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, while 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, which helped them hunt and eat other sea creatures.

Scientists think placoderms were some of the first fish to have pelvic fins, which 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 were very successful and became top predators in the ocean, with some very big species like Dunkleosteus.

Eventually, though, all placoderms went extinct about 359 million years ago. After they disappeared, other types of fish were able to grow and spread in the oceans. Today, we know about these amazing 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 during the Silurian and Devonian periods. Many of them had tough, bony plates covering their heads and chests, which is why they were named "placoderms," meaning "plate skin." These fish were among the first to have jaws and true teeth, which helped them hunt and eat other sea creatures.

Some placoderms were large predators, like Dunkleosteus, which could grow up to 4 metres long and was one of the first big hunters in the ocean. Others were smaller and had streamlined bodies for swift swimming. Scientists discovered a special fossil of a small placoderm, Materpiscis attenboroughi, that showed it gave birth to live young, which was a surprising finding about ancient fish reproduction.

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 closely. He carefully removed layers from the fossils to create detailed models of their skulls. This helped show that placoderms were actually jawed fish related to sharks.

Even later, amazing fossils from Gogo reef showed that placoderms had features shared with many different kinds of fish, helping scientists understand their place in the history of life.

Evolution and extinction

See also: Evolution of fish

It was once believed that placoderms disappeared because they could not compete with the first bony fish and early sharks. Now, scientists think they vanished because of big changes in the oceans and rivers during the Late Devonian and end-Devonian extinctions.

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

Taxonomy and phylogeny

Currently, Placodermi are divided into eight recognized orders. There are two further controversial orders: One is the monotypic Stensioellida, containing the enigmatic Stensioella; the other is the equally enigmatic Pseudopetalichthyida. These orders are considered to be basal or primitive groups within Placodermi, though their precise placement within the class remains unsure. Fossils of both are currently known only from the Hunsruck lagerstatten.

Arthrodira ("jointed neck") were the most diverse and numerically successful of the placoderm orders, occupying roles from giant apex predators to detritus-nibbling bottom dwellers. The largest member of this group, Dunkleosteus, was a true "superpredator" of the latest Devonian period, reaching 3 to as much as 8 metres in length.

Antiarchi ("opposite anus") were the second most successful order of placoderms known, after the Arthrodira. The front portions of their bodies were heavily armoured, resembling a box with eyes. The pair of pectoral fins were modified into a pair of caliper-like, or arthropod-like limbs.

Brindabellaspis was a long-snouted placoderm from the Early Devonian.

Phyllolepida ("leaf scales") were flattened placoderms found throughout the world. Like other flattened placoderms they were bottom-dwelling predators that ambushed prey.

Ptyctodontida ("folded teeth") were lightly armoured placoderms with big heads, big eyes and long bodies.

Rhenanida ("Rhine fish") were flattened, ray-like, bottom-dwelling predators with large, upturned mouths that lived in marine environments.

Acanthothoraci ("spine chests") were a group of chimaera-like placoderms closely related to the rhenanid placoderms.

Petalichthyida ("thin-plated fish") were small, flattened placoderms, typified by their splayed fins and numerous tubercles that decorated all of the plates and scales of their armour.

Pseudopetalichthyida ("false petalichthyids") is a group of elongated, possibly flattened fishes comprising three, poorly preserved and poorly studied genera.

Stensioellida ("[Heintz's] little Stensio") contains another problematic placoderm of uncertain affinity, known only from the Lower Devonian Hunsrück slates of Germany.

Cladogram

See also: Agnatha § Groups

The following cladogram shows how scientists used to understand the relationships between different placoderms. Over time, scientists have learned more, and now think that placoderms might not all be closely related to each other. Some placoderms seem to be closer to a group called Eugnathostomata than others. This means our understanding of these ancient fish continues to change as we discover more.