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Science Olympiad
Class Placodermi

Gnathostomes (Infraphylum Gnathostomata) include all of the jawed vertebrates. Jaws opened up many adaptive pathways including predation. Jaws allow prey to be secured, manipulated and eaten. Evidence suggests that jaws formed from modified gill arches. Placoderms (class Placodermi "tablet or plate skin") were heavily armored jawed fish that range from the Silurian to the Devonian.

Armored Jawed Fish

Placoderms reached their greatest diversity during the Devonian, often referred to as the "Age of Fish", and were in fact the dominant fish group during this time (Prothero, 1998, p. 345). Placoderms possessed articulated bony plates over their head and shoulder regions. A neck joint allowed placoderms to lift the anterior portion of their head shield. A cartilaginous skeleton supported the placoderm body. Placoderms had scales or skin covering the rest of their body and tail. Placoderms had a long dorsal fin and a heterocercal tail (an asymmetrical tail in which the dorsal lobe is extended due to an upward flexion of the spine). Placoderms had paired pectoral fins and were the first vertebrates to evolve paired pelvic fins (Benton, 2005, p. 55). The majority of placoderms did not have true teeth, their head shield and mandibular shield were modified into sharp-edged cutting surfaces. Most placoderms were small fish reaching lengths of 15 cm; however, a few species reached lengths of 4 to 10 meters. Placoderms lived in both marine and freshwater environments. Currently, Placodermi includes seven clades.


Arthrodires (order Arthrodira "jointed neck") make up more than half of all known placoderms and reached their greatest diversity in the Devonian. Although sometimes heavily armored these fish diversified into most marine ecological niches. Arthrodires had a ball-and-socket joint between their head and neck armor. This joint allowed the upper jaw to swing upwards, while the lower jaw dropped creating a wide mouth opening. Arthrodires had paired pectoral and pelvic fins. The edges of their bony plates acted as a cutting surface, although there is evidence that some had teeth inside their mouth. Arthrodires have a ring of bony plates surrounding their eyes. Some Arthrodires reached lengths of up to 10 meters. Dunkleosteus was the largest predator known from the Devonian seas (Benton, 2005, pp. 55-57). Some arthrodire plates have puncture wounds from other arthrodires (Prothero, 1998, p. 346).


Acanthothoracids (order Acanthothoraci "spiny chests") are basal placoderms that resemble early arthrodires. As adults their armor plates fused together. Like typical placoderms they had beak-like tooth projections and bone enhanced eye sockets.


Rhenanids (order Rhenanida "Rhine Fish") had scales and tubercles covering the head and neck region instead of heavy armor. Rhenanids looked like modern day skates orrays as their body was flattened and possessed broad pectoral fins. This is a good example of convergent evolution as rhenanids were placoderms not chondrichthyans.


Antiarchs (order Antiarchi "opposite anus") were mostly small, freshwater forms and retained heavy armor. The thorasic shield covered almost half of their body. They had a flattened head shield with eyes pointing upward and lived along the bottom swallowing sediment and extracting organic matter. Their pectoral fin was enclosed in bone and was jointed. This arthropod-like limb may have been used to crawl along the bottom and cover the fish with sediment. Antiarchs were the second most successful group of placoderms.


Petalichthyids (order Petalichthyida) were small bottom-dwelling forms. They had splayed fins and numerous tubercles decorating the plates and scales of their armor.


Phyllolepids (order Phyllolepida "leaf scales") are flattened bottom dwelling freshwater placoderms. The armor of phyllolepids were made of whole plates. Phyllolepids had vestigial eyes and were most likely blind.


Ptyctodonts (order Ptyctodontida "beak teeth") are small (20 cm) placoderms with reduced armor plating. They had large heads and eyes with long whip-like tails. The anterior dorsal spine was supported by long spine. The posterior dorsal fin was long. Some ptyctodonts had claspers associated with their pelvic fins that may have been used for internal fertilization (Benton, 2005, p. 58). Ptyctodonts had thick tooth plates for crushing molluscs and looked superficially like the modern day chimaera or ratfish.

Placoderms were one of the first groups of fish to evolve jaws and reached their greatest diversity during the Devonian. Placoderms did not survive the extinction event at the end of the Devonian.

Science Olympiad Fossil Event

The 2016 Science Olympiad Fossil List includes the class Placodermi (Armored Fish). Two genera are included within this category Dunkleosteus and Bothriolepis.


Benton, M.J. (2005) Vertebrate Palaeontology [3rd Edition]. Blackwell Publishing: Main, USA.

Janvier, Philippe. 1997. Gnathostomata. Jawed Vertebrates. Version 01 January 1997 (under construction). in The Tree of Life Web Project,

Paleos Achanthothoraci page:

Paleos Antiarchi Page:

Paleos Anthrodira Page:

Paleos Petalichthyida Page:

Paleos Phyllolepida Page:

Paleos Placodermi Page:

Paleos Rhenanida Page:

Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill.

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