Tag: placoderm

Spectember 2025 #02: A Little Shell-Fish

Another anonymous request asked for a “terrestrial placoderm“:

A digital sketch of a speculative terrestrial placoderm fish. It has only two limbs, with a bony carapace covering its head, body, and legs. Its boxy head has large frog-like eyes, its back is high-domed like tortoise's shell, and a scaly tail protrudes behind it for balance. Its legs are made up of two segments each, somewhat resembling the hind legs of a grasshopper, ending in blocky triangular "feet".

Keluphichthys pezoporus is a descendant of Bothriolepis-like Devonian placoderms. Inhabiting shallow freshwater environments, they often used their rigid jointed fins to scramble short distances over land to reach new isolated ephemeral pools, and they developed convergently lung-like structures that could exchange gases directly from air, allowing them to survive in poorly-oxygenated waters and make even longer terrestrial journeys.

While the end-Devonian extinction devastated all other placoderms, this odd lineage survived into the Carboniferous, eventually raising themselves up to walk fully on their two limbs using a heavily scaled tail for balance.

Due to the relative weight of its bony carapace Keluphichthys is a fairly small animal, standing about 10cm tall (~4″). Its high domed body shape allows it to right itself when overturned, and resists the bite forces of larger predators such as the early tetrapods it lives alongside.

Its jaws are protrusible, with bony blades fused into a serrated “beak” used to snatch up invertebrate prey.

A digital sketch of a speculative terrestrial placoderm fish's head, detailing its protrusible jaws made up of serrated beak-like bony blades.

It’s still reliant on wet environments, needing to stay moist and returning to pools of water in order to reproduce. Juveniles start out aquatic and gradually transition to terrestrial habits as they mature.

Dunkleosteus

With its armored head and blade-like jaws, the placoderm fish Dunkleosteus terrelli is an iconic Paleozoic animal.

Living during the Late Devonian, about 375-359 million years ago, in subtropical waters covering parts of what are now North America and Europe, this species is known mostly just from the bony plates that covered its head and thorax. The rest of its skeleton was cartilaginous and rarely ever fossilized (only a few vertebrae and the pectoral fin are currently known), so its full body shape and size is poorly understood, and previous length estimates have ranged all the way up to 10m (33′).

…Except it turns out it wasn’t nearly that big.

Based on its head proportions, along with comparisons to more complete remains of other arthrodire placoderms, recent studies instead come up with a maximum length of about 4m (~13ft) – giving Dunkleosteus a much shorter-but-heavier chunky body shape, more like a tuna than a shark.

But even after this size revision Dunkleosteus would have still been one of the largest animals around at the time, with the ability to snap its jaws open at high speed and an incredibly strong bite force. It was probably specialized to mainly prey on other heavily-armored animals such as other placoderms and shelled cephalopods, and was likely a strong swimmer with a shark-like tail fin.

Preserved stomach contents in one fossil show remains of the fast-swimming cartilaginous fish Orodus – suggesting that much like the modern tuna it resembled, Dunkleosteus was also capable of bursts of high speed.

Groenlandaspis

The armor-headed placoderms were the dominant fish during the Devonian period, evolving a wonderfully diverse range of shapes and sizes, and occupying ecological niches in both marine and freshwater habitats.

Groenlandaspis antarctica here lived during the mid-to-late Devonian, about 383 million years ago, in the Oates Land region of Antarctica – at that time located further north than it is today, with the local climate being warm and subtropical.

It was a moderately-sized river-dwelling placoderm, around 50cm long (1’8″), and its bony armor formed a sort of pyramid shape with wing-like projections at its sides, a structure that would have acted as a hydrofoil and made it an efficient swimmer. Most of the armor plates were rigidly fused together, except for a hinge point between its head and thorax that allowed it to open its jaws, but unlike its more famous relative Dunkleosteus it couldn’t gape its mouth open particularly wide. It may have been a bottom-feeder, grubbing around in muddy riverbeds and using its small but strong jaws to crush hard-shelled prey.

Various other species of the Groenlandaspis genus have been found all around the world, but there’s something incredibly rare and special about Groenlandaspis antarctica in particular:

We actually know what color it was.

Preserved pigment cells in its fossils indicate that it was red on top and silvery-white on its underside in a countershaded pattern, camouflaging it in the murky silty waters of the ancient Antarctic rivers.

…And also made it look a bit like a prehistoric goldfish.

Brindabellaspis

The placoderms are most famous for some of the biggest members of the group such as the giant blade-jawed Dunkleosteus. But these ancient armored fish were actually incredibly diverse in their time, occupying many different ecological niches and developing a wide range of body shapes.

Perhaps one of the most unusual was Brindabellaspis stensioi from the Early Devonian of New South Wales, Australia. Living around 405 million years ago in a tropical reef ecosystem, this early placoderm was quite small, only about 45cm long (1′6″), and it was recently revealed to have had an especially weird head.

Its skull was flattened with its eyes facing upwards on top, its nostrils came out of the corners of its eye sockets, and its jaws were positioned very far forward. It also had a long flat snout packed full of sensory nerves, sort of like the bill of the modern platypus but using a modified form of the pressure-sensing lateral line system instead of electroreception.

It was probably some sort of bottom-feeder, using its bill to feel around on the seafloor for small prey – and there may even have been a longer and wider soft tissue extension to its sensitive snout, giving it even more of a duck-like shape.