Prehistoric Fish - Devonian

Devonian

Devonian (419–359 mya): Several major groups of fish evolved during the Devonian, so the Devonian is often referred to as the "Age of Fish".

The placoderms, a group of jawed fishes, appeared by the beginning of the Devonian, about 416 million years ago, and became extinct at the end of the Devonian or the beginning of the Mississippian (Carboniferous), about 360 million years ago. Recent studies suggest that the placoderms are a possibly paraphyletic group of basal gnathostomes, and the closest relatives of all living jawed vertebrates. Some Placoderms were small, flattened bottom-dwellers, such as antiarchs. However, many, particularly the arthrodires, were active midwater predators. Dunkleosteus was the largest and most famous of these. The upper jaw was firmly fused to the skull, but there was a hinge joint between the skull and the bony plating of the trunk region. This allowed the upper part of the head to be thrown back and, in arthrodires, this allowed them to take larger bites.

The first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating almost every known aquatic environment.

The ancestors of all tetrapods began adapting to walking on land, their strong pectoral and pelvic fins gradually evolved into legs (see Tiktaalik). In the oceans, primitive sharks became more numerous than in the Silurian and the late Ordovician. The first ammonite mollusks appeared. Trilobites, the mollusk-like brachiopods and the great coral reefs, were still common. The Late Devonian extinction severely affected marine life, killing off all placoderms, and all trilobites, save for a few species of the order Proetida.

The Devonian Period is formally broken into Early, Middle and Late subdivisions. The rocks corresponding to these epochs are referred to as belonging to the Lower, Middle and Upper parts of the Devonian System.

The Early Devonian lasts from 419.2 million years ago to 393.3 million years ago and begins with the Lochkovian stage, which lasts until the Pragian. This spans from 410.8 million years ago to 407.6 million years ago, and is followed by the Emsian, which lasts until the Middle Devonian begins, 393.3± 2.7 million years ago. The Middle Devonian comprises two subdivisions, the Eifelian giving way to the Givetian 387.7± 2.7 million years ago. During this time the armoured jawless ostracoderm fish were declining in diversity; the jawed fish were thriving and increasing in diversity in both the oceans and freshwater. The shallow, warm, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop essential characteristics such as well developed lungs, and the ability to crawl out of the water and onto the land for short periods of time.