Placoderms: Knights of the Aquatic Table

Article by: J. D. Dixon
Edited by: Adam Manning and Harry T. Jones

Placoderms (Class Placodermi) are a group of prehistoric armoured fish known from global sediments. They are some of the earliest recognised gnathostomes (jawed vertebrates), and occupy a temporal range of at least 70 million years. While they first occurred in the Early Silurian of China, the group diversified massively in the Early Devonian, becoming the dominant aquatic vertebrate across marine and freshwater ecosystems.

Dunkleosteus, a large Devonian placoderm. Artwork by Julio Lacerda.

Placoderms are characterised by their distinctive frontal armour, most likely a method of protection against predation. In primitive forms, this is restricted to a single row of plates encircling the body behind the skull. However, as the group became more derived, they developed body scales along with bony plates making a head and thoracic shield. Their armour even had a neck joint which allowed movement of the head independent of the body. The armour of placoderms is tough and subsequently easily preserved, meaning they have a very robust fossil record (there are around 335 identified genera). The scales of placoderms may even serve a purpose in biostratigraphy, as some unique forms could be used to date sediment boundaries.

The group consisted of several clades, some of which had varying modes of life. Placoderms such as Lunaspis and Gemuendina had flattened bodies, and so are thought to have lived on the sea floor. Coccosteus, a genus first described in 1841, had large fins and a powerful tail, meaning it was probably a capable swimmer. The placoderms may have reproduced by viviparity (live birth), as embryos reported within the skeletons of Materpiscis and Incisoscutum indicate this was the case.

Most placoderms are interpreted as carnivores that evolved powerful jaws in order to capture and crush live prey rapidly and prior to ingestion. Instead of teeth, these predators had a series of razor-sharp bony plates forming a beak-like structure used to squash their prey. This was thought to be the case across the group, but some placoderms, such as Compagopiscis, did have teeth as well.

Holonema, a small Devonian placoderm. Artwork by Joschua Knüppe.

Many placoderms were between 10-40 cm in length. These included the primitive Dicksonosteus and the long-bodied but small-headed Pterichthyodes. The most successful placoderm was Bothriolepis, as this genus contained over 100 species distributed globally.

Some placoderms adopted very bizarre bodies. Rhamphodopsis was not heavily armoured, but had a whiplike tail and is known to have exhibited sexual dimorphism. Males of the genus had claspers (perhaps to assist in internal fertilisation), while females had pelvic fins covered in significantly sized scales. Rolfosteus was another peculiar placoderm, with a headshield that formed a tapering tube at the snout, the use of which is undecided.

The most famous placoderm is probably Dunkleosteus, a Late Devonian fish that could grow up to six metres in length and had armoured plates up to two inches thick. These apex predators have been found across Europe, the USA, and Morocco, and had an estimated bite force of 4400-5300 N, the strongest of any living or extinct fish.

The End-Devonian Mass Extinction saw the final demise of the placoderms. Nevertheless, while this strange group of fish may be lost to time, their fossils still hold interest for scientists around the world.

Image References
[1] Dunkleosteus, a large Devonian placoderm. Artwork by Julio Lacerda.
[2] Holonema, a small Devonian placoderm. Artwork by Joschua Knüppe.

Information References and Further Sources
[1] Anderson, P. S. L., and Westneat, M. W. (2007). ‘Feeding mechanics and bite force modelling of the skull of Dunkleosteus terrelli, an ancient apex predator’, Biology Letters, 3 (1), pp. 77-80. Accessed 15th November 2020. Click Here.
[2] Benton, M. J. (2014). Vertebrate Palaeontology. 4th ed. Oxford: Wiley-Blackwell. pp. 60-65.
[3] Benton, M. J., and Harper, D. A. T. (2009). Introduction to Paleobiology and the Fossil Record. Wiley-Blackwell. pp. 435-438.
[4] Burrow, C. J., and Turner, S. (1999). ‘A review of placoderm scales, and their significance in placoderm phylogeny’, Journal of Vertebrate Paleontology, 19 (2), pp. 204-219. Accessed 15th November 2020. Click Here.
[5] Coenraads, R. R. (2004). ‘Ancient Worlds’ in Rocks & Fossils: A Visual Guide. The Reader’s Digest Association Limited. pp. 78-81.
[6] Dorling Kindersley. (2009). ‘Devonian’, in Prehistoric. Great Britain: Dorling Kindersley Limited. pp. 108-139.
[7] Goujet, D. (2001). ‘Chapter 13: Placoderms and basal gnathostome apomorphies’, in Ahlberg, P. E. Major Events in Early Vertebrate Evolution. CRC Press. pp. 209-222. Accessed 15th November 2020. Click Here.
[8] Johanson, Z., and Smith, M. M. (2005). ‘Origin and evolution of gnathostome dentitions: a question of teeth and pharyngeal denticles in placoderms’, Biological Reviews, 80 (2), pp. 303-345. Accessed 15th November 2020. Click Here.
[9] Young, G. C. (2010). ‘Placoderms (Armored Fish): Dominant Vertebrates of the Devonian Period’, Annual Review of Earth and Planetary Sciences, 38, pp. 523-550. Accessed 15th November 2020. Click Here.