Placoderm

Placoderms were an interesting group of ancient fish that lived long ago during the Silurian and Devonian periods. Their name means "plate skin" because they had hard, bony plates covering their heads and chests. The rest of their bodies had scales or no covering at all. These fish were special because they were among the first to have jaws and teeth. This helped them hunt and eat other sea creatures.

Scientists think placoderms were some of the first fish to have pelvic fins. These are the back pair of fins that later helped fish and even land animals move. Some placoderms could even give birth to live young, which is rare for fish. They became top predators in the ocean. Some were very big species like Dunkleosteus.

All placoderms went extinct about 359 million years ago. After they disappeared, other types of fish grew and spread in the oceans. Today, we know about these ancient fish from fossils that scientists have found all over the world.

Characteristics

Placoderms were an ancient group of fish that lived in the seas long ago. Many had tough, bony plates on their heads and chests. That is why they were called "placoderms," which means "plate skin." These fish were among the first to have jaws and teeth. This helped them catch and eat other sea creatures.

Some placoderms were big hunters, like Dunkleosteus. It could grow up to 4 metres long. Others were smaller and could swim quickly. Scientists found a special fossil of a small placoderm, Materpiscis attenboroughi. The fossil showed that it gave birth to live young. This was an exciting discovery about how ancient fish had babies.

History of study

The earliest studies of placoderms were done by Louis Agassiz between 1833 and 1843. At first, people thought placoderms were jawless fish or even invertebrates.

Later, in the late 1920s, Erik Stensiö at the Swedish Museum of Natural History in Stockholm studied placoderm fossils. He looked closely at the fossils to learn more about them. This helped show that placoderms were jawed fish related to sharks.

Amazing fossils from Gogo reef showed that placoderms had features like many other kinds of fish. This helped scientists learn more about their place in the history of life.

Evolution and extinction

See also: Evolution of fish

Scientists used to think placoderms disappeared because they could not compete with the first bony fish and early sharks. Now, we know they vanished because of big changes in the oceans and rivers during the Late Devonian and end-Devonian extinctions.

The oldest placoderm fossils come from China. They date back to the early Silurian period. These fossils show that placoderms were already many different types. Some of the earliest known placoderms include Silurolepis, Entelognathus, and Qilinyu. Placoderms lived in almost every kind of water environment during the Devonian period. But, like many other animals, their numbers greatly reduced during the Late Devonian extinctions. They all went extinct by the end of the Devonian, with none surviving into the Carboniferous period.

Taxonomy and phylogeny

Placodermi are divided into eight groups. There are two more groups that are not fully understood: Stensioellida and Pseudopetalichthyida. These groups are thought to be very basic types of placoderms, but we do not know exactly where they fit. Fossils of both are found only in the Hunsruck lagerstatten.

Arthrodira were the most successful group of placoderms. They were many different kinds of animals, from large apex predators to smaller detritus-eaters that lived on the bottom dwellers. The largest member of this group, Dunkleosteus, was a big predator in the latest Devonian period, growing up to 8 metres long.

Antiarchi were the second most successful group of placoderms. The front of their bodies was very heavily armoured, looking like a box with eyes. Their pectoral fins were changed into caliper-like or arthropod-like limbs.

Brindabellaspis was a placoderm with a long nose from the Early Devonian.

Phyllolepida were flat placoderms found all over the world. Like other flat placoderms, they lived on the bottom and waited to catch their food.

Ptyctodontida were lightly armoured placoderms with big heads, big eyes, and long bodies.

Rhenanida were flat, ray-like placoderms that lived on the bottom. They had big, turned-up mouths and lived in the sea.

Acanthothoraci were a group of chimaera-like placoderms that were closely related to the rhenanid placoderms.

Petalichthyida were small, flat placoderms with wide fins and many small bumps on their armour.

Pseudopetalichthyida is a group of long, possibly flat fish that are not well known.

Stensioellida is another placoderm that is hard to understand, known only from the Lower Devonian Hunsrück slates of Germany.

Cladogram

See also: Agnatha § Groups

The following cladogram shows how scientists used to think about how different placoderms are related. As we learn more, we now believe that placoderms may not all be closely related. Some placoderms appear to be closer to a group called Eugnathostomata than others. This means our knowledge about these ancient fish is still growing as we find out more.