Ellimmichthyiformes were a group of ray-finned fish known from the early Cretaceous to the mid-Oligocene, about 140-30 million years ago. For much of that time they were quite widespread, found in various marine, estuarine, and freshwater environments across Africa, Eurasia, and the Americas.
Closely related to modern clupeiformes (herrings, sardines, and anchovies), and characterized by two rows of bony scutes – one in front of the dorsal fin and the other along the belly – they’re also known by the nickname “double‐armored herrings”.
Rhombichthys intoccabilis was a rather unusual-looking ellimmichthyiform from the mid-Cretaceous, around 95 million years ago. Living in shallow reef and lagoon waters covering what is now the West Bank in the Middle East, it was about 20cm long (~8″) and had a tall narrow dorsal fin along with incredibly elongated belly scutes that gave its body a rhombus-like profile.
Juveniles of this species seem to have lacked the extended belly scutes, instead having a much more rounded body shape. This may indicate that adults and juveniles occupied very different ecological roles, or that the distinctive scutes might have been a secondary sexual characteristic involved in displaying for courtship and reproduction.
Modern flatfish are characterized by their highly asymmetrical skulls, with both eyes positioned on just one side of their head. They aren’t actually born this way, but instead they undergo “eye migration” as juveniles, twisting up their skulls to bring one eye across the top of the head.
This bizarre arrangement is the result of flatfish adapting to life laying flat on the seafloor, but instead of slowly widening and flattening themselves out they took an evolutionary “shortcut” by simply tipping their tall narrow bodies over onto one side. Initially this would have left one of their eyes unusable, but random mutations causing slightly asymmetrical skulls would have rapidly become highly advantageous to the earliest members of this lineage – and over time they just got wonkier and wonkier.
We’ve even found fossils of early flatfish in the “halfway there” stage of their evolution!
Amphistium paradoxum lived in what is now northern Italy during the Eocene, around 50-48 million years ago. About 20cm long (~8″), it had one eye partially migrated towards the top of its head, but not all the way around yet, showing a transitional state between its bilaterally symmetric ancestors and its more twisted-skulled modern relatives.
Unlike most modern flatfish Amphistium came in both “right-eyed” and “left-eyed” forms in equal numbers, suggesting that a genetic preference for a specific side also hadn’t developed yet.
Most of them also had jaws full of round flat teeth used to crush hard-shelled prey, but some may instead have been herbivorous grazers similar to parrotfish.
And a couple of lineages even became carnivores.
Serrasalmimus secans lived in what is now Morocco during the late Paleocene, about 59 million years ago. Although only known from its jaws, the size of the fossil material suggests it was fairly large for a pycnodont, possibly around 80cm long (~2’8″).
This is an especially remarkable example of convergent evolution because on land placental carnivorans were developing their own carnassials at the same time, just a few million years after the K-Pg mass extinction. Both mammals and pycnodonts were simultaneously taking advantage of the vacant predatory roles in their respective ecosystems, and ended up with incredibly similar tooth adaptations as a result.
First discovered in Jurassic-aged fossil deposits in Europe in the 1830s, this genus was quickly turned into a notorious wastebasket taxon for any similar-looking fossil fish. Over time dozens of different Lepidotes species were named, many of them rather dubious, from locations all around the world and spanning a time period of over 100 million years.
But despite Lepidotes being a wastebasket for almost two centuries, it wasn’t until surprisingly recently that any real progress began to be made on cleaning it all up.
In the early 2010s a large-scale review of ginglymodian relationships found that many “Lepidotes” species were either invalid or polyphyletic, belonging in completely different genera or families. True Lepidotes were restricted down to just the original type speciesLepidotes gigas and a few of its closest relatives, all from the early Jurassic of Europe, while some other forms were moved into the newer genera Scheenstia and Callipurbeckia. Since then some other “Lepidotes” have also been reclassified, creating new names like Macrosemimimus, Occitanichthys, and Quasimodichthys.
There’s still work needing to be done on untangling all these Lepidotes-like fish – Scheenstia might actually now represent several different lineages, for example – but at least Lepidotes itself is now in a much better situation than it was just a couple of decades ago.
Ghost pipefishes are close relatives of pipefish and seahorses, and today are represented by six different species found in shallow tropical waters of the Indo-Pacific. But while this lineage is estimated to have originated around 70 million years ago in the Late Cretaceous, their fossil record is very sparse – only three fossil representatives are currently known from the entire Cenozoic.
Up to about 9cm long (3.5″), it was already very similar in appearance to modern ghost pipefishes, with a long tubular snout, star-shaped bony plates in its skin, two dorsal fins, and fairly large pelvic fins that formed an egg-brooding pouch in females. It probably had the same sort of lifestyle as its modern relatives, floating pointing downwards and camouflaging itself among seagrasses, algae, and corals.
One specimen preserves a small amount of color patterning, showing hints of dark banding on the pelvic and tail fins. But since modern ghost pipefish can change their coloration to better mimic their surroundings, it’s unclear whether these markings were common to all Calamostoma or were just part of this particular individual’s camouflage.
Non-tetrapod vertebrates like fish have spines that are much less differentiated, with just body and tail segments. So for a long time multiple distinct spine regions were thought to be something completely unique to tetrapods – a specialization developed early in their evolutionary history that served to better support their weight when moving around on land.
But one little fossil fish makes this idea… problematic.
Tarrasius problematicus lived during the early Carboniferous, about 345 million years ago, in shallow tropical marine waters in what is now southern Scotland. Around 9cm long (3.5″), it was an early type of ray-finned fish with a scaleless body and a long scaled eel-like tail with a single continuous dorsal fin.
Its spine shows five different regions all corresponding to those seen in tetrapods, despite it not being closely related to them. But unlike early tetrapods Tarrasius was no land-walker, with its lack of hind fins indicating it was instead a streamlined fully aquatic fast swimmer.
It’s not clear why this fish developed such an incredibly convergent backbone, but it may have helped to stiffen its body so its more flexible tail could provide more efficient thrust, swimming like a modern tadpole.
It also suggests that a pre-existing genetic basis for regionalization – specific patterns of Hox gene expression – was actually an ancestral trait for all bony fish or jawed vertebrates. Tarrasius and early tetrapods may have just happened to specialize their spines in the same way for different purposes, with only the tetrapods going on to see long-term evolutionary success with it.
For the final entry in this series, let’s take a look at a modern weird-headed species – and where better to find some of the strangest and most unique-looking animals alive today than the deep sea?
Malacosteus, also known as the stoplight loosejaw, is a 25cm long (10″) genus of dragonfish found at depths of over 500m (1640′) in oceans all around the world, with the exception of the Mediterranean and polar waters. Two different species are currently recognized, with Malacosteus niger here known from just below the Arctic Circle down to the southern reaches of the subtropics, and Malacosteus australis ranging from there to around 45°S, and up towards the equator in the Indian Ocean.
And there’s a lot to unpack here with the anatomy of this one.
First of all, there’s the fact that its entire head can hinge away from its body, gaping enormous jaws with long fang-like teeth.
The bottom of its lower jaw has no skin membrane connecting the two sides, attached to the rest of its bizarre head only by the hinges and a single exposed muscle, reducing water resistance so it can shoot its trap-jaws out extra fast to snare prey.
Once it catches something it retracts its head, and several sets of pharyngeal teeth further back grab hold of its prey and direct it down its throat.
(Let me remind you that this isn’t an early April Fools joke. This thing is completely real.)
In addition to all that anatomical weirdness, it’s also one of the only deep-sea fish that can both see and produce red-colored light. Most creatures living at that depth have lost the ability to see red since that frequency doesn’t penetrate so far down through water, but the stoplight loosejaw has evolved to take advantage of that by using bioluminescent red light as its own personal night vision goggles.
Using large red photophores under each eye, it can shine a spotlight out ahead of itself and see other deep-sea animals all clearly lit up, while remaining completely invisible to both them and any nearby larger predators. It’s able to perceive the color red thanks to a pigment in its eyes modified from chlorophyll, a visual setup unique to this fish and not known from any other vertebrate.
It also has a smaller green photophore further down on its head – inspiring its common name thanks to the resemblance to traffic lights – and many smaller blue and white ones over its head and body.
So, with its highly specialized jaws and ability to see things other deep-sea animals can’t, the stoplight loosejaw must be hunting something pretty impressive, right?
And as it turns out, it eats… plankton.
The vast majority of its diet appears to be copepods, small zooplanktoic crustaceans that are incredibly common in the waters the loosejaw inhabits. It may simply be “snacking” on such a convenient food source in-between rare encounters with larger prey – but it may also be getting the chlorophyll-based pigment needed for its night vision from eating them.
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.
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.
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.
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.