More than 400 million years ago, Earth’s waters were home to some very unusual fish. Many were covered in heavy armor, some had powerful jaws, and a few grew into formidable predators.
Meet the placoderms, one of the most distinctive groups of fish from the Devonian Period.
The Age of Fishes
The Devonian Period began about 419 million years ago and ended about 359 million years ago. It is often called the “Age of Fishes” because fish underwent an extraordinary period of diversification during this time.
Among the most recognizable inhabitants of Devonian waters were the placoderms, an extinct group of armored, jawed fish.
Placoderms appeared before the Devonian, but they became remarkably diverse during this period. Fossils have been discovered in ancient marine and freshwater environments around the world.
What Is a Placoderm?
The name Placodermi refers to the distinctive bony armor that covered much of these fishes’ heads and the front portions of their bodies.
Instead of being covered entirely in heavy armor, many placoderms had a heavily protected front section paired with a more flexible rear portion of the body. Their armor came in a variety of shapes depending on the species and its way of life.
Placoderms were also among the earliest vertebrates with jaws, an evolutionary development that opened up entirely new ways of feeding.
Meet Bothriolepis
Not every placoderm was a giant predator.
Bothriolepis was a relatively small placoderm with an armored head and body and distinctive armored pectoral appendages extending from its sides.
These appendages even had joints, giving them an appearance that can look surprisingly limb-like. Rather than being built like the fins we usually picture on a modern fish, they were covered in bony armor.
Bothriolepis lived near the bottom of aquatic environments, where it could move along the substrate while searching for food.
Meet Dunkleosteus
At the other end of the placoderm spectrum was the much more imposing Dunkleosteus.
Dunkleosteus was a large predatory placoderm that lived during the Late Devonian. Its massive armored skull and powerful jaws have made it one of the best-known animals from the Age of Fishes.
But there is something particularly interesting about its mouth.
Dunkleosteus did not have conventional teeth. Instead, portions of its bony jaw plates formed sharp cutting edges. As the jaws opened and closed, these surfaces could maintain effective cutting edges capable of processing tough prey.
Combined with a powerful bite, this made Dunkleosteus a formidable predator in Devonian seas.
What Did Dunkleosteus Eat?
With jaws like those, Dunkleosteus was equipped to tackle a variety of prey.
Evidence suggests its diet could have included other armored fish and other marine animals available in its environment. Fossil evidence also indicates that at least some large placoderms could process heavily armored prey.
Rather than relying on individual teeth, Dunkleosteus used its sharpened jaw plates to seize and cut through food.
What Happened to the Placoderms?
Placoderms flourished for millions of years, but their story came to an end during the Late Devonian extinction events.
By the end of the Devonian Period, placoderms had disappeared from the fossil record. Other groups of jawed fish survived and continued to diversify, eventually giving rise to much of the incredible variety of vertebrate life found in the world's waters today.
Their fossils give paleontologists an extraordinary glimpse into an important chapter in vertebrate evolution, when armored fish occupied ecosystems very different from anything we see today.
Explore the Age of Fishes
From small bottom-dwelling forms like Bothriolepis to formidable predators like Dunkleosteus, placoderms show just how diverse life had already become hundreds of millions of years ago.
The Devonian truly earned its nickname: the Age of Fishes.
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