Weird Heads Month #31: What Even Is This Fish

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.

Diagram showing how the stoplight loosejaw's jaw parts articulate.
From Kenaley, C. P. (2012). Exploring feeding behaviour in deep-sea dragonfishes (Teleostei: Stomiidae): jaw biomechanics and functional significance of a loosejaw. Biological Journal of the Linnean Society, 106(1), 224-240. doi.org/10.1111/j.1095-8312.2012.01854.x

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.

Weird Heads Month #23: Dome-Headed Claw-Horses

Much like Platybelodon from a few entries back, chalicotheres look like a fictional creature design rather than something that actually existed.

These animals were odd-toed ungulates related to modern horses, tapirs, and rhinos, who ranged across Africa, Eurasia, and North America for a large chunk of the Cenozoic. Instead of hooves they had large claws on their feet, and they appear to have occupied the same sort of ecological niche as ground sloths or therizinosaurs – sitting or rearing up on their hind legs to browse on high vegetation, using the hook-like claws on their forelimbs to pull down and strip branches.

There were two different lineages of chalicotheres which developed along slightly different evolutionary paths: the knuckle-walking gorilla-like chalicotheriines and the more goat-like schizotheriines.

Tylocephalonyx skinneri here was one of the latter group, known from the Miocene of North America about 16-13 million years ago. Standing about 2m tall at the shoulder (6’6″), it had the same sort of chunky body as other schizotheriines and walked around with its large front claws held up to keep them raised away from the ground.

But there was also an unusual feature on its otherwise rather horse-like head – a large bony dome on top of its skull, like a mammalian version of a pachycephalosaur.

It probably used its dome in the same way as the dinosaurs it convergently resembled, headbutting or flankbutting in fights with each other.

Weird Heads Month #20: Shovel-Tuskers

With their odd-looking skulls, long tusks, and their noses and upper lips modified into tentacle-arm-like trunks, modern elephants are the sort of animal that would seem completely unbelievable if we only had fossils of them.

But not nearly as strange as some of their ancient relatives.

Platybelodon is probably the most famously weird member of the proboscideans (the group that contains both modern elephants and their extinct cousins), looking like some sort of deliberately outrageous speculative creature design.

Living during the mid Miocene, around 15-4 million years ago, several different species of Platybelodon ranged across Africa, Europe, Asia, and North America, with Platybelodon grangeri here known from abundant fossils in Asia.

These strange-looking proboscideans stood around 2.2m tall (7’3″) and had fairly standard elephant-like bodies, but also heads with bizarre-looking elongated lower jaws that ended in a wide flat shovel-like shape tipped by two flat tusks, leading to their nickname of “shovel-tuskers”.

It was originally interpreted as a swamp-dwelling animal using its weird jaw to scoop up soft aquatic vegetation, with a fairly short flat trunk. But more recent studies of the wear patterns on its teeth suggest it actually used them more like a scythe than a shovel, cutting through tough grasses and branches – a feeding style that would also require it to have a much more modern-elephant-like trunk, using it to hold on to plants while it was sawing through them.

Weird Heads Month #10: Permian Crowns

The tiny-headed Cotylorhynchus we saw earlier in this series wasn’t the only synapsid with a weird head.

A little more closely related to modern mammals, the dinocephalians were a a diverse group that were found across Pangaea during the middle of the Permian period. Many of them had thickened skulls that may have been used for headbutting each other, and some also developed bony horn-like projections around their faces.

And Estemmenosuchus mirabilis here was particularly elaborately ornamented, earning it a name meaning “wondrous crowned crocodile”. It lived in the Perm region of Russia during the mid Permian, about 268-265 million years ago, and was one of the largest dinocephalians, reaching at least 3m long (9’10”).

It had two big antler-like structures on its head, two wide cheek flanges, and a small nose horn, almost looking like the synapsid version of a ceratopsid dinosaur – and with its big bulky body, fairly erect-legged posture, and herbivorous-or-omnivorous diet it may have been a fairly close ecological equivalent to them, too.

But it’s also possible it was semi-aquatic, and it certainly does have a very hippo-like appearance when reconstructed with a decent amount of soft tissue.

One specimen of Estemmenosuchus even preserved skin impressions around its face, which were described in Russian in the early 1980s. They show scaleless glandular skin with a slightly bumpy texture, similar to that of hairless mammals or some amphibians. Since it occupied a point in the synapsid family tree close to where hair may have originated (somewhere in the Permian therapsids), it’s not clear if it was entirely hairless or if it had just secondarily lost some of it.

Weird Heads Month #09: Butterfly Faces

The nose-forks and head-crests we saw last time weren’t the only unusual headgear in ancient ruminants.

