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Arthrodira

Adapted from Wikipedia · Discoverer experience

Scientific illustration of Amazichthys, an ancient armored fish from the Late Devonian period discovered in Morocco.

Arthrodira means "jointed neck" in Greek. They were a group of armored fish that lived a very long time ago during the Devonian period. These special fish had jaws and were part of a larger group called Placodermi.

Arthrodires were very successful and lived in many different parts of the ocean. They were the biggest and most varied group of placoderms, which means they came in many shapes and sizes. These fish lived for about 50 million years before they suddenly disappeared from the Earth.

Description

Arthrodire placoderms are known for the special joint that let the armor around their heads move with their bodies. They didn’t have real teeth; instead, they used sharp edges on their jawbone bones to bite. Their eyes were protected by a special bone ring, similar to what birds and some ancient sea reptiles had.

Evolution and extinction of placoderms. The diagram is based on Michael Benton, 2005.

Early arthrodires, like the fish Arctolepis, had strong armor and flat bodies. The biggest arthrodire, Dunkleosteus, was a giant fish from the Devonian period, growing up to 6 meters long. Smaller types, like the long-nosed Rolfosteus, were only about 15 centimeters.

Some scientists once thought arthrodires were slow fish that couldn’t compete with newer fish. But we now know they were very successful and filled many different roles in the ocean, from top hunters to small fish that ate bits on the sea floor. Sadly, arthrodires disappeared during a big change in Earth’s environment at the end of the Devonian period. After they were gone, other fish like sharks began to spread and grow in new ways during the Carboniferous period.

Phylogeny

The order Arthrodira belongs to the class Placodermi, a large group of extinct armored fish. These fish lived long ago and are thought to have split away from the ancestors of sharks, bony fish, and all animals with four limbs, like mammals, birds, reptiles, and amphibians.

Recent studies have shown that Placodermi might not form a natural group but instead represent steps in evolution leading to a larger group that includes sharks, bony fish, and four-limbed animals. Arthrodira used to be divided into three groups, but new research shows that two of these groups do not form natural categories either.

Classification

The pelagic selenosteid Amazichthys trinajsticae
Artist's reconstruction of the arthrodire placoderm Coccosteus cuspidatus
Lower jaw plate of Eastmanosteus pustulosus from the Middle Devonian of Wisconsin
Heterosteus ingens from the Middle Devonian of Estonia

Order Arthrodira Woodward, 1891

Images

An artist's drawing showing what the ancient armored fish Kudjanowiaspis might have looked like when it lived millions of years ago.
An artist's drawing showing how the ancient fish-like creature Ailuracantha might have looked millions of years ago.
An artist's drawing showing what the ancient armored fish Cartieraspis might have looked like millions of years ago.
An artistic reconstruction of the ancient armored fish Groenlandaspis antarctica, showing its distinctive red and silver coloring as inferred from fossil pigment cells.
A scientific illustration of Titanichthys clarki, an ancient armored fish from the age of the dinosaurs, showing its body and fins as imagined by modern paleontologists.

Related articles

Devonian
Dunkleosteus
Shark
Carboniferous

This article is a child-friendly adaptation of the Wikipedia article on Arthrodira, available under CC BY-SA 4.0.

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