Tag: cartilaginous fish

Spectember/Spectober 2024 #06: Death Ray

Yeah, I’m keeping this stuff going again for another round of Spectober!

An anonymous request asked for a “big macropredatory ray”:

A shaded sketch of the head of a speculative descendant of pelagic stingrays. Its a large ray with triangular wing-like fins, a slightly shark-like pointed snout, protrusible sharp-toothed jaws, and a vaguely skull-like pale marking on its underside.

Speirobatis thanasima is a large ray found in open oceanic waters, reaching sizes comparable to the modern giant manta ray at around 3m in length (~10′) with a wingspan of over 5m (~16’5″).

But despite its body shape closely resembling that of mobulids or myliobatids, its closest present-day relative is actually the pelagic stingray. Already being active hunters with mouths full of sharp pointed teeth, these ancestral rays gradually evolved bigger body sizes and began occupying an apex predator niche similar to that of large sharks and toothed whales.

Speirobatis is a strong swimmer, using flapping motions of its triangular wing-like pectoral fins to travel at high speed and make acrobatic jumps out of the water. Highly intelligent and social, it lives in family groups that hunt cooperatively – encircling and herding schools of fish tightly into bait balls, taking turns stunning prey with slaps of their fins, and then grabbing incapacitated individuals with snaps of their protrusible jaws.

Groups will also tackle larger prey such as marine mammals and sharks, harassing and chasing targets to exhaustion while taking quick opportunistic bites out of them.

Lessiniabatis

Lessiniabatis aenigmatica was a rather strange stingray.

It lived around 50-48 million years ago during the early Eocene, in a shallow warm sea covering what is now Italy, with its three known fossil specimens all coming from the fish-rich Monte Bolca fossil beds.

About 60cm long (~2′), it had a round pancake-like body similar to many modern seafloor-dwelling stingrays – but uniquely it was also almost tailless, with only a tiny, slender, stingless tail.

It wasn’t a particularly strong swimmer, instead probably spending most of its time buried in the muddy seafloor sediment. When on the move it likely swam along just above the surface of the seafloor using undulations of its fins, foraging for smaller bottom-dwelling animals like worms, molluscs, crustaceans, and fish.

Maghriboselache

Maghriboselache mohamezanei was a cartilaginous fish from the late Devonian Period, about 369 million years ago, living in the shallow marine waters that covered what is now the Anti-Atlas mountain range of Morocco in northwest Africa.

Up to around 2.5m long (~8′), it’s known from several exceptionally well-preserved and near-complete skeletons.

It had a streamlined body with large pectoral fins, small pelvic fins, and a strongly keeled crescent-shaped tail fin. And although it was superficially shark-like in appearance, it was actually part of a lineage known as cladoselachids, which were much closer related to modern chimaeras than to sharks.

It’s unclear if Maghriboselache had two dorsal fins like its close relative Cladoselache, but some specimens preserve evidence of a chunky spine where the front dorsal fin would have been. Others show no sign of a front dorsal fin or spine at all, suggesting there may have been some sexual dimorphism going on in this species, with males having a spine (and possibly also an associated front dorsal fin) and females only having a rear dorsal fin.

But the most unusual feature of Maghriboselache was its nose.

It had a very broad snout with large and unusually widely-spaced nostrils, which would have given it the ability to “smell in stereo” and determine the direction of scents carried through the water much more precisely – making it the earliest known example of that sort of sensory specialization.

Strange Symmetries #15: Serrated Saw-Snoots

Long flattened snouts lined with pointy tooth-like denticles have convergently evolved at least three separate times in cartilaginous fish: in modern sawsharks and sawfish, and in the extinct sawskates.

This repeated “pristification” suggests that saws are just incredibly useful and relatively “easy to evolve” structures for these types of fish, being both highly sensitive to bioelectric fields and able to physically slash and stab to kill prey.

Onchopristis numida was a sawskate known from what is now Northern and Western Africa during the mid-Cretaceous, about 95 million years ago. Up to about 3m long (~10′), it lived in both saltwater and freshwater, and was probably a bottom-dwelling ambush predator similar to modern angelsharks.

Whenever a denticle was lost from its saw, a larger one would grow to replace it, and over the life of an Onchopristis this resulted in an increasingly extreme amount of saw asymmetry.

Modern pristified fish also have rather asymmetrical saws. Sawfish are commonly born with a different number of denticles on each side, while sawsharks add extra denticles of varying sizes as they age, with the ongoing replacement of lost denticles resulting in more uneven arrangements over their course of their lives.

It’s not clear if the asymmetry gives any sort of advantage to these fish – but if nothing else it probably doesn’t cause them any disadvantage, so there’s no evolutionary pressure to stay more symmetrical.

Ornithoprion

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.

1890s-2000s

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.

2020s

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

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.

Squaloraja

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 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!