Tag: fish

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.

Holopterygius

Coelacanths are represented today by just two surviving species, one in East Africa and one in Indonesia, both very similar in appearance and ecology to each other.

For a long time their lineage was thought to be all “living fossils“, retaining the same basic body plan for the last 400 million years – but more recent discoveries have revealed that these fish were actually much more diverse over the course of their evolutionary history.

Holopterygius nudus was a fairly early member of the group, living during the mid-Devonian about 385 million years ago. The only known fossil specimen was discovered in Germany in the 1970s, but it was originally thought to be a different type of fish entirely and wasn’t identified as being a coelacanth until over 30 years later.

And compared to its living relatives it was tiny, just 7cm long (2.75″), with a distinctive tapering eel-like tail. Its convergent close resemblance to modern cusk-eels suggest it may have occupied a similar ecological niche, living near the sea floor and hiding in tight spaces like crevices and burrows.

Rostropycnodus

The extinct pycnodonts were a group of mostly circular-shaped fish, convergently similar to modern reef fish like marine angelfish or butterflyfish – but some of them developed much much weirder appearances.

Rostropycnodus gayeti here was one of the especially odd-looking forms, known from the mid-Cretaceous of Lebanon about 100-95 million years ago.

It had an elongated snout with the upper jaw longer than the lower, a pointed spiky horn on its forehead, and a massive pectoral region that bulged out at the front of its body. Meanwhile its pectoral fins were modified into big immobile spines, and its pelvic fins were highly reduced and positioned beneath another set of large spines.

And it was tiny, only about 5.5cm long ~(2″).

It would have been a slow swimmer, relying on its spikiness to deter larger predators, and it’s currently unclear what it ate with its unusual spiny snout. Many other pycnodonts had mouths full of round crushing teeth, but Rostropycnodus’ jaws seem to have been mostly toothless – so perhaps it used its snout to probe around in cracks or sediment for small soft-bodied invertebrates.

Falcatus

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.

Almost-Living Fossils Month #06 – Circle Fish

Pycnodonts were a group of fish that originated in the western Tethys Sea during the Late Triassic (~215 mya), and later spread to most of the rest of the world with the exception of Antarctica and Australia. Ranging in size from a few centimeters to around 2m (6′6″), they had deep vertically-flattened bodies and almost circular silhouettes. Although they somewhat resembled modern marine angelfish or butterflyfish, they weren’t actually very closely related, instead being part of a much older branch of neopterygian fish.

They inhabited a range of shallow coastal waters from marine to freshwater environments, and most of them had jaws full of round flat teeth used to crush hard-shelled prey – but some may have been herbivorous grazers similar to parrotfish, and one lineage even became sharp-toothed piranha-like predators.

Some also developed quite elaborate appearances, such as Hensodon spinosus here. Living during the peak of the pycnodonts’ diversity in the mid-to-late Cretaceous, its fossils are known from Lebanon and date to about 100-95 million years ago.

It was only about 7cm long (2.75″) but it was bristling with various small spines and large “horns”, with different specimens showing two distinct arrangements. One type had double-pronged forward-facing horns, while the other had two horns one after the other – this may be evidence of sexual dimorphism, with the “bull horned” form thought to be male and the “rhino horned” form thought to be female.

Hensodon was also probably stripy in life, since one fossil preserves faint evidence of a light-and-dark stripe pattern on its dorsal fin.

Only a few pycnodonts survived into the Cenozoic, and their last appearance in the fossil record was in the mid-Eocene (~40 mya). Since this was at about the same time that more modern types of reef fish began to evolve, it’s likely that a combination of new competition and changing climate conditions resulted in the last pycnodonts going extinct by the end of the Eocene around 33 million years ago.

Almost-Living Fossils Month #02 – The Saber-Toothed “Herring”

First appearing in the mid-Cretaceous, about 113 million years ago, Enchodus was a small-to-medium-sized genus of predatory fish. Different species ranged from a few centimeters to up to 1.5m in length (4′11″), with Enchodus gladiolus here being an averaged-sized example at about 60cmlong (2′).

The most distinctive feature of these fish were the enlarged fang-like teeth in both their upper and lower jaws, over 6cm (2.4″) long in the largest individuals, which may have been a specialization for feeding on soft-bodied cephalopods.

Despite having been nicknamed “saber-toothed herrings”, they weren’t actually closely related to herrings at all, instead being part of the aulopiformes – a group also containing modern lizardfish, lancetfish, and a different type of sabertooth fish.

Fossils of various Enchodus species have been found all over the world, and they seem to have been very common and important members of ancient marine ecosytems, occupying mid-level carnivore niches and in turn being eaten by other predators. Their remains have been identified within the preserved stomach contents of marine reptiles such as plesiosaurs and mosasaurs, as well as sharks and hesperornithean birds.

These toothy fish survived through the end-Cretaceous mass extinction and continued their success for almost 30 million years into the Cenozoic, with the last known fossils dating to just 37 million years ago in the Late Eocene. They probably didn’t survive much longer beyond that date, since there was an extinction event at the Eocene-Oligocene boundary (~34 mya), a period of sudden cooling that affected many marine animals at the time.

Rebellatrix

Coelacanths are famous for being “living fossils”, completely disappearing from the fossil record at the end of the Cretaceous but then being rediscovered alive just 80 years ago. But although they’re often thought to have physically changed very little over the last 300 million years or so, more recent discoveries are starting to show that coelacanth body forms and lifestyles were actually more varied in the distant past.

Meet the wonderfully-named Rebellatrix divaricerca, from the Early Triassic of British Columbia, Canada (~251-247 mya). Measuring around 1.3m long (4′3″), its body shape and large symmetrical forked tail suggest it was adapted for fast swimming. Unlike its slow-moving deep-water modern relatives this coelacanth was a speedy oceanic active predator, convergently similar to tuna or some sharks.

Since it lived in the immediate wake of the end-Permian “Great Dying” mass extinction, Rebellatrix may have rapidly evolved from more standard-looking coelacanths to take advantage of a suddenly vacant ecological niche – or it might be part of a more extensive unusual lineage whose other members simply haven’t been discovered yet.