actinopterygii – Nix Illustration

Luoxiongichthys

Luoxiongichthys hyperdorsalis was an early ginglymodian ray-finned fish, related to the ancestors of modern gars, living during the Middle Triassic (~244 million years ago) in coastal tropical marine waters covering what is now southwestern China.

About 15cm long (~6″), it had a large hump on its back shaped like a backwards shark fin. Its deep body was wider at the base, giving it a triangular shape when viewed from the front similar to some modern tropical fish – and suggesting it may have had a similar sort of ecology as a slow but highly maneuverable swimmer feeding on small seabed invertebrates.

Dibango

In the late 18^th century a strange fish fossil from the Monte Bolca deposits in northern Italy was described and named as Pegasus volans. This name had actually already been assigned to the living longtail seamoth (today known as Pegasus volitans), but despite this the fossil continued to be referred to as “Pegasus volans” for well over 200 years.

Now, finally, it’s been redescribed and given a proper genus name of its own: Dibango volans.

Living during the early Eocene, around 50-48 million years ago, in what was then a warm shallow reef in the western Tethys Ocean, Dibango was probably around 7-10cm long (~3–4″). It had a long flag-like first ray of its dorsal fin, a very reduced and compact abdominal region, an extremely elongated pelvic bone that appears to have supported an exterilium (external gut), long pelvic fins, and a long slender tail.

This bizarre combination of features is often seen in fish larvae, but Dibango’s level of skeletal development shows it was fully grown – suggesting it was actually an unusually neotenic fish, retaining its larval anatomy all the way into adulthood.

This also makes it very difficult to figure out what kind of fish it actually was, with the current best guess being “some sort of percomorph“.

Dorypterus

Dorypterus hoffmanni was a stem–actinopterygian fish that lived during the late Permian, around 259-254 million years ago, in shallow warm lagoons covering what is now northwestern Europe.

About 13cm long (~5″), it had a tall narrow disc-shaped body convergently similar to modern reef fish, and it was mostly scaleless with only a few scales on its underside, below its pectoral fins, and along the top of its tail. It also appears to have been toothless, and probably used its large scissor-like jaws to snip off mouthfuls of soft food such as algae.

But its most distinctive feature was its highly elongated pennant-like dorsal fin, which may be an example of sexual dimorphism – fossils of short-finned individuals have also been found, and although they were originally named as a separate species (Dorypterus althausi) they probably actually represent female D. hoffmani.

Hemingwaya

Hemingwaya sarissa here was one of the earliest known billfish, related to modern sailfish and marlin. Living during the late Paleocene, about 58 million years ago, it inhabited the area around what is now Turkmenistan, in the warm shallow waters of the western Tethys Sea that covered much of central Asia at the time.

It was rather small compared to its modern relatives, just 30-40cm in length (~1′-1’4″), with a long streamlined body armored with six rows of scutes. Its slender snout was lined with tiny teeth, and both its first dorsal fin and first anal fin were tall and elongated.

It probably wasn’t a very active swimmer, instead hovering near the surface and catching smaller prey with quick bursts of speed.

Rhombichthys

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.

Strange Symmetries #18: Flat Fish Friday

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.

Progressive eye migration in a developing Summer Flounder, *Paralicthys dentatus*.
From Helfman et al (2009). The diversity of fishes. 2nd ed., Wiley-Blackwell.

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.

Serrasalmimus

The pycnodonts were a diverse group of ray-finned fish that were found in shallow coastal waters from the late Triassic to the late Eocene (~215-37 million years ago). They usually had deep but very narrow body shapes with a disc-like appearance, convergently similar to modern reef fish like marine angelfish or butterflyfish – but some looked much weirder, with elaborate horns and spines, lon g snouts, or vertically-stretched bodies.

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″).

It had sharp flesh-cutting teeth similar to those of modern piranha, but with a surprising evolutionary twist. Unlike any other known ray-finned fish, Serrasalmimus‘ teeth were true shearing carnassials anchored into bony sockets, with new replacement teeth forming directly below each current tooth – a very specific arrangement of features previously only known in mammals.

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.

It Came From The Wastebasket #18: Lots And Lots of Lepidotes

Lepidotes was a ray-finned fish that lived during the Mesozoic, found in both freshwater and shallow marine environments. It was a member of the ginglymodian fish lineage, related to modern gars, and along with distinctive thick enamelled scales on its body it was also one of the earliest types of fish able to protrude its jaws for suction feeding.

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.

Lepidotes gigas

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 species Lepidotes 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.

Calamostoma

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.

Calamostoma lesiniforme is one of the oldest of these, dating to the early Eocene around 50-48 million years ago. Known from the Monte Bolca fossil beds in northern Italy, it lived in a warm shallow reef environment during a time when that region of Europe was covered by the western Tethys Ocean.

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.

Tarrasius

The spinal column in tetrapods is made up of five different regions of distinctly-shaped vertebrae: cervical (neck), thoracic (upper back attached to ribs), lumbar (lower back without ribs), sacral (pelvic) and caudal (tail).

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.

And it also had some very unusual vertebrae for a non-tetrapod fish.

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.