Placodermi had first jaws

Placodermi had strong armored front. Artist view
Image armadaproject.org 2004

This fish looked already like a real fish with jaw, gills and cartilage skeleton. It is considered the first vertebrate on Earth that had jaws. It first appears in Late Silurian and becomes very common during the Devonian period. Adam has called these placodermi (Gr. plax derma plate skin). These creatures were very successful and constitute the most common fish species in the Age of Fish. Placodermi could be a real threat to other fish and animals living in the waters as they were aggressive hunters and would even fight each other when aggravated. Although most of them were only some 15 cm long there were species that grew up to 4 or even 10 meters long sea monsters.

Placodermi did not survive the night that fell at the end of the Devonian period in massive extinction of life forms.

Mike Viney (2008) tells that among all creatures on Earth these animals were the first to have jaws and a pair of fins. The tail resembles that of a shark so they must have been effective swimmers. The dorsal fin assisted in moving in waters. The head area was protected with massive armour while the rear part was covered only by scales or skin as with ostracodermi. They usually had sharp edges on the armour plates around the mouth and only some had teeth.

Placodermi inhabited both salty seas and fresh water regions. Adam has classified seven sub-species listed below with links to PALAIOS.org pages. There you can find drawings and more information about these important devonian fish.

__________________________________________________

Acanthothoraci - spike chest. Eyes supported by bones, adults had the plates grown together.

Arthrodira - joint neck. The most common placoderm consisting about half of all the fossils discovered so far. Some arthodira could grow to even 10 meters (Dunkleosteus). The ball-joint in the neck allowed it to raise the front of the head armour and to drop the jaw into an enormous gaping mouth. Some had also teeth in addition to the sharp edges of the bone plates around the mouth.

Dunkleosteus. Late Devonian.
Reconstruction Karen Carr The Royal Tyrrell Museum of Paleontology

Antiarchi - anti anus. The second most common placoderm species. It was a small strongly protected fish as the armour covered most of its body. The eyes were looking upwards so it probably lived near the muds in search of food. Antiarchi had front fins with joints and were able to use them to crawl on the bottom.

Antiarchi had fins equipped with joints
Picture uua.ca

Petalichthyida was a small fish living at the bottom of lates. Its armour was covered with knobs.

Phyllolepida - leaf scale. These flat fish were protected by bony plates and apparently lived only in fresh waters. The species may have been blind suggesting life in darkness (caves?).

Ptyctodontida - beak tooth. This was a small placoderm with big eyes and heavy mouth plates used to break shells. The fossils resemble the rat fish (in picture) living in northern Pacific Ocean.

Spotted rat fish. Hydrolagus colliei
Image Fish index

Rhenanida - Rein fish. This was very flat resembling rays. Instead of bony plates the front of its body was covered with scales.