Placodermi (Armoured Fishes)

Abstract

Abstract Placoderms are a group of fossil true jawed fishes restricted to the Middle Palaeozoic period (Silurian and Devonian). Their head and body are covered with an assemblage of bony plates in a box‐like manner up to the anal region. They were the dominant forms in the seas of the time, with more than 300 species present in all continents. Nine groups have been presently distinguished within the whole ensemble, considered according to different specialists either as a monophyletic or a paraphyletic group. They retain in their anatomy some features that are present in their most closely related agnathan precursors. They show specific adaptations to various modes of life, including the first active vertebrate predators and numerous bottom‐feeders. Most recent discoveries have shown that some of them had an internal fertilisation and viviparous reproduction has been demonstrated with the discovery of embryos within the body of presumed female individuals. Key Concepts Placoderms were the dominant group in the Devonian waters. They are the earliest true jawed vertebrates that show a number of anatomical features, illustrating the primitive condition in forthcoming other vertebrates. They also retain some features present in fossil agnaths. They are the first fish to adapt to different modes of life and to various reproductive strategies.