Placodermi (Armoured Fishes)

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.

Keywords: fossil fishes; gnathostomes; palaeozoic; armour; evolution

Figure 1. Placoderm basic design. (a) General view of the skeleton of a placoderm: the arthrodire ; modified with permission from Miles and Westoll (). © Royal Society of Edinburgh. (b) Dermal armour of an arthrodire from the Gogo formation; modified with permission from Long (1995) © The Paleontological Society.
Figure 2. Diversity of forms in placoderms. (a and b) A dolichothoracid arthrodire, . The dermal armour in dorsal view (a); cast of the endocranial cavities (b). Reproduced with permission from Goujet (1984) © CNRS. (c) An antiarch, ; adapted with permission from Young and Zhang (1992) © The Palaeontological Association. (d) An acanthothoracid, . (e) A ptyctodont, ; adapted with permission from Forey and Gardiner (1986) © John Wiley and Sons and Ørvig (1960) © Springer.
Figure 3. Cladogram summarising the interrelationships of placoderms as a monophyletic group; modified with permission from Goujet and Young (2004). © Verlag Dr. Friedrich Pfeil.
Figure 4. The two main interpretations of the relationships of Placoderms relative to other vertebrates. (a): Placoderms as a monophyletic group (modified with permission from Goujet and Young (). © Verlag Dr. Friedrich Pfeil). (b): Placoderms as a paraphyletic group (data from Brazeau, 2009, Zhu et al., 2013, Brazeau and Friedman, 2014).
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Further Reading

Janvier P (1996) Early Vertebrates. Oxford: Clarendon Press.

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Stensiö EA (1969) Arthrodires. In: Piveteau J, (ed.) Traité de Paléontologie, vol. 4 2, pp 71–692. Paris: Masson.

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How to Cite close
Goujet, Daniel F(Feb 2015) Placodermi (Armoured Fishes). In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001533.pub2]