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What are Placoderms?

Michael Anissimov
Michael Anissimov
Michael Anissimov
Michael Anissimov

Placoderms are an extinct class of armored fishes, the first known animals of any type to evolve true jaws. Placoderms evolved from agnathan (jawless) fishes in the Silurian period, about 425 million years ago. This is around the same time as the first terrestrial animals, such as millipedes, are known to have ventured on the land.

Placoderms (class Placodermi) diversified in the mid-Silurian or possibly earlier, an early pioneer in vertebrate evolution that would continue with the Devonian period immediately afterwards, which featured so much fish diversification that is has been dubbed "The Age of Fishes." Unfortunately, no complete placoderm fossils are known — the species have been described from fragments of their body armor that are preserved when the fishes' body breaks apart after death.

Frog
Frog

The characteristic feature of placoderms is a layer of segmented armor across the head and thorax. Depending on species, the remainder of the body would be scaled or naked. Instead of true teeth, placoderms grinded their food using sharpened points of bone which protrude from the head. This method of feeding is a clear evolutionary intermediate between jawless fish and the highly adapted toothed jaws of modern fish groups.

Placoderms ranged in size from a few inches to the huge 6 m (20 ft) Dunkleosteus telleri, which lived about 360 – 415 million years ago and weighed a ton. Dunkleosteus is considered the first vertebrate superpredator, and probably took over some segments of the evolutionary niche previously occupied only by large predatory invertebrates such as sea scorpions. These animals demonstrated the ability of members of phylum Chordata (vertebrates) to occupy the apex predator niche, which they would continue to dominate for the rest of history. Dunkleosteus is thought to have had a bite force similar to that of the strongest biters in history, like Tyrannosaurus rex modern crocodiles.

Placoderms were also the first animals to internalize egg fertilization, leading to the first live births in evolutionary history. They perished in the late Devonian extinction, which also wiped out practically all jawless fishes as well as numerous other species.

Frequently Asked Questions

What are Placoderms?

Placoderms were a diverse group of armored prehistoric fish that lived during the Devonian period, around 416 to 359 million years ago. They are distinguished by their heavy bony plates covering the head and thorax, and they were among the first jawed vertebrates to appear in the fossil record. Placoderms ranged in size from a few centimeters to several meters in length.

How did Placoderms differ from modern fish?

Placoderms were quite distinct from modern fish, primarily due to their armored plates. Unlike the scales of modern fish, these plates were made of bone and provided significant protection. Placoderms also had simple jaws compared to the more advanced and varied jaw structures seen in contemporary fish species. Additionally, their fins were often spiny and not as flexible as those of modern fish.

What was the largest known Placoderm?

The largest known placoderm was Dunkleosteus terrelli, which could grow up to 6 meters in length and weigh up to a ton. This formidable predator had powerful jaws with sharp bony plates that acted like teeth, capable of exerting incredible bite force to crush its prey, which included other armored fish and even its own kind.

Did Placoderms have any unique behaviors?

While specific behaviors of placoderms are not well-documented due to the limitations of the fossil record, some species exhibited unique features suggesting specialized behavior. For instance, certain placoderms like the ptyctodontids showed evidence of live birth, indicating a level of reproductive sophistication. This viviparity is a rare glimpse into the complex life history of these ancient fish.

Why did Placoderms go extinct?

Placoderms went extinct at the end of the Devonian period, approximately 359 million years ago. The exact reasons for their extinction are not fully understood, but it is believed to be part of a larger mass extinction event that affected many marine organisms. Changes in the environment, such as fluctuations in sea levels and climate, likely played a significant role in their demise.

What significance do Placoderms hold in evolutionary history?

Placoderms are highly significant in evolutionary history as they were among the earliest jawed vertebrates, marking a major evolutionary advancement. Their emergence represents a pivotal moment in the vertebrate lineage, leading to the evolution of more complex jawed vertebrates, including the ancestors of all current fish, birds, reptiles, and mammals, including humans.

Michael Anissimov
Michael Anissimov

Michael is a longtime AllThingsNature contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

Learn more...
Michael Anissimov
Michael Anissimov

Michael is a longtime AllThingsNature contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

Learn more...

Discussion Comments

chrisinbama

As the article stated, the very first record of the jawed Placodermi was from the Early Devonian, about 400 million years ago. The placoderms flourished for almost 60 million years and were almost completely gone at the end of the Devonian.

There is nothing known of their ancestors, who must have existed in the Silurian. After the placoderms, the evolution of several other better adapted fish groups followed.

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