The giraffoids are represented today by just pronghorns, giraffes, and okapi, but in the past they were much more diverse, modifying their prongs and ossicones into multiple sets of horns, or into deer-like and moose-like antler shapes.  

And Prolibytherium was probably the most striking of the lot.

Two different species have been identified, with Prolibytherium magnieri here living in North Africa during the early-to-mid Miocene, about 17-16 million years ago. Its exact evolutionary relationships are uncertain but it was probably part of a group called climacoceratids, deer-like giraffoids which often had thorny branching ossicones that resembled antlers.

It stood around 1.2m tall at the shoulder (~4′), and exhibited dramatic sexual dimorphism – females had slender forked horn-like ossicones, while those of the males flared out into large wide flat shapes that resembled butterfly wings.

Heavy reinforcement in the bones of the back of the males’ skulls helped to support all the extra weight of those huge ossicones, and if they actually used the structures to fight with each other then this may have also provided some protection or shock absorption.

Weird Heads Month #01

It’s been a whole four years since Weird Backs Month, so we’re long overdue for a companion series:

Weird Heads Month!

Ever since heads first evolved as a defined body part, over 500 million years ago, evolution has been experimenting with them. There are many modern examples of animals that have modified parts of their heads and faces in a variety of strange-looking ways – elephants, deer, narwhals, hornbills, sawfish, bats, stalk-eyed flies, hammerheads, barreleyes, and star-nosed moles, to name only a few – and species in the fossil record were just as diverse and weird.

So let’s start off with…


Weird Heads Month #01: Wacky Flying Headgear

One of the most immediately recognizable examples of extinct animals with strange head structures are the pterosaurs, almost always depicted in pop culture with a large Pteranodon-like head crest.

But that wasn’t anywhere near as weird as pterosaur crests got.

Nyctosaurus gracilis here had an absolutely ridiculous elaborate crest, sporting an enormous antler-like structure on the back of its skull that grew to lengths longer than its own body.

Living around the Western Interior Seaway of the Midwestern United States during the Late Cretaceous, around 85 million years ago, it was a fairly small pterosaur standing about 40cm tall without the crest (1’4″) and with a 2m wingspan (6’6″). Its wings were long and narrow, and had completely lost the three small clawed fingers seen on other pterosaurs, suggesting it may have been less capable of moving around on the ground. It’s thought to have been a specialized soaring flier that spent most of its life on the wing at sea, much like a modern albatross.

Made up of two long thin spars arising from a common base, Nyctosaurus‘ crest has sometimes been reconstructed with a large sail-like membrane of skin – but since there’s no evidence at all of soft-tissue attachment on the bones, this seems unlikely. Juveniles were crestless, with only fully mature adults developing their spectacular headgear, so it was probably some sort of display structure.

It’s also not clear whether there was any sexual dimorphism in Nyctosaurus, since well-preserved skulls with intact crests are incredibly rare. But as with most other crested pterosaurs it’s likely that all mature individuals had crests, just with a difference in size and shape between sexes.

Clausocaris

While this might look like a sci-fi alien design, it was actually a very real Earth animal!

This strange-looking creature was Clausocaris lithographica, a member of a group of unique marine arthropods known as thylacocephalans. Only about 3.5cm long (1.4″), it lived in a shallow tropical lagoon environment during the Late Jurassic of Germany, about 150-145 million years ago.

Like most other thylacocephalans it had a narrow flattened shield-like carapace, three pairs of large grasping limbs, and a battery of swimming appendages further back – along with absolutely enormous bulbous eyes. Based on this anatomy it would have been a highly visual hunter, using its huge eyes to locate prey items and then snagging them with its long spiny limbs.

And we’re not even entirely sure what type of arthropods thylacocephalans actually were. They’re generally thought to be some sort of crustacean, but their highly modified anatomy makes linking up their exact evolutionary affinities very difficult. Whatever they were, they must have been incredibly successful as a group because they first appeared in the early Cambrian (~518 mya) and survived all the way into the Late Cretaceous (~94 mya).

Erlikosaurus

Erlikosaurus andrewsi, a therizinosaur from the Late Cretaceous of Mongolia (~90 mya).

Named after Erlik, the Turko-Mongolian god of death, it’s only known from partial remains – but it was the first therizinosaur ever found with a preserved skull, helping to fill in some of our knowledge of these oddball dinosaurs’ anatomy.

It was closely related to Therizinosaurus, but was only about half the size, estimated to have measured around 4-5m long (13′-16’4″). It would have had a toothless beak at the front of its jaws, an adaption for a herbivorous diet, along with long claws on its hands and a coat of fluffy down-like feathers. I’ve also given it some longer quill-like feathers here, similar to those known in Beipiaosaurus.