist predator eating whatever it could catch. We do know its diet at least included the fast-swimming squid-like belemnites, since some fossils preserve clusters of their internal hard skeletons in Hybodus’ stomach region.

Towards the end of the Jurassic neoselachians began to diversify and take over most of the marine shark ecological niches, and the hybodontiformes became increasingly restricted to freshwater. During the Cretaceous they continued to do fairly well in those environments, but most of them still disappeared around the time of the end-Cretaceous extinction (~66 mya). Since most other sharks weren’t actually particularly affected by the extinction event, it’s not clear whether the hybodontiformes were more vulnerable for some reason or whether it was the ongoing competition from neoselachians that drove the majority of them extinct at that time.

Still, a few of them did seem to make it through to the Cenozoic, although they were absent from the fossil record until the Miocene. Freshwater deposits in Sri Lanka have evidence of a late-surviving member of the group living perhaps as recently as 5 million years ago – so they would have only gone completely extinct sometime after that, and we probably missed seeing them alive by only a few million years at most.


Edestus, a holocephalan fish from the Late Carboniferous (~315-299 mya) of Eurasia and North America. A relative of the “spiral-saw-mouthed” Helicoprion, it continuously grew a single row of teeth in each jaw, creating an arrangement often compared to a pair of pinking shears.

Multiple species of this genus have been named, with varying degrees of tooth bracket curvature, and the largest may have had body sizes similar to modern white sharks – about 6m long (19′8″).

Since Edestus is only known from fossilized tooth brackets, how exactly its jaws worked and what it ate with them is still a mystery. Many reconstructions end up either goofy or horrifying as a result, and so I’ve attempted to make this one look a bit more “normal”. And capable of closing its own mouth.

Edit: This reconstruction was based more on chimaeras than on other eugeneodontids, and is therefore probably very inaccurate. When I originally did this image I wasn’t aware body outlines were known for the group. For a much more accurate version see my 2020 version in the “Weird Heads” series.