Monday, February 3, 2014

Content Highlights – Placoderms

Did you know that in some placoderms the structure of their brain is described, although these animals are completely extinct? This can be possible due to the extraordinarily preserved internal parts of their braincase.

Bothrioloepis - dermal plates.CC BY-NC of Institute of Geology at TUT.

Placoderms are extinct paleozoic (Upper Silurian – Upper Devonian) vertebrates which inhabited oceans and fresh-water. They were characterized by the presence of articulated dermal plates covering their head and the front part of the thorax. While the plates consisted of true bone, the internal skeleton was most probably cartilaginous, sometimes with calcified parts. The placoderms had no true teeth – only ornamented dermal plates instead. The size span of placoderms was variable. The average length of their body was several tens of centimeters, but the largest could measure six meters in length or more.

Coccosteus - dermal plates. CC BY-NC of Institute of Geology at TUT.

There are two main groups of placoderms - Antiarchi and Arthrodira. Antiarchi were usually small (about 1 m long) and quite specialized. They lived mostly in freshwater environments and often possessed long limbs covered with small dermal plates which probably served for holding their body up straight in the substrate. A typical representative of Antiarchi is the Middle to Upper Devonian genus Bothriolepis. Arthrodira were placoderms of variable size from very tiny ones to the largest ever. These placoderms lived in the oceans and some of them could be active swimmers and even dangerous predators. Among the well-known representatives of the group is the genus Coccosteus.

Coccosteus - internal side of the skull roof. CC BY-NC of Institute of Geology at TUT.

In general, placoderms lived at the bottom or swam close above it. Only some of them could have been good swimmers. The bottom dwelling placoderms were most likely scavengers or fed on different kinds of animals such as mollusks or trilobites. One of the largest placoderms ever, Dunkleosteus, was certainly among the top predators in the Devonian seas.

Coccosteus (top left) and Bothriolepis (bottom right) from BHL.

Placoderms were very successful during the Devonian period (ca. 420 – 360 Ma), but later they were replaced by more successful cartilaginous and bony fishes. This was probably the reason why they became extinct. Stay tuned